Map, Compass, and GPS Basics

Search and Rescue… When Accuracy Counts

Brad Rounds, Registered Maine GuideWilderness Rescue Team LeaderEducation Director, MASAR

With illustrations fromKjetil Kjernsmo's illustrated guide

on How to use a compass(Used With Permission)

Peter H. Dana

The Geographer's Craft Project, Department of Geography, The University of Colorado

GPS World

and the U.S. Geological Survey

When A Life’s In The Balance

• Accuracy = Effectiveness

• Effectiveness = Maximum Advantage to Victim

Not Even In The Ball-Park

• Finding south using a watch

• Shadow methods

• Moss on Trees

• Direction of lean of branches

• Sunrise and Sunset

• Pixie Dust and Natural Instinct

These skills require practice

You will not be proficient or confident otherwise…

You are likely to become part of the problem instead of part of the

solution !

Hand In Hand

GPS to report findingsand confirm coverage

Compass to make map calculations andto accurately deliver in the field

“Map” for calculating search activity and as the basic Information system in the field

MAPS

• Simply a representation of the land area of part of the earth on a piece of paper or screen (chart if ocean)

• Different Projections• Different Scales Larger Number = Smaller Features• Topographic are the most useful

DistancesDescriptionsDeclinationDetails (color coded symbols)DirectionDesignations (place names)

Maps in the Field

• Folding to optimize usefulness

• Weather Protection

Baggie

Waterproofing solution

Remember, the ink on print-outs of maps is

not weather-resistant at all !

Scale

• You must know the scale to work a map

• 1:12,000 occasionally available, usually as a printout from a mapping program – very rarely increases the features from a 1:24 …just spreads them out more

• 1:25,000 metric maps, several years still in circulation• 1:24,000 most common topo portrayal, 7.5 minute

quads. About 2.5 inches = mile• 1:62,500, or 15 minute quads consist of the area

covered by 4 7.5-minute quads (1 inch = 1 mile)• 1:125,000, or DeLorme Atlas maps

Details

• Black = Manmade objects

• Blue = Water features

• Brown = Elevation

• Pink = Heavily populated areas

• Green = Woodlands

• Purple = Change since last full map made

• Red = Major roadways

Land Features – Tan/Brown

Land Features – Green/Blue

Water Features – Blue

Water Features - Blue

Manmade Features Black and Pink

Manmade Features Black and Red

Manmade Features – Black & Red

Directions

• Top of the map is true north• There is also grid north• There is also magnetic north

Declination (on ocean it’s variation)The agonic line

Declination in MaineDeclination by roseDeclination by compass measure

The Compass Rose on Your Map

Designations

Different style print for town/city/stateNames

Different style print for boundary linesand then there’s benchmarks

Contour Lines

• Drainages• Ridges• 10-20-50-100 foot increments• Index contours• Concept of closer = steeper

Practicing visualizing in the field so you can get a knack of what the portrayal should look like in reality

The Brown Contour Lines

The Compass

• Pocket

• Lensatic

• Mirror

• Optical

• Orienteering

Important Features

• Fluid or induction dampingThe infamous “air bubble”

• Base-plate creates right angles and a distance measure…may also give GPS/UTM scale(s)

• Bezel creates a compass – the bigger the bezel, the easier and more accurate reading

• How to tell north on the protractor and north on the compass needle

• Deviation problems• Complications of adjustable declination devices

Concept of Direction

• You must distinguish between a true direction and a magnetic direction

• Direction is a line of travel or sight from point A to point B – called a bearing

• Expressed in 360 degrees of a circle in which you are in the center and “north” is at 360 degrees

• If you go exactly opposite in direction it is 180 degrees off your bearing and called a back-bearing

Critical Need for AccuracyIn One Mile of Travel, if you are off by:

2 degrees (typical difference of grid north in Maine) = 190 feet to the side of destination

3 degrees (typical error factor for a compass reading in 5 degree increments) = 475 feet to the side of destination

18 degrees (typical error failing to correct for declination in some parts of Maine) = 1,710 feet to the side of destination

36 degrees (typical error made correcting for declination east instead of west = 3,420 feet to the side of destination

Parts of the Orienteering Compass

• Base-Plate

• Direction of Travel Arrow

• Compass Housing (bezel)

• Magnetic Needle

• Orienteering Arrows/lines

Parts of the Orienteering Type Compass

Function of the Compass

• “Always” points magnetic north• Create an imaginary angle between north and

your destination

Taking a field bearingTaking a back bearingGoing point to pointBoxing around a house/boulder, etc.Working around water bodies

Taking A Bearing In the Field

Time to Rest the Mindand Exercise the Butt

Combining Map and Compass

Making Oranges and Apples

Into

Apples and Apples

Orienting the Map

• Visually• Using the Compass• How and Why to Convert the Map to Magnetic Direction

Using bordersUsing Tick-marks

• How not to…using the grid lines

extending a line from the “compass rose”

Taking a Map Bearing

• Remember, the compass is a protractor, the magnetic needle has NO function here

• Remember, use Tick-marks or bordersOR magnetic north-south lines

• Extending out your lines• Make sure where the “Direction of Travel”

arrow is pointed

Why Not Use Grid Lines ?

You can, but remember, they are a different projection for GPS purposes

For use in Maine, you will need to forget True North, use magnetic north declination number, subtract grid north factor and use the result to compute bearings

Why bother ??? Well, most people won’t, but it does enable you to then use the vertical grid lines instead of drawing lines on the map

Taking a Map BearingStep One

Taking a Map BearingStep Two

Routefinding with Map and Compass

• Whenever using field bearings think whether what it says makes sense to you

• But don’t doubt the compass

• Handrails

• Offsetting

• Backstops and Baselines

• Checkpoints

• Best Travel Routes are not usually an option in SAR

that doesn’t compromise safety

it does compromise comfort

Triangulation

• By visual map orientation

• Much more accurate using the compass

Requires two visible points

Best between 90 and 120 degrees apart

OK – How Do I Calculate Distance In The Field ?

• Pedometers

• Pace is the basic unit of measure

but it’s too small !!!

• Pace becomes chains

• 66 feet = a chain

• 80 chains = a mile

• Chain counters

Global Positioning System

This is a Very Brief Overview to Introduce You to The Concepts of

GPS

GPS

Global Positioning System

• Started in 1960 as a USAF Project called NavStar

• Declared fully operational in 1995

• The result is we common folk benefit directly from a $12 billion project

3 Components to GPS

• 24 satellites in geo-synchronous orbit 12,500 miles up

• Government base stations that operate the system

• May include “Differential transmission facilities”

• Your receiver holds satellite I.D. and locations in memory & synchronizes with the satellites’ atomic clocks

P and CA Codes

• CA or coarse acquisition codes are affected by the ionosphere

• P or precision codes are restricted to the military

• CA codes can be enhanced by DGPS, but this application is limited to proximity to a DGPS transmission facility

Other Limitations

• It uses batteries• It does not normally replace the compass

some GPS units have electronic compass function• It doesn’t do altitudes very well at all (normally within 100- 500 feet) some GPS units have digital altimeters that are quite accurate• The more you ask it to do, the heavier the

battery drain

WAAS

• Developed for aviation purposes• Like DGPS it involves 25 ground stations

and two extra equatorial satellites • Corrects for ionespheric errors in CA code

transmissions

• Works only in US• Because satellites are equatorial, areas

such as Maine may not fully benefit

It just keeps getting better

                                 

           

How Does WAAS Affect Accuracy ?

• 100 meters:Accuracy of the original GPS system, which was subject to accuracy degradation under the government-imposed Selective Availability (SA) program. 

• 15 meters:Typical GPS position accuracy without SA.3-5 meters:

• Typical differential GPS (DGPS) position accuracy.

• < 3 meters:Typical WAAS position accuracy.

Still More

• A major use in SAR work is to create “tracks” that show, one computer mapping, exactly where you have been

• Tracks and indeed, the ability of the GPS unit to lock onto signals, don’t “do well” under canopy

• They are line of site, and so may have limited ability to lock satellites in cols, valleys, etc.

• It must be set on the right map datum to work• It uses batteries

Most receivers work on 12 channels…”looking” for 12 satellites at once

A location requires a 3-way lock, a 4-way lock gives much more accurate altitudes and less

horizontal error

Unpredictability then comes from a combination of receiver limitations and satellite locations

An additional concern is LCD screen operation in colder temperatures – many don’t do well under 15 degrees

So Why Bother ?

• Accuracy is NOT in question with a GPS – if you get a lock and a

reading, it is right horizontal accuracy is typically between 45 and 150 feet

• Provides a backup method of orienting yourself

• Visibility is not required

• It is used by IFW to provide track data and pinpoint locations

OK on the batteries already…

It does hold data if off or changing batteries but may not hold it over

long term battery removal

So Where’s the map is all of this ?

Good question…

A map must have coordinate grids to be useful with a GPS unit and you MUST know the map datum or the same reading will bring you to a different place in the field

The default Datum is WGS 84

Many 7.5 maps use NAD 27

Lat-Long Coordinate System

Latitude

Lines parallel to the equator

equator = 0 degrees

north pole = N 90 degrees

South Pole = S 90 degrees

Expressed in 3 formats

• Hemisphere-degrees-minutes-seconds N 38 27’ 54”• Hemisphere-degrees-minutes

N 23 27.3”• Hemisphere-degrees N 58.385

1” of latitude = 1 nautical mile

Longitude

• Lines run pole to pole• West or east of the prime meridian• Expressed in 3 formats Hemisphere-Degrees-Minutes-Seconds W 140 54’ 09” Hemisphere-Degrees-Minutes W 67 28.75’ Hemisphere-Degrees W 86.8241” on the map equals nothing consistent

UTM Coordinate System

• Splits the world into 60 6-degree zones

• Zone 1 starts at 180 degrees W/E longitude

• Because of distortion from this projection, UTM can’t be used N/S 80+ degrees

• You have two readings, northings and eastings…the letter shown on eastings isn’t really the zone

                      Figure 1

                                         

More on UTM

• Northing values start at the south pole with the equator being 10000000mn (meters

north) or more often expressed as 0000000mn

Screens

Commonly, screens will include:• Satellite Status• Location • Navigation• Plot/Track

Waypoints can be marked (in the field) or entered as computer input

Plotting in UTM requires a scale device

can be a template or the base-plate of the compass in some cases

Confidence circles are used to indicate the “area” of the location

GPS, like map and compass, requires practice for

proficiency