T H E M O U N T A I N E E R S
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`GPS
` SPS
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`M A D E E fl S V
`MHHEEHSS
`USING GLOBAL POSITIONING
`SYSTEMS IN THE OUTDOORS
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`IJSINS SLIIHHL PIISITIIININS
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`SYSTEMS IN THE fl|JT[|fl[lHS
`SECOND EDITION
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`LAURENCE LETHRM
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`V *
`M
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`MOUNTAINEERS
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`For Tanya
`i_ The only thing left to map is life
`and that only in retrospect
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`Table of Contents
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`Front Cover: Using a GPS receiver to mark the position of a camp in Ellesmere Island,
`Nunavut, Canada. Photo: Tony Daffern.
`
`&J\AI
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`Published by
`The Mountaineers
`1001 SW Klickitat Way, Suite 201
`Seattle, WA 98134
`
`Copyright © 1995, 1998 by Lawrence Letham
`Second edition, 1998
`
`All rights reserved. No part of this book may be reproduced in any form, or by
`any electronic, mechanical or other means, without permission in writing from
`the publisher.
`
`Published simultaneously in Canada by Rocky Mountain Books, Calgary
`Distributed in Europe by Cordee, 3A De Montfort St.,
`Leicester LE1 7HD, Great Britain
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`Manufactured in Canada
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`Library of Congress Cataloging-in-Publication Data
`
`Letham, Lawrence,
`GPS made easy: using global positioning systems in the outdoors /
`Lawrence Letham.-- 2nd ed.
`p. cm.
`Includes index.
`ISBN 0-89886-592-1
`1. Outdoor recreation-Equipment and supplies. 2. Global
`Positioning System. 3. Orienteering-Equipment and supplies.
`I. Title.
`GV191.623.L48 1998
`796.58'028-dc20
`
`98-38724
`CIP
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`1 Introduction to the Global Positioning System (GPS) — 5
`2 Why Buy a GPS Receiver? — 15
`3 Backing up Your GPS Receiver — 21
`4 About GPS Receivers — 25
`5 Using UTM Coordinates on a Hiking Trip — 73
`6 More UTM and Collecting Water Samples in the Desert — 89
`7 GPS Navigation in a Whiteout — 101
`8 Latitude, Longitude and a Kayaking Trip — 109
`9 More Latitude, Longitude and a Sailboat Rally — 128
`10 Small Scale Maps and a Road Trip — 139
`11 Recovering from Disaster — 152
`12 GPS Receivers and Personal Computers — 158
`13 Another Input Device — 169
`14 Other Grids — 173
`15 Differential GPS — 183
`16 Degrees, Minutes, Seconds and Mils — 187
`
`Glossary — 194
`Resources — 200
`Other Books — 201
`GPS Receiver Information — 202
`Index — 206
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`GPS Made Easy
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`Speed
`A receiver measures the time and distance between the point you were
`at a few seconds ago and where you are now, then it divides the distance
`by the time to get the speed.
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`There are two different ways to measure speed:
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`• speed over ground
`• velocity made good
`Both are explained below. Selective Availability does affect the speed
`measurement's accuracy, but newer receivers use averaging algorithms
`to make the calculation more precise. Even if a receiver averages, the
`receiver may report you are going some slow speed when you are
`standing still. At slow speeds, just ignore the random fluctuations.
`Higher speeds are measured more accurately because the variations
`caused by Selective Availability are a smaller part.
`Receivers have upper speed lim­
`its, so if you plan on using it at
`really high speeds, be sure it will
`work at the speeds you want to go
`before you buy it. All speeds are
`reported in the units you select:
`miles per hour, kilometers per hour
`or knots.
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`23.4
`TIME TO DE
`021
`BEARING
`090
`AVG ."FEE
`8.4
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`Garmin compass screen showing both speed
`and average speed.
`
`Speed Over Ground: Speed over
`ground (SOG) or ground speed is
`just like the speed given by the
`speedometer in a car; it simply
`measures how fast you are going at
`that very moment. SOG does not
`care if you are on course or off
`course, so it is a measurement of
`your speed irrespective of direction.
`
`Velocity Made Good: Velocity
`made good (VMG) is the speed at
`which you approach your destina­
`tion. VMG does take into account
`your present course and your desti­
`nation. If you are directly on
`course, VMG is the same value as
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`VMG = SOG
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`Destination
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`M
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`SOG
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`Relationship between SOG and VMG.
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`T
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`4-About GPS Receivers
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`SOG, but if you stray from course, VMG decreases and is less than SOG.
`The figure shows how VMG and SOG relate.
`The calculations for ETA and ETE are based on VMG. If you are headed
`directly on course, ETE will decrease until it is zero at the destination. If
`you are slightly off course, the ETE will decrease until the receiver
`determines you will never arrive at the destination, then it either begins
`to increase or it disappears altogether. Refer to the section Estimated
`Times on page 61.
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`Average Speed: Average speed tells you how fast you really go in
`heavy traffic. It is not the same as speed over ground. SOG is your speed
`at any second in time. If one second you go 25 mph (40.2 kph), the SOG
`shows 25 mph. If a second later you go 50 mph (80.5 kph), the SOG
`instantaneously changes to 50 mph. Average speed divides the distance
`you have gone by the amount of time it took. Suppose you have driven
`your car for a long time and the average speed is 25 mph. When you
`suddenly accelerate to 50 mph, the average speed does not immediately
`change, but slowly starts to rise. After you have traveled 50 mph for as
`long as you did 25 mph, the average speed is only 37.5 mph (60.4 kph).
`Any receiver that provides average speed must also measure the
`distance you have gone and the time in transit. Usually, the timer measur­
`ing the duration of the trip counts only when you are moving. In a traffic
`jam, the timer does not increment, so the average speed can be deceptively
`high if you go fast for a while then wait at a stop for even longer.
`
`Maximum Speed: Maximum speed is the fastest instantaneous speed
`reached by the receiver. If you flail your arms with the receiver in your
`hand, the maximum speed represents how fast you moved your arms.
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`Direction Indicators
`All directions calculated by the receiver are expressed as a bearing.
`Bearings are described above and in Chapter 16. Two directions are
`always provided by a receiver: the bearing of the direction you are
`currently heading and the bearing you should be going to get to your
`destination. The bearing you are currently going has names like:
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`• Course Made Good (CMG)
`• track (TRK)
`• heading (HDG)
`The bearing you should be going to get to your destination may be called:
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`• desired track (DTK)
`• bearing (BRG)
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