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Navigation Systems – Basics of Maps

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How do we measure Latitude and Longitude?

Longitude: Just like a circle, any longitude line (or ring) on the earth is separated into

360º. It is easiest to visualize along the equator. Looking at the longitude map there are

usually 24 sections (divided by the lines), 12 each for the west and east sides. Taking

360º and dividing by 24, we know that each section is 15º across. The reference point

(0º) is the prime meridian, at Greenwich, England. From here you can measure 180º to

the west, 0 to –180º. You can also measure 180º to the east, 0 to 180º. Western degrees

are always negative, and eastern degrees are always positive. As an example, Newcastle,

Australia is at 151.46º longitude. Hence it is east of the Greenwich Meridian.

Latitude: Starting from the equator, you can measure 90º northward and 90º southward.

As an example, Newcastle, Australia is at 32.55º S. The South Pole is at 90º S, and the

North Pole is at 90º N.

The distance covered by a single degree is too large for practical use, so the system of

minutes and seconds was developed. A degree is divided into 60 minutes. For even

more detailed use, a minute is divided into 60 seconds. One second of latitude

corresponds to about 30meters on the surface of the earth.

1 degree = 60 minutes

1 minute = 60 seconds

1 degree = 3600 seconds (60 minutes x 60 seconds)

Symbols forº, minutes and seconds:

° Degrees

' Minutes

" Seconds

The three common formats:

DDD° MM' SS.S" Degrees, Minutes and Seconds

DDD° MM.MMM' Degrees and Decimal Minutes

DDD.DDDDD° Decimal Degrees

Degrees, Minutes and Seconds

DDD° MM' SS.S"

32° 18' 23.1" N 122° 36' 52.5" W

This is the most common format used to mark maps. It's also the most cumbersome to

work with. It's a lot like telling time…


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There are sixty seconds in a minute (60" = 1') and

There are sixty minutes in a degree (60' = 1°).

Keeping in mind a few easy conversions between seconds and decimal minutes will help

when working with maps that useº, minutes and seconds.

15 seconds is one quarter of a minute or 0.25 minutes

30 seconds is one half of a minute or 0.5 minutes

45 seconds is three quarters of a minute or 0.75 minutes

Degrees and Decimal Minutes

DDD° MM.MMM'

32° 18.385' N 122° 36.875' W

This is the format most commonly used when working with electronic navigation

equipment.

Decimalº

DDD.DDDDD°

32.30642° N 122.61458° W

or +32.30642, -122.61458

This is the format used by most computer-based mapping systems.

Often the N-S and E-W designators are omitted. Positive values of latitude are north of

the equator, negative values to the south. Watch the sign on the longitude, most programs

use negative values for west longitude, but a few are opposite. This saves a lazy western

hemisphere programmer from having to type in a minus sign before most of their

longitude values.

Which format should you use?

First off, if you are working with other people who have agreed upon a format to use,

then you should probably use that format. In this course you will be required to work

extensively with Global Positioning Systems (GPS), which have a default setting of

degrees and decimal minutes, but you will be made aware of how to use all formats


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Plotting and Measuring Latitude

Lines of latitude are parallel and evenly spaced;

therefore a degree of latitude represents a constant

distance on the ground. This makes plotting latitude

very straightforward.

Place the ruler so that it spans the lines of latitude and

the point to be measured or plotted falls between.

Orient the ruler north to south. The zero minute end of

the ruler should be on the northern line of latitude,

when you're in the southern hemisphere (and vice versa

for the northern). To measure the latitude of a point on

the map, read the value from the ruler at the point, and

add it to the latitude of the line at the zero end of the

ruler. On the picture to the left, the ruler indicates the X

is at the 4' mark, for a resulting latitude of 37° 34'

N. To plot the location of given coordinates, make a

small mark on the map to indicate the line of

latitude the coordinates fall on.

Plotting and Measuring Longitude

Longitude represents an east-west position on the earth. Longitude increases as you move

away from the prime meridian, or 0°, in

Greenwich, England. Because the lines of

longitude converge at the poles, a degree of

longitude represents a varying distance on the

ground, depending on the latitude.

Place the ruler so that it spans the lines of

longitude the point to be measured or plotted

falls between. The ruler will need to be on a

diagonal to fit. To measure the longitude of a

point on the map, slide the ruler vertically,

keeping the ends on the lines of longitude

marked on the map, until the edge of the ruler

touches the point to be measured. You may

need to extend the lines of longitude above

or below the map to properly position the

ruler. On the picture to the left, the ruler

indicates the X is at the 3.5' mark, for a

resulting longitude of 122° 3.5' W. To plot a longitude coordinate, make a small tic on the

map to indicate the line of longitude. The point of interest is located where the plotted

lines of latitude and longitude cross.

Figure 2.5 – Measuring latitude (from Maptools.com)

Figure 2.6 – Measuring longitude (from Maptools.com)


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Figure 2.7 – Measuring longitude part 2 (from Maptools.com)

Test Your Knowledge

This exercise will test your ability to plot Latitude/Longitude coordinates onto a map and to determine the coordinates of features on the map.

The page following contains a very simple 1:100,000 scale map, the instructions for the exercise, and a 1:100,000 scale Lat/Lon ruler. Cut the ruler out, and use it for the exercise.


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Cut out the 1:100 000 scale ruler at the bottom of the page and use it with the map to

complete the exercise.

Determine the coordinates for the points labeled A, B, and C.

Find the features on the map at the following coordinates:

37º 35´ N 122º 05.8´ W

37º 31´30˝ N 122º 02´ 10˝ W

37º 33.6´ N 122º 03.7´ W


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Answers to the Lat/Lon Exercise

Measuring coordinates for points on the map:

A – 37° 35' N 122° 05' W B – 37° 32.5' N 122° 06.5' W or 37° 32' 30" N 122° 06' 30" W

C – 37° 36.8' N 122° 02.3' W or 37° 36' 45" N 122° 02' 15" W

Plotting coordinates onto the map:

37° 35' N 122° 05.8' W is the camp site

37° 31' 30" N 122° 02' 10" W is the mine

If you don't have a map or ruler to make accurate estimations of latitude and longitude,

you can still make good guestimates by interpolating and estimating between the lines.

Just remember to allow for the convergence of lines of longitude as you get closer to the

poles.

Test Your Knowledge

Use the map below to complete the table following.


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Place Latitude Longitude Place Latitude Longitude

Seymour 36º 58´ S

145º 10´ E

Presenting Information on Maps - Symbols and Colors

Symbols Map symbols are signs or colours drawn on the map to represent features on the earth’s

surface. Every symbol used on a map must be listed in a key or legend next to the map.

Symbols must be simple yet easily identified in what they represent.

Colour Maps also use colour as a type of symbol. Standard colours are used to represent water,

buildings and land features. Details of these are listed below.

Most topographic maps use the following symbols and colors to designate different

features (see Figure 2.8)

• Black - man-made features such as roads, buildings, etc.

• Blue - water, lakes, rivers, streams, etc.

• Brown - contour lines

• Green - areas with substantial vegetation (could be forest, scrub, etc.)

• White - areas with little or no vegetation; white is also used to depict permanent

snowfields and glaciers

• Red - major highways; boundaries of public land areas

• Purple - features added to the map since the original survey. These features are

based on aerial photographs but have not been checked on land.


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Figure 2.8 – Topographic map Symbols (from US Geological Survey)


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Map Legend The map legend contains a number of important details. The figures below display a

typical topographic map legend. In addition, maps generally include latitude and

longitude and the names of the adjacent maps (depicted on the top, bottom, left side, right

side and the four corners of the map).

The major features on the map legend are shown in Figure 2.9 and labeled below.

1. Map Name

2. Year of Production and Revision

3. General Location in State

4. Next Adjacent Quadrangle Map

5. Map Scale

6. Distance Scale

7. Contour Interval

8. Magnetic Declination

9. Latitude and Longitude


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Figure 2.9 – Topographic map legend features (from US Geological Survey)

how do we measure latitude and longitude? - cffet.net · latitude: starting from the equator, you...

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