About this book

There have been many books on the subject of Overland Navigation and it’s true to say that

the subject requires some dedication to become proficient; however, once you are proficient

you will gain a self confidence when moving through hostile terrain that others will envy.

I have deliberately not listed the individual headings under page numbers; the reason is that

most people tend to skip to the bits they feel they want to know about. Often, this leads to

misunderstandings and a lack of correlation between the individual skills. The book is not

particularly long and by reading it from the beginning you will find that everything falls into

place with each other. It’s this continuity that will make practical navigation much easier and

understandable; after all, it’s the way you’ll interpret the theory that will make the practical

navigation skills easier to master.

As you progress through the book you will learn how to find your way across all types of

terrain using a number of methods. You will learn how to find out where you are, how to plan

a route; you will learn about attack points and aiming off in bad weather. You will learn how

to perform an expanding spiral search, learn about slope aspects, pacing, handrailing and

much, much more. The book refers to the Ordnance Survey 1:50,000 ‘Landranger’ series to

explain many aspects but the concept is the same wherever you are in the world.

To the overland adventurer, navigation is only one of the many skills that are required. The

skill is no more or less important than many of the other skills an adventurer must possess to

remain safe in what can sometimes be extremely dangerous environments. Many adventurers

travel alone requiring all of the basic skills to be of an extremely high standard in order to

maintain a healthy safety margin

Of course, reading this book won’t make you a good navigator overnight but it will give you

all the basic information to become very proficient. The practical experience will be gained

on your adventures and as you become more confident you will plan more remote adventures.

Although there is always more to learn you will get to the stage where extreme weather will

only ever be a consideration from an additional skills & equipment point of view, never from

a navigational one.

I hope that this book will inspire you to explore the subject further and to use your newfound

skills to fulfil your outdoor dreams and ambitions, wherever they may be. An advanced

overland navigation book should be available shortly covering the navigational

considerations for group leaders, Special Forces and escape & evasion.

Our journey begins with a look at the system used in making many of our maps. Although the

system is quite complex a general understanding will be sufficient for our needs. We also

need to look at the terms used and how to find all the information that we may require from

the map we are using at the time. Once you have a good basic understanding on how maps

depict the surface of our planet it’s amazing what can be done with such a small amount of

information. A map generally contains as much information that’s unwritten as is written! It’s

this basic understanding that is so often lacking in many overland navigation courses, the

emission often causing life threatening problems under certain conditions.

The Mercator Projection

Gerardus Mercator, a Flemish geographer employed by the Emp. Charles V to draw maps,

first used this system in 1568 and it has since been known by his name.

The Mercator projection is a cylindrical projection, meaning the globe is encircled by an

imaginary cylinder touching at the equator; the Earth is then projected onto the cylinder.

The term ‘map projection’ actually means ‘any transformation of the globe onto some other

surface’ but strictly speaking, the Mercator projection is obtained through mathematical

formulas and as such is a representation rather than a projection. The Mercator projection is

termed a ‘conformal’ projection, meaning that angles and small shapes on the globe project

as the same angles or shapes on the map; however, the downside of all conformal projections

is the extreme scale distortions away from the central origin.

The Transverse Mercator Projection

Within 10 degrees or so of the equator the scale error of a Mercator projection would only be

around 1 percent or so; however, outside this band distortions increase dramatically.

There is no reason that the cylinder has to touch at the equator; we can turn the cylinder so

that it touches on a meridian of longitude. Such a projection is known as a ‘Transverse’

Mercator projection. There are also ‘Oblique’ Mercator projections, but these are only used

for specific applications such as long, thin islands running at an angle to the meridians.

Because the Transverse Mercator projection is very accurate in narrow zones it has become

the basis for a global co-ordinate system known as the ‘Universal Transverse Mercator

System’ or UTMS. The globe is divided into narrow bands of longitude each with an angular

displacement of 006 degrees and then projected. A grid is then constructed on the projection

and is used to locate specific points. The advantage of the grid system is that since it is

rectangular and decimal, it is far easier to use than latitude and longitude. The disadvantage is

that, unlike latitude and longitude, there is no way to determine grid locations independently;

however, this is a small price to pay for such a universally accepted system. This is covered

later in more detail.

A Simplified Diagram of the Mercator Concept

THE NORTH POLE THE NORTH POLE

NARROW BANDS OF

LONGITUDE

EQUATOR MERIDIANS OF LONGITUDE

A NORMAL MERCATOR PROJECTION A TRANSVERSE MERCATOR PROJECTION

Types of Maps

Bathymetric maps. These maps are more commonly referred to as charts and are for

navigating on water. They contain very limited information about the land but contain a vast

amount of information about the sea in the form of depths, currents, overfalls, rocks, buoys,

wrecks, traffic lanes, bottom composition etc. The word ‘Bathymetric’ derived from the

Greek ‘Bathus’ meaning deep and ‘Metron’ meaning measure.

True North to Magnetic North on charts = Magnetic Variation.

Planimetric maps. These maps depict only a flat surface with no contours or height

recognition at all. The word ‘Planimetric’ derived from the Latin ‘Planus’ meaning flat and

‘Metrum’ meaning measure.

Topographic maps. These maps provide detailed information on rivers, mountains, valleys,

etc. and contain full contour information. The word ‘Topographic’ derived from the Greek

‘Topos’ meaning ‘place’ and ‘Graphein’ meaning to write.

True North to Magnetic North on topographical maps = Magnetic Declination.

Note that it is ‘Magnetic Declination’ on topographic maps and ‘Magnetic Variation’ on

bathymetric maps. When working with bathymetric maps we use the word ‘declination’ in

another method of navigation.

We use a topographical map to navigate in open country, often with the help of a baseplate

compass; it’s this that we’ll cover in some detail. Firstly, we need to look at the different

types of North along with the correct terminology; this is important as a good understanding

will allow you to visualise what’s actually happening. We are looking at the British Ordnance

Survey system here but the theory holds true with any grid overlay system.

Using the British National Grid

The British National Grid, like its military predecessors consists of a systematic breakdown

of the grid area into progressively smaller and smaller squares, identified first by letters and

then numbers. The largest units of the grid are 500 km squares; twenty-five in number, each

designated a prefix letter alphabetically from A - Z omitting ‘I’, the first letter to be quoted in

today’s British National Grid Reference. Great Britain is covered by only four of these 500

km squares, H, N, S and T. The 500 km squares are then further broken down into twenty-

five 100 km squares which are identified by a letter, again A - Z omitting ‘I’, the second letter

quoted in a reference.

On Ordnance Survey maps these squares are further divided into smaller squares by grid

lines, each square representing 10 km, each numbered 0 – 9, from the south west corner in an

easterly and northerly direction. These are known as ‘Eastings’ and ‘Northings’. So, a 10 km

grid square can be identified using the two letters i.e. TQ followed by the Eastings and

Northings i.e. TQ63.

On Ordnance Survey ‘Landranger’ maps the two grid letters i.e. TQ may be found on the

legend or the corners of the map. The 10 km squares are further broken down into 1 km

squares, again numbered 0 – 9. So, a 1 km square can be identified by the two letters and the

Eastings and Northings i.e. TQ6237.

By using the compass romer to sub-divide a 1 km grid square further into tenths a full six

figure grid reference can be specified that expresses a position to 100 m on the ground.

Grid reference precision is as follows:

Important. When giving or using a British National Grid reference the Eastings have to be

quoted first and the Northings second.

Grid Reference Precision

T 500 km. (250,000 sq. km.).

TQ 100 km. (10,000 sq. km.).

TQ37 10 km. (100 sq. km.).

TQ3877 1 km. (1 sq. km.).

TQ389770 100 m. (10,000 sq. m.).

TQ38937706 10 m. (100 sq. m.).

TQ3893377066 1 m. (1 sq. m.).

Where is the Origin of the British National Grid?

The true origin of the British National Grid is 049 degrees North: 002 degrees West.

The false origin is 049 degrees 45 minutes and 58 seconds North: 007 degrees 33 minutes and

23 seconds West.

The false origin which lies slightly southwest of the Scilly Isles was devised to ensure that all

British National Grid co-ordinates were positive (i.e. to the East and North of origin 00).

400 km were added to all Eastings and 100 km were subtracted from all Northings. If co-

ordinates were calculated from the true origin, the positions lying west of the central meridian

would be negative and the Northings although positive would exceed 1000 km for some parts

of northern Scotland.

Vertical Datum

We must now take a look at our vertical datum; this is not quite so straight forward.

The vertical datum on our O.S. Landranger map is ‘mean sea level’ and is the level of the sea

after averaging out the influence of tides (this is not used for marine navigation)….This is

derived from the Geoid.

The Geoid is defined as the equipotential surface of the Earth’s gravitational field.

Because the Earth is not a uniform lump of rock, its gravitational field varies from place to

place.

The Geoid ranges from over 100m below the W.G.S. Spheroid in the Indian Ocean to around

80m above it in the western Pacific. In the U.K. the Geiod is about 50m above the Spheroid.

Accurate map making and satellite positioning rely on mathematical foundation models that

are neat, consistent and simple. These smooth, symmetrical models are called Spheroids.

There are many different Spheroids in use, the O.S. use the Airy Spheroid.

The Three Types of North

There are three types of North on an Ordnance Survey map, True North, Grid North and

Magnetic North. The diagrammatic view of the three types of North shows their relative

position to each other on the Landranger map; this relationship may not always be the case in

a different area. We can now continue with a small explanation on each ‘North’.

True North

True North is the ‘Geographical North Pole’ where all the meridians of longitude intersect at

the top of the geoid (the Earth is slightly flattened at the poles and not a true sphere, neither is

it a spheroid or ellipsoid). It is the geoid which is defined as the true shape of the surface of

our planet if we discount all elevations above sea level.

Grid North

Grid North is the direction indicated by the grid lines that run North/South on an Ordnance

Survey map or indeed any other map; remember, this grid is completely flat, it is

superimposed over the Transverse Mercator Projection to give us the means to locate any

designated points with a reference but it cannot allow for the curvature of the Earth. Grid

North will only be the same as True North on the grids longitudinal origin, in the case of the

British Isles this would be the O.S.G.B. British National grid. Its true longitudinal origin

being 002 degrees West of the prime meridian (Greenwich).

The difference between True North and Grid North is given in the legend and is expressed as

a quantity at the sheet corners.

Magnetic North

Magnetic North is the direction that the needle inside our compass module swings to. Its

location is in the North Eastern part of Canada and its position changes by a small amount

each year; it can be calculated by using the information contained within the legend on our

Landranger map.

Orienting the Map Using the Compass

First set the compass to the G.M.A. (Grid Magnetic Angle found at the top of your

Landranger map, this is the difference between Grid North and Magnetic North) by turning

the compass housing. Now align the edge of the compass so that it runs along any

North/South grid line with the ‘direction of travel’ arrow on the compass’ baseplate pointing

to the top of the map (Grid North). Now, gently turn the map and compass until the red end

of the compass needle lines up with the ‘NORTH’ marker in the compass housing. The map

is now oriented.

Taking a Grid Bearing and Changing it Into a Magnetic Bearing to Walk On

Firstly, there is no need to orient a map when taking a grid bearing from it. Place the compass

on the map so that one edge passes through your present position and your next destination

with the ‘direction of travel’ arrow pointing in the direction of your destination. Now turn the

compass housing so that the ‘NORTH’ marker in the housing is pointing to the top of the

map (Grid North) and the lines in the housing are running parallel with the North/South grid

lines on the map. You now need to adjust the grid bearing by an amount equal to the G.M.A.;

this will turn the ‘grid bearing’ into a ‘magnetic bearing’ to walk on. Hold the compass level

and at waist height so that the ‘direction of travel’ arrow is at 090 degrees to your body, turn

your body and compass until the red end of the compass needle aligns with the ‘NORTH’

marker in the housing. The ‘direction of travel’ arrow now points to your destination.

Finding Your Position Using the Map & Compass

If you’ve lost your way and are unsure of your location, providing you have a reasonable

amount of visibility you can find your position by a method called ‘resection’; G.P.S. uses a

variation on this called ‘trilateration’.

First, find a prominent landmark that you can see but not far enough away as to be off your

map, say a large lake or mountain. Point the ‘direction of travel’ arrow at your landmark and

turn the compass housing until the ‘NORTH’ marker is under the red end of the compass

needle. Before you can transfer your bearing to the map you need to adjust it allowing for the

G.M.A. With this done your next step is to find your chosen landmark on the map, then place

the edge of the compass on your landmark with the ‘direction of travel’ arrow also pointing

toward it. Now, slowly pivot the whole compass around your landmark until the North

marker in the base of the compass housing points to the top of the map and aligns with the

North/South grid lines. Now draw a line from the centre of your landmark in the opposite

direction to the ‘direction of travel’ arrow… You are somewhere along this line!

You need to complete this three times in all, at approximately 120 degrees to each other; the

three lines that are produced should intersect each other and give your position. However, this

rarely happens and your lines will normally end up forming a small triangle; this triangle is

referred to as a ‘Cocked Hat’. Don’t take it for granted that you’re inside this triangle, you’re

probably not! There is a mathematical reason for this that is not important at this level but the

probability of you being inside the triangle is only 25%. That means that the probability of

being outside of the triangle is 75%!

Use the position to navigate to a fixed point such as a trigonometrical station, a stream

junction, prominent knoll etc.

As you can see from the diagram below, the chosen landmarks are a telecommunications

mast, a lake and a church with a spire. They are approximately 120 degrees apart giving us,

more often than not a triangle (cocked hat) where the lines cross.

Performing a Resection Using a Baseplate Compass

What Type of Compass is Best

For general map work use and overland navigation a compass like the one below is the best.

The three best makes are Silva, Suunto and Recta, all of which will give years of precision

service if treated with respect regardless of the model purchased. When you buy your new

compass a quick check to make sure that the needle settles to position quickly and that there

is no bubble in the housing is always wise. The Silva Type 4 or type 54 is a good one to go

for, it has a base plate romer, magnifying lens, silicone feet, and can be in mils, degrees or

both. It is worth purchasing your compass in the region of the world that it will be used as

they are ‘tuned’ to different global areas so as not to suffer from magnetic inclination

sometimes called ‘dip angle’.

There are also compasses that have a ‘global needle’; this eliminates the need to buy a

‘zoned’ compass. These compasses will work anywhere in the world.

Your compass is a precision instrument, treat it with care and it will give years of faultless

service.

Grid Magnetic Angle – Is it Important?

Many outdoor enthusiasts say that adding and subtracting the G.M.A. is insignificant. When

you are navigating with a map & compass the vast majority of the navigating is done with the

map so the argument against does hold some merit. Purists will argue the point and I have to

admit that I fall more to that side; however, a balance between the two is without doubt the

best solution. I do feel however, that on navigation courses it should be covered in its entirety

and the choice of when and where to use it left with the student.

1 km Grid Square Breakdown

15 16

08

1km

100m

07

100 1km

The 1 km grid square above has been divided into 100 smaller squares each having sides of

100m. These smaller squares are not depicted on the actual map and are only drawn in to

demonstrate how to visualise the 1 km square. The markings on your compass (the romer)

will divide this for you to give an accurate position.

You will find the spot height of 775m at Beinn Loinne on the Ordnance Survey Landranger

34 map, (Fort Augustus & Glen Albyn area) at NH 152078.

The position from The British National Grid datum 00 would be: 215200mE 807800mN

The map contains a vast amount of information; we can now look at how best to evaluate this

information. We’ll also look at how to use a compass in conjunction with the map, not only

to guide us toward our destination but also to find our position.

Although all navigational maps are a variation on a theme regardless of manufacturer, the

main thing that you’re looking for when purchasing a map of your chosen area is the scale of

the map. The scale is expressed as a ratio, and as such has no units. 1:50,000 simply means

that 1 unit on the map represents 50,000 units on the ground.

A scale of 1:25,000 may give very good detail but if you are touring in your car you could

drive across the map in only a few minutes needing many, many maps for your journey; just

as a scale of 1:625,000 would not be detailed enough to venture into the mountains. However,

it would be ideal if we used the 1:25,000-scale map for the mountains and the 1:625,000-

scale map for touring in the car.

The Ordnance Survey Landranger series covers the whole country at a scale of 1:50,000, this

scale gives good detail definition but also covers a reasonable area and is ideal for use by

hikers, backpackers, off-road vehicles etc. It now seems logical to use the 1:50,000

Landranger series for our needs here but the methods used will work just as well with any

scale.

When you purchase your landranger map and open it up you will find a Legend, this is the

part along one of the edges of the map itself and explains how certain features such as roads,

churches, paths, water features etc. are depicted along with other useful information. These

are generally self-explanatory and need take up no further space here. That said, armed with

only a mug of tea and a few of your favourite biscuits you should be able to recognise most

of the symbols on the map itself in an hour or so without any great difficulty.

Before moving on to actually finding our way with the map we need to look at a few areas

that may need further explanation or clarification.

Gradients

You will notice that a single arrow marked on the road indicates a gradient of between 1 in 7

and 1 in 5 on the road and that double arrows indicate a gradient of 1 in 5 or steeper. There is

no need to study the contour lines on the map to find out which way the incline goes; as a

quick reference the arrows always point down the slope.

1 UNIT

5 UNITS

A gradient of 1 in 5 is just another way of saying that the slope rises or falls vertically 1 unit

in every 5 units of distance. This can also be expressed as a percentage. A 1 in 5 gradient

being 20%

Heights

Spot heights marked on the map like the one below are not evident on the ground in any way

but may be used for reference in certain circumstances. They refer to the height in metres

above the maps vertical datum and are accurate to the nearest metre.

258

Some Interesting Extra Information Contained in the O.S. Landranger Map

On many ‘Landranger’ maps there is also a box with a blue outline and blue writing in it. It’s

not important but while we are on the subject we may as well know about it. Written in the

box is: Series M 726, Sheet 34, Edition 4-GSGS. The Sheet 34 refers to the number on the

front of the map; no mystery there. The Edition 4-GSGS holds a little interesting information.

GSGS stands for Geographical Section General Staff and the 4 indicates the edition; GSGS

was also known as MI4 (Military Intelligence 4). The M 726 part also contains some hidden

information; the M part indicates that it is from a series in the European part of the world, the

7 part denotes that the scale of the map is between 1:35000 and 1:70000, the 2 denotes a sub-

region (the British Isles in this case) and finally the 6 which indicates the series.

Contours

As you can see, the simple contour illustration below shows two hills of different heights

joined by a lower piece of ground, we call this a saddle. However, there are two sides to

every story, if we had the same contour pattern but with a minus sign before the heights we

would have two holes in the ground joined by a higher piece of ground, a mirror image in

fact. Of course if it were a minus sign it would be below the level of the map datum, this

being quite possible. Remember that this can happen without the minus sign if we start on a

plateau of say 200m and then reduce our contours in the same pattern to 150m.

You may also notice that the height lines do not reach the top of the hills, if the contours rise

in 10m increments the hill could be as much as 9m higher before the next contour is

appropriate, also it’s worth remembering that the ground can take many forms between two

10m contour lines.

A contour line does no more than mark the shape of the land at that particular level, it may

help to think of them as 10m thick slices of land placed on top of each other; whichever

method you find best, it will take time to recognise all the curves and kinks associated with

contour lines and what they actually depict. A good method is to go out with a map and

compass on foot so that you can relate what you see to what is on the map.

Contour Illustration

0m Contour

10m Contour

20m Contour

30m Contour

40m Contour

50m Contour

30m

SIDE ELEVATION

0m

10m

20m

40m

50m

The Compass Rose

Although not an essential part of navigation nowadays the compass rose is, without doubt, an

interesting item. If asked, many would say that it has 4 points, North, South, East and West.

Some may say that it has 8 points adding North East, South East, South West and North

West. In fact it has 128 points, giving a difference in angular displacement between points of

2.8125 degrees. There is something about a compass rose that makes the hairs on the back of

an adventurers neck stand up; it is the very essence of adventure.

The points are described as follows:

Cardinal points – North, South, East & West.

Half Cardinal points – North East, South East, South West & North West.

Intermediates – NNE, ENE, ESE, SSE, SSW, WSW, WNW & NNW.

By points – N by E, NE by N, NE by E, E by N etc.

Half points – N ½ E, N by E ½ E, NNE ½ E, NE ½ N, NE ½ E etc.

Quarter points – N ¼ E, N ¾ E, N by E ¼ E, N by E ¾ E etc.

Degrees or Mils

You will have no doubt noticed the term ‘mils’ being used on Ordnance Survey maps, we

need to take a look at the mil in more detail; it is an extremely useful tool to the navigator.

We are all familiar with degrees, we know there are 360 of them in a circle, we also know

that there are 60 minutes in a degree and 60 seconds in a minute. Unfortunately, to use this

system further we need to use trigonometry and a calculator, but now let’s take a look at the

mil.

The mil is an abbreviation for a unit of angular measurement called the Millieme and is used

by the army, mainly the artillery. It is defined as a unit of angular displacement equal to

1/6400th

of a circle. In other words there are 6400 mils in a circle (360 degrees).

It is at this point that we should take a look at the origin if the Millieme.

The unit is based on the milliradian (1/1000 of a Radian). A Radian being the angle

subtended at the centre of a circle by an arc of length equal to that of the radius of the circle.

There are 2 pi Radians in a full circle (6.283 Radians). The number of milliradians in a full

circle is 6283 (6.283x1000). Now, because of this odd number it was decided to round things

up or down, other values such as 6000, 6200 and 6300 have also been used but 6400 is now

the accepted standard. It’s extremely important to remember that the Millieme is not a

milliradian, it is only based on that unit of angular measurement.

An angular displacement of 1 mil at a distance of 1000m is equal to 1m.

An angular displacement of 1 mil at a distance of 2000m is equal to 2m.

An angular displacement of 10 mils at a distance of 3000m is equal to 30m.

1000m 3000m

1m 30m

1 mil 10 mils

This system can be used to solve all sorts of distance, elevation and navigational problems

and you don’t need a calculator or be a mathematician of such genius that you’re in danger of

being headhunted by N.A.S.A.; any information contained in the map concerning the mil has

been added by Ordnance Survey at the request of the Ministry of Defence.

Pylons

It is worth noting that the network of 415kv pylons that transport our electricity may often be

seen when in remote locations in the U.K. They are placed (ground permitting) at a pitch of

200m.

The fact that these are on our maps means that when we are using mils we can take advantage

of the pylons by using a bearing and calculated distance to give our position; this method

being useful should we not be able to see much else for a full resection.

We need to look at the pylons a little more closely, this will enable us to recognise the correct

ones on the map; this involves taking a look at the way the HT cables are supported. If the

insulators are horizontal the cables are in tension and if the insulators are vertical the cables

are in suspension.

As a general rule straight line runs are in suspension and changes of direction are in tension.

Each pylon has its own unique number; as a very last resort you may call this reference in to

the police who can then call the pylons owners who will in turn give the location of the pylon

to an accuracy of less than 10m. This information in turn would then be given to the rescue

team.

If You Remember the Following You Won’t Go Far Wrong.

Grid Magnetic Angle

The angle between Grid North and Magnetic North.

Magnetic Declination The angle between True North and Magnetic North on topographic maps.

Magnetic Variation The angle between True North and Magnetic North on bathymetric maps.

Grid Convergence Angle The angle between Grid North and True North.

Magnetic Inclination The angle that the magnetic flux enters the Earth’s surface (dip angle).

Magnetic Deviation

The angle that the compass needle deviates from true magnetic position due to magnetic

influences other than that of the Earth’s magnetic field.

The Universal Transverse Mercator System (U.T.M.S.)

The U.T.M.S. divides the globe into zones, 60 in total like the segments of an orange, each

extending from North to South poles and covering 006 degrees of longitude. Zone 1 starts

from the International Date Line 180 degrees West and extends to 174 degrees West. Zone 2

from 174 degrees West to 168 degrees West and so on. Zone 60 runs from 174 degrees East

to the International Date Line. A grid comprising squares of 1000m overlays each zone using

Cartesian coordinates that are metric based.

A North/South meridian through the centre of each zone is given a value of 500,000m, the

easting’s of a position is measured in reference to this line. A position to the west of the

central meridian would be expressed as less than 500,000m and a position to the East as more

than 500,000m. It is worth remembering that the position of a point is expressed as the

distance in metres East from the western edge of the zone and not East or West from the

central meridian.

As the maximum distance at the equator is around 667,000m our maximum grid distance of

1000.000m (2 x 500.000m) is more than adequate.

All northern positions are in relation to the equator, which has a value of zero. All Southern

positions are in relation to the equator which has a value of 10,000,000m.

Some positions North or South of the equator will be the same numerically, so we need to

specify if they are North or South of the datum. This is usually done by including the letter

for the latitude band.

Along the central meridian of each zone the grid coincides with the meridian but East and

West of this it does not, this is because the meridians are converging towards the poles and

the grid is perfectly symmetrical. This produces a convergence and is known as the ‘Grid

convergence angle’.

A U.T.M. grid allows for a positional accuracy of 1m to be achieved; the full, unique

reference of a position being the zone number, followed by the latitude band, followed by the

Eastings and then the Northings.

Note: You may have more digits in the North/South part of the reference as the distance is in

metres North or South of the equator. The East/West reference is limited to six digits as each

zone is only 667,000m wide at the equator. As we are working with grids, we always give the

position East first and the position North second.

The Poles

The poles (North of 84 degrees N and South of 80 degrees S) cannot be accurately depicted

in U.T.M are covered with a system called Universal Polar Stereographic (U.P.S.) and uses

the same Cartesian/metric system.

How North, South, East and West are Defined

We can now take a look at the official definitions of North, South, East and West which are

all based on the Earth’s rotation.

EAST is the direction in which the Earth rotates.

WEST is the opposite of EAST.

NORTH is the direction from a point on the Earth’s surface towards the pole that would be

on the left hand side of an observer facing EAST.

SOUTH is the opposite of NORTH.

Bearings

Bearings can be expressed as three types, Magnetic, True and Grid. They should always be

expressed using the full digit allocation so as to eliminate as much as possible unnecessary

errors.

Magnetic

A magnetic bearing is a bearing taken straight from or taken straight to the compass. It relates

to magnetic North and should be described as ‘090 degrees Magnetic or 1600 mils

Magnetic’.

True A true bearing is relates to true North or polar North, true North/South are at opposite ends of

a meridian of longitude and should be described as ‘090 degrees True or 1600 mils True’.

Grid A grid bearing is relates to the North/South grid lines on a map and should be described as

‘090 degrees Grid or 1600 mils Grid’.

Reciprocal Bearings

A ‘Reciprocal Bearing’ will take you back to where you have come from. Why? You may

ask. Suppose you are starting out from your camp and you take a bearing to a cabin 3

kilometres away to collect some equipment, you then want to head back to your camp. Why

completely reset your compass again using the map? You could make a mistake and get lost!

You know that the bearing to the cabin was correct because you have arrived there; you could

just add or subtract 180 degrees from your bearing, but that involves a calculation and

resetting the compass, another opportunity to make a mistake. The best way is to just align

the white end of the compass needle with the ‘North’ marker in the compass housing. No

calculation... No re-setting of the compass... No mistake!

The navigational methods we have covered so far have been the ‘bread and butter’ of getting

from one place to another. The following few subjects fine tune the basics to give easier and

more accurate navigation.

Handrailing

When we are navigating across open land and we notice that a road, river, cliff or some other

visible feature is running parallel with our intended route we can use this feature as a guide.

We call this a “Handrail” and using the feature is called “Handrailing”.

Aiming Off

The term “Aiming Off” is used when we deliberately set a bearing to miss our intended

objective, usually, but not always in bad visibility. We may use this method if we are heading

for a footbridge to cross a river in 20m visibility. If we take a bearing directly toward the

bridge and arrive at the river 100m out of position we would not see the bridge but more

importantly, we would not know which way to go to find it. Depending on the direction of

our approach we could “aim off” to one side of the bridge so that when we arrived at the river

we would know which way to go. The amount of “aiming off” would depend on the length of

your last navigational leg and your ability to follow a bearing in various conditions.

Expanding Spiral Search

The expanding spiral search is not the only search pattern we can use but if done properly it is

one of the most detailed, in addition to this it can be performed with a single person making it

ideal for the lone traveller.

The search is conducted as follows:

1. Search on a bearing for a distance equal to half the limit of your visibility. Although any

bearing will be acceptable, the fact that you will have to add 090 degrees each time you

change direction, starting on 000 degrees magnetic will make it easier.

2. Turn 090 degrees to the right and search for a distance equal to the visibility.

3. Turn 090 degrees to the right again and search for a distance equal to one and a half

times the visibility.

4. Continue turning 090 degrees at the end of each leg and increasing the distance by half

the limit of visibility. Continue until the object has been found.

Many other books recommend increasing the distance by the limit of the visibility. The

problem with that method is twofold. Firstly there is no overlap in visibility so that if you’re

slightly off on the bearings you could easily miss the person or item and secondly, the

distance across the corners is 1.414 times the width across the square leaving a ‘dead zone’

that has not been searched. If you are going to search an area it’s important that you are 100%

certain that you have searched the total area before moving on to the next area; be sure to

overlap the areas in the same way.

Diagram of Expanding Spiral Search Plan

Slope Aspect

If you are lost, using the slope aspect can often provide very useful information on your

position. Although using the slope aspect will only give you a position along a line, combined

with some additional information it may well give you a fairly accurate position. Being able

to use the information gained with this method may be extremely useful under certain

circumstances.

On the illustration you can see that slope aspect can give you a position line. Take a bearing

straight down the slope if it is S.E. you are somewhere along the S.E. line, if it is N you are

somewhere on the N line, if it is N.N.W. you are somewhere on the N.N.W. line and if you

are on flat ground you are on the col.

If you have no additional information you will not be able to pinpoint your position more

accurately, if this is the case move on with care noting the direction and distance until you

come to a feature you can identify; this will then give you a better fix on your position.

HALF THE

LIMIT OF

VISIBILITY

HALF THE

LIMIT OF

VISIBILITY

MAGNETIC

NORTH

SEARCH

START

POINT

ROUTE TO

SEARCH

AREA

Diagram of Slope Aspect in 1 km Grid Square

Pacing

Pacing is a method used to increase the accuracy of navigation in conditions of very poor

visibility. It involves counting your steps to give an accurate distance along with a direction

from your compass. When this method has been mastered you can accurately navigate in near

zero visibility with ease. However, the down side is that it takes much practice and hard work

to attain a good standard. Firstly you will need to find out how many paces you take to cover

100m on the flat (take an average over five runs). Then repeat this with a 10kg rucksack, then

a 20kg, then a 30kg. Log the results on a card and laminate. Ascents and descents don’t

matter too much because if the ground is that steep you would be using the slope aspect

method along with re-entrants. Mental allowances and adjustments can be made en-route to

cover terrain etc.

Counters are available that register one unit each time you press a lever on the side. These are

an invaluable piece of equipment for the serious navigator.

Solar, Lunar and Astral Navigation

We have now looked at Mercator projections, spheres, ellipsoids, geoids, grids and the like.

We have looked at maps, scales, contours, compasses and how to use them. Now we can look

at navigation without a map or compass, solar, lunar and astral navigation.

N

S.E

.

N

N.N.W

When we venture into the wilderness we always take our map and compass; they provide the

most simple, straightforward and foolproof method of finding our way around. However,

there may be a time when our normal navigational tools are unavailable; you may find

yourself in a situation where you don’t have a map or a compass, perhaps an aeroplane crash

or vehicle fire where everything has been destroyed. For various reasons rescue may be

weeks away if at all, so the normal procedure of staying where you are is not an option. We

must now look to the sun, moon and stars to find our way, this form of direction finding is

approximate at best but it will keep you pointing in the direction that you wish to travel.

So let’s start with finding North using the sun.

Solar Navigation

We all know that as a rough guide the sun rises in the East and sets in the West; we can

expand on this knowledge with no more than a little common sense. If our location is in the

northern hemisphere the sun’s path will be from East to West through South and so it follows

that if our location is in the southern hemisphere the sun’s path will be from East to West

through North. The hemisphere you are in would be indicated by the rotation of the shadows,

this being clockwise in the northern hemisphere and counter clockwise in the southern

hemisphere. We can also deduce that the sun’s apparent progress around our planet is 015

degrees per hour. We now have a reasonable amount of information with which to find our

way.

The Shadow Stick Method

Find a long straight stick around 1m in length, push it into a flat patch of ground so that it

stands upright and then mark the tip of the shadow with a small stone. Wait around 20

minutes and then mark the tip of the new shadow with another small stone.

Now draw a line through the two stones, this is an East/West line and the position of the first

stone will be West. Now draw a line at 090 degrees through the centre of the first line. This is

the North/South line.

Diagram of Shadow Stick Method

EAST /

WEST LINE

Using Your Watch to Find North

If you have an analogue watch you can easily find North with a little help from the sun; if

you have a digital watch you can still manage but you will have to draw the analogue watch

on the ground to give the hour hand or 12 o’clock position. The only down side to this

method is that the nearer you are to the equator the less accurate it will become. It is

extremely important that the watch is set to the correct local time and any adjustments made

to compensate for daylight saving etc.

The Northern Hemisphere

Hold your watch horizontal and rotate it until the hour hand points toward the sun, then bisect

the angle between the hour hand and 12 o’clock. This is the North/South line.

The end of the line nearest to the sun is South. If you mark this line on the ground the

remaining cardinal points can be drawn in easily.

Diagram for Using Your Watch in the Northern Hemisphere

The Southern Hemisphere

Hold your watch horizontal and rotate it until the 12 o’clock mark points toward the sun, then

bisect the angle between 12 o’clock and the hour hand… This is the North/South line. The

end of the line nearest the sun is North. As before, if you mark this line on the ground the

remaining cardinal points can be drawn in.

12

S

Diagram for Using Your Watch in the Southern Hemisphere

Lunar Navigation

We have looked at how to navigate using the sun, but what do we do if we want to move at

night? This would be favourable in desert areas, as it is cooler at night. Well, we have the

moon and the stars; let’s take a look at the moon first.

As a General Guide

If the Moon rises before the Sun sets the illuminated side will be on the West.

If the Moon rises after the Sun sets the illuminated side will be on the East.

If the Moon rises at the same time as the Sun sets it will be a full Moon and be due South at

midnight local time.

You may also use the quarter Moons but these will only provide a very rough guide, but then

if you don’t have much else it’s a bonus

Quarter Moons

If you join the horns of either of the quarter moons with an imaginary line down to the

horizon the point it touches will be roughly South if you are in the northern hemisphere and

will be roughly North if you are in the southern hemisphere as indicated below. You may

have to estimate as to where the line would touch the horizon if in mountainous terrain.

N

12

The illustration below shows the direction of the moons 28-day orbit around the Earth; its

apparent phases are also shown.

The moons apparent shape changes with its position, when the moon is directly between the

Earth and the sun the light reflected from the moon is not visible to us on Earth; this is what

we call a ‘New Moon’. As the moon waxes the visible crescent on the right-hand side gets

larger. When the moon reaches a position on the opposite side of the Earth to the sun, the

moon is full. As the moon starts to wane the visible area reduces to a small crescent on the

left-hand side.

Astral Navigation

EARTH

It is now the turn of the stars to help us find our way and are, in general, a much better guide

than the moon. As you are probably aware the stars appear to turn above us around a central

point, we can use this as our first directional aid.

This method of finding your way is ideal in partly cloudy conditions, as you do not need to

pick out any particular star or constellation. Stars that are about 45 degrees from the horizon

are better as the movement becomes much more apparent.

For this method you will need to set up a simple sighting device to monitor the movement of

your chosen star. The diagram below suggests a means of sighting your star with two sticks

but any two fixed points will suffice.

Diagram for the Sighting of a Star

If your star appears to be following an arc to the left you will be facing approximately North.

If your star appears to be following an arc to the right you are facing approximately South.

If your star appears to be rising you are facing approximately East.

If your star appears to be falling you are facing approximately West.

As we have already said the stars appear to be revolving around a central point, in actual fact

the stars are staying where they are and it is the earth’s rotation that gives this illusion.

Nevertheless, the central point on which they appear to be rotating is conveniently marked for

us by the star Polaris, also known as the Pole Star or the North Star. This star is positioned

above the geographical north pole and as such serves as a useful marker to indicate True

North. Polaris will indicate True North all year round and its deviation at any time is less than

001 degree. Unfortunately Polaris is only of use if you are in the northern hemisphere, in the

southern hemisphere we are not so fortunate in as much as there is no particular star that

marks the centre of rotation. However, the Southern Cross can be employed with a little

additional knowledge.

LINE OF

SIGHT

When you have found the direction that you wish to travel in, mark it with a row of stones or

a long stick, you may only be checking your direction at night and travelling in the day.

We can now look at how to locate our markers in the night sky and how to use them.

How to Find Polaris

Below is a diagram to help you locate Polaris. The constellation on the left can be picked out

easily on a clear night, it is commonly known as The Plough or The Big Dipper.

Of the seven stars that make up The Plough, a line drawn through the two lower ones in the

diagram and extended will direct you to Polaris. It is about five times the distance between

these two stars away and is just to one side.

Cassiopeia is ‘W’ shaped and lies on the opposite side to The Plough. A line can almost be

drawn from the lower star of Cassiopeia through Polaris to the upper star of The Plough.

Diagram Showing How to Find Polaris in the Northern Hemisphere

POLARIS

CASSIOPEI

A

THE PLOUGH OR BIG

DIPPER

How to Find South Using the Southern Cross

When in the southern hemisphere we need to locate the Southern Cross to help us find South.

If it’s a dark night and you can find the Milky Way, look for a dark patch known as The Coal

Sack, the Southern Cross is on one side of this. There are two other cross formations but both

are larger than the Southern Cross. Once found, imagine a line through the two stars with the

largest centres and extend this distance five times from the lower star to an imaginary point,

then take the line vertically down to the horizon… This is your South indicator.

Diagram Showing How to Find the Southern Cross in the Southern Hemisphere

SOUTH

THE

SOUTHERN

CROSS

FIVE TIMES

THIS

DISTANCE

HERE

The Global Positioning System

Whether we like it or not, the use of global positioning systems for outdoor use is becoming

ever more popular. They are an extremely useful tool to have in your box; however, it

shouldn’t be your only tool. Even the most modern G.P.S. units that incorporate full mapping

systems are still way behind the versatility of a map & compass when in the hands of a good

navigator in mountainous terrain. They are very useful for checking a position and navigating

across open, desolate areas. Never allow your skills with a map & compass to deteriorate in

favour of a G.P.S. on your adventures. The sensible option is to think of it as another tool in

your navigation toolbox and when you consider it’s the best tool for that particular job...bring

it out.

Although the idea of a Global Positioning System came in the 1970’s, it would not be until

the mid 1990’s that the system would become fully functional although it was useable in

limited areas long before this.

Let’s take an extremely basic look at the world of the G.P.S. and see how it works. It must be

stressed that this is a simplified overview of a constantly changing project; its purpose is no

more than to provide an understanding of the concept.

The space segment consists of 24 Navstar satellites orbiting the Earth at an altitude of around

20200 km and a speed of 2200 km/h. The satellites are on 6 equally spaced circular orbits

around the Earth with 4 satellites in each orbit. The satellites are inclined at around 55

degrees to the equator and have a twelve hour period.

The heart of each Navstar satellite is its atomic clock; this is an atomic oscillator with a

rubidium cell and caesium beam gives an accuracy of around 1 second in 300,000 years.

Without these atomic oscillators the Navstar system could not exist; an error of only 1

millisecond would result in a positional inaccuracy of more than 300km!

The G.P.S. system can be divided into three main segments, the space segment, the control

segment and the user or receiver segment.

The Space Segment

This segment consists of 24 ‘NAVSTAR’ satellites.

(‘NAVSTAR’ being NAVigation, Satellite, Timing And Ranging).

The satellites circle the earth on 6 equally spaced orbits, 060 degrees apart with 4 satellites

equally spaced in each orbit. They are angled at 055 degrees from the equator as this ensures

that the receiver can pick up a minimum of 5 and a maximum of 8 satellites at any time

anywhere on earth.

It takes each satellite 12 hrs to complete a full orbit at an altitude of around 20200km and a

speed of around 2200 km/h.

The Control Segment

The control segment consists of a number of main ground stations that constantly monitor all

the satellites. Information is constantly exchanged between the ground stations and the

satellites, any adjustments to the orbit of each satellite, atomic clock synchronisation’s, coded

data emissions etc. would be made as and when required. The master control station is at

Schriever Air Force Base in Colorado.

The User or Receiver Segment

This segment consists of the many hundreds of thousands of G.P.S. receivers world-wide.

The receiver units, as the name implies are only receivers and do not transmit any

information within the system. With this being the case there is no limit to the amount of

receivers in the system and performance is unaffected.

The Accuracy of the System

The original system ran on two levels of accuracy, one for the military and one for civil use.

The first was the ‘Standard Positioning Service’ (S.P.S.). This was made available to all

civilian users free of charge; however, the U.S. Department of Defense did not want the

civilian users to obtain the same level of accuracy provided by the P.P.S. system.

They decided that the best way of reducing the accuracy would be to ‘dither’ the signals from

the satellites randomly. They referred to this as ‘Selective Availability’ (S.A.), with S.A. only

a ‘Course Acquisition’ (C.A.) was attainable.

The accuracy under S.A. is around 100 metres horizontally, around 160 metres vertically and

a time accuracy of around 350 nanoseconds.

The second was the ‘Precise Positioning Service’ (P.P.S.). The P.P.S. could only be accessed

by authorised personnel, mainly government and military although some civilian users were

licensed. Special receivers were required to analyse the ‘dithered’ signal from the S.P.S.

transmissions and by using a ‘P’ code encrypted into a ‘Y’ code transformed the S.A. signal

into the original data.

However, on 2nd

May 2000 S.A. was switched off enabling all G.P.S. receivers to produce

P.P.S. accuracy.

The accuracy of P.P.S. is around 20 metres horizontally, around 30 metres vertically, and a

time accuracy of around 200 nanoseconds.

Limitations of the G.P.S.

The G.P.S. is without doubt a fantastic aid to navigation and could even be described as

invaluable. Whether you are on foot, in a boat or in a vehicle, the G.P.S. should only be

regarded as a back-up system for navigation. Your skills with map, compass, natural

navigating methods or indeed solar, lunar and astral navigation should always be maintained

to a high standard and a quick check should be made to confirm the information from the

G.P.S. as often as you consider necessary.

There are some other limitations that may be worth a look before we move on.

G.P.S. receivers need a clear unrestricted view of the sky to give good results so you will

generally struggle inside buildings, underwater, in caves, in dense tree cover, surrounded by

high buildings, deep gorges or valleys etc. If using a G.P.S. in a vehicle an external antenna is

a wise choice as the receiver can only see a small piece of the sky through the windscreen.

Additional Sources of Positional Error

A number of small errors are present in the system that affects the overall accuracy of the

calculated position. Some of these are due to the interference of the signals by some means

while they are on their journey from the satellite to the receiver; others include slight

variation in satellite orbits, slight atomic clock inaccuracies, tracking errors etc.

The information sent by the chosen satellites travels to the receiver in the form of a radio

signal. On its journey it travels through the ionosphere, as it does so the charged particles

deflect the signal slightly and as the signal continues on through the troposphere the water

molecules produce a slight refraction. All this adds up to the signals path not being a straight

line, this reverts to a timing error resulting in a positional inaccuracy. Later receivers can

combat this problem in two ways, the first is known as Duel Frequency Measurement and the

second as Error Modelling.

The duel frequency measurement is the most accurate and is generally used by the military, it

utilises the fact that the satellites transmit information on two different frequencies exactly at

the same time, L1 transmitting on a frequency of 1575.42 MHz and L2 transmitting on a

frequency of 1227.60 MHz. L1 carrying the navigation information along with the S.P.S.

code signals and the information carried by L2 is used to calculate the ionospheric and

tropospheric delays in equipped receivers.

It is well known that different frequencies refract by different amounts, by measuring the

difference in time between when the two signals arrive at the receiver the amount of

refraction can be calculated in the receiver and the appropriate correction made.

The error modelling solution is less accurate and uses a mathematical model to predict any

refraction on a typical day and make the necessary adjustments.

The satellites are equipped with atomic clocks, indeed this is the basis of the systems

accuracy. The ground stations monitor this and make program adjustments as and when

required. The cost of an atomic clock would be upwards of $80,000. With this in mind it is no

doubt obvious that the receiver does not have an atomic clock. However, when receiving the

information from the satellites the receiver can synchronise its own internal clock with that of

the satellite.

This is extremely important as an error of 1 millisecond would result in a positional error of

more than 300km.

The other main inaccuracy is Multipath Error, this is when the radio signals bounce off

objects on their way to the receiver, these objects may be high-sided valleys, buildings, cliffs

etc. The problem is that as the signals are being bounced of these objects the receiver may

pick the same signal up more than once, this results in positional calculation inaccuracies.

Similarly, if your television signal is bounced about before it reaches the antenna the picture

is said to be ‘ghosted’.

The remaining error is associated with the satellite geometry itself.

The way the receiver works is almost the same as finding your position with a map and

compass using the resection method, but in this case it is called ‘Trilateration’. Nothing

changes eh! When you turn on the G.P.S. receiver it searches the sky for all of the available

satellites that are in view, it then chooses the four best satellites it needs to calculate a 3

dimensional position. The receiver will make its choice of satellites from their position

relative to itself; however, if the receiver cannot see all of the sky because it is in a deep

valley or forest clearing it will have to take the signals from the satellites it can see. In this

situation the calculated position through trilateration will be less accurate due to poor satellite

geometry. This is known as ‘Geometric Dilution of Precision’ or G.D.O.P.

Ensuring Accuracy

When you have made the decision to purchase a G.P.S. receiver and you have decided on the

best one for your application, the excitement starts to build. You can contain it no longer and

you make your way down to the store to make your intended purchase. You can’t wait to get

home and have a play with your new toy. You finally get home, make yourself a nice drink of

tea and as it’s a special occasion you decide to go the whole hog and break out the biscuits.

You carefully ease back the paper, gently open the box and what’s this? There must be some

mistake. It appears that some wag has replaced the two-page instruction manual with the

latest edition of War & Peace!

It is with some trepidation and a heavy heart that I must insist you read it from cover to cover.

First! Then read through it again, but this time have the receiver turned on in your hand

following the receiver set up sequence, this is called ‘initialisation’ and is best done outside if

possible. You will have to collate certain information depending on where you are, what

maps you intend to use and what you wish to use the receiver for. Don’t rush this stage; make

THE L1 CARRIER

FREQUENCY OF

1575.42 MHz

DELIVERS

NAVIGATIONAL

AND S.P.S. CODE

SIGNALS.

THE L2 CARRIER

FREQUENCY OF

1227.60 MHz

DELIVERS

INFORMATION ON

IONOSPHERIC

DEFLECTION AND

TROPOSPHERIC

DELAYS

sure that it’s absolutely correct. If you make an error at this stage and do not spot it, your

positional accuracy will suffer to some extent.

On first initialisation or when you have moved the unit more than 500 miles in the ‘switched

off’ position it will take around five minutes to find its position; this is called ‘Auto location’.

After the initial auto location any subsequent starts will be much quicker, around a minute or

so. This is called a ‘Cold start’.

If you are using the receiver intermittently throughout the day for positional information, the

start-up will only be around 20 seconds. This is called a ‘Warm start’.

Map Datum

The term map datum is used to express the shape of the earth as a mathematical model. The

map datum you use will be dependent on where you are and what you wish to do.

Examples of map data are as follows:

WGS 84 (WORLD GEODETIC SYSTEM 1984).

NAD 27 San Sal (NORTH AMERICAN DATUM 1927 San Salvador Island).

RT 90 (Sweden).

ORD SRVY GB (ORDNANCE SURVEY GREAT BRITAIN).

AUS 84 (AUSTRALIAN GEODETIC1984.

The list goes on and on, be certain to choose the correct datum.

The other information that goes with this is the co-ordinate system you wish to use. A co-

ordinate system consists of a number of lines that form a grid; this is then used to relocate a

position by giving a series of numbers we refer to as co-ordinates.

Examples of co-ordinate systems are as follows:

Latitude/Longitude.

U.T.M./U.P.S. (Universal Transverse Mercator/Universal Polar Stereographic) Grid.

You may also set user-defined co-ordinate systems on some receivers.

Familiarity

The next step is to practice with the instructions until you become fluent with its operation.

Make sure you can obtain any piece of information you require quickly and without trouble.

Be fluent with all the user changeable features such as units of measurement, trip times,

distance trips, altitude settings etc.

Additional Accuracy

If additional accuracy is required and your receiver is D.G.P.S. friendly you may try this

system. It stands for Differential Global Positioning System and works by using many ground

stations that receive satellite transmissions, refine them and then transmit the refined &

corrected signal back out again, this in turn is picked up by your receiver and is used to

calculate a position of greater accuracy. With D.G.P.S. an accuracy of 3 metres is easily

achievable and often improved upon.

Differential Global Positioning System

In the Differential Global Positioning System the L1 & L2 signals from the orbiting

NAVSTAR satellites are picked up by a number of reference stations, the exact position of

which are known. The signals are then analysed and any errors such as deflection, refraction

etc. are corrected. The corrected signal is then transmitted so that any able G.P.S. receiver

that has the D.G.P.S. function set to ‘ON’ will receive the corrected signals. D.G.P.S.

coverage is not complete and only covers important areas such as busy shipping ports etc.

where a more accurate position is required.

There has been a charge for the use of this more accurate system but since the S.A. has been

switched-off most receivers give reasonable accuracy in good satellite geometry conditions. It

is not uncommon to see an E.P.E. (Estimated Positional Error) reading on your receiver of 5

metres or less. This as we know is not a true figure.

By knowing a little about how the G.P.S. operates will enable you to get the best from it by

understanding what your receiver needs to calculate an accurate position on command. We

can now look at some of the terminology used in G.P.S.

Common Terminology Used in G.P.S. Navigation

Bearing: The compass direction from your position to a destination.

Course Made Good: The bearing from the ‘active from’ position (your starting or last

waypoint ) to your present position.

Crosstrack Error: The distance you are either side of your desired course.

Desired Track: The compass course between the ‘from’ & ‘to’ waypoints.

Differential G.P.S.: An extension of the G.P.S. system that uses land based radio beacons to

transmit position corrections to G.P.S. receivers.

Estimated Time of Arrival: The time of day that your arrival at a waypoint or destination is

expected, based on your current speed and direction.

Estimated Time En-route: The time remaining to your next waypoint or destination based

on your current speed and direction.

Grid: A co-ordinate system consisting of square zones that are overlaid onto the map

projection.

Ground Speed: The speed you are travelling relative to a ground position.

Latitude: The North / South measurement of position perpendicular to the Earth’s

Polar axis.

Longitude: An East / West measurement of position in relation to the prime meridian, an

imaginary circle that passes through the north and south poles.

Navigation: The process of travelling from one place to another and knowing where you are

in relation to your desired route at any time.

Position: An exact, unique location based on a geographic co-ordinate system.

Route: A route is two or more waypoints joined together.

Velocity Made Good: The velocity you are travelling in the direction of your next waypoint

or destination.

Waypoint: A specific location stored in a G.P.S. receiver’s memory.

Note: Routes are broken down and navigated in smaller segments called ‘legs’. The waypoint

you are travelling to is called the active to waypoint and the waypoint you are travelling from

is called the ‘active from’ waypoint.

The line between the ‘active from’ waypoint and the ‘active to’ waypoint is called the ‘active

leg’.

Different Types of Direction

When travelling through air or water things can get a little more complicated. This is due to

the fact that the air or water can (and almost certainly will) be moving in its own right relative

to the Earth. This can also happen on surfaces that are floating such as icecaps. When you see

the terms below being used just be aware that they may mean something other than the

obvious.

Bearing: The bearing is the direction from one object to another.

Course: The course is the direction in which the vessel is intended to be steered.

Heading: The heading is the direction in which the vessel is actually pointing at any given moment. It

is seldom exactly the same as the course.

Track Angle: The track angle is the direction in which the vessel is actually moving over the surface of the

Earth. The effects of the wind, tide, currents etc. mean that it is not usually the same as the

heading.

In practice the word ‘angle’ is often omitted. Ground Track, Course Made Good and Course

Over Ground all mean the same thing.

The information contained in this book is related to basic overland navigation, indeed, the

subject of navigation is almost endless and to many quite infectious. Don’t just take the

information in, use it to further your dreams and adventures as you travel through life.

About the author

Don Russell FRIN FRGS FRSA

Don Russell served a Tool & Prototype Apprenticeship at Rolls-Royce Motors Ltd. in the

U.K. and has subsequently held positions as a Toolmaker, Production Planning &

Programming Engineer, Sub-Contracts Manager, Works Manager, General Manager,

Technical Director and Managing Director. Don also owned Seatech Marine, a company

specialising in specific skill diving instruction, search & recovery projects and sub-surface

engineering. He now runs an outdoor pursuits company, writes for various websites and

magazines along with having written for a well known 4x4 magazine on a regular basis.

When not writing or conducting courses he often puts his engineering knowledge and

experience to good use, taking on electro-mechanical engineering projects, diving projects,

technical report preparation and project management contracts. Don is a Fellow of the Royal

Institute of Navigation, of the Royal Geographical Society and also of the Royal Society for

the encouragement of Arts, Manufactures and Commerce along with having been an

Executive Member of the Professional Bodyguard Association. He has trained with the

British Special Forces (volunteers) and is conversant in the use of many assault rifles, sub-

machine guns and pistols. His passions are adventure, exploration and researching lost

treasure, be it on land or under the sea and would like to pursue these passions further by

working on joint projects with enthusiastic, like-minded people. Don is a very 'hands-on'

person and is always interested in being involved with any expedition or adventure in any

capacity that may be required; he has a 'can do' attitude and is happy to work on his own or as

part of a team. Don is always looking for exciting new challenges, opportunities and projects

with which to become involved; his professionalism, substantial skill base, relentless

enthusiasm and sense of humour being appreciable assets within any team.