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While every increase in electronic technology will improve the accuracy of 
GPS position measurements part of the problem with using a GPS for an 
odometer is inherent in how it computes the distance.

I just looked up Magellan's top of the line WAAS enabled specifications.  It 
takes a reading every second.  Lets assume the following conditions for 
grins and giggles:

1. Each position it calculates is right on the money. (we do know that there 
is random error, but they may average out.)
2. The distance is calculated by assuming that you go on a straight line 
between each measurement.
3. The horse is moving at 16 K per hour (10 mph).  In one second it will 
move at 4.4 meters (~ 4.8 yards).  Exactly 0.5 seconds after the fix it 
makes an abrupt right angle turn and continues without slowing down.

Using elementary geometry, we have described a right triangle with legs 2.2 
m long and a hypotenuse of 3.11 m long.  That means that during the time of 
this turn 1.3 m have disappeared.  Or looking at things another way,  the 
measured distance is only   ~71% of the actual distance traveled.

How can this error be reduced?  The slower you go, the less error you will 
get.  Simple geometry.  The fewer turns, the less error you will get.

I suspect that when my old Magellan is used some more errors are included 
due to the electronic/physical limitations Truman posted about.  I have 
considered getting a newer unit, but since the old one works fine for my day 
to day purposes, I am too cheap.  Last spring while laying out a compass 
course for a practical map course, I did notice that under ordinary mountain 
GPS conditions, it was advantageous to wait 30 sec to a minute at each 
important point.  The reported position would slowly drift and then 
eventually settle down.  I suspect that a "better"  or newer unit would have 
exhibited less drift.

As far as trusting a GPS unit for normal foot, horseback, or automobile land 
navigation, I have never seen positional errors that were bad enough to 
cause problems.  Before "selective availability" was turned off, I did get a 
fix that said I was about waist deep in the Minnesota River when I was 
really on the bank.  Now even in the mountains, where reception is less than 
ideal, I rarely have a fix that is even 20 meters different than my USGS 
map.  I would hope that if I were lost, and called rescue on a satellite 
phone, they would have enough sense to look around when they got to my 
reported position.

The real problem, as I see it, is those folk who go into remote areas and 
depend on modern electronics.  They do not consider what would happen if 
their horse stepped on a dropped GPS.  Even worse, they assume that they can 
use their cell phone to call for help.  If going into a remote area a 
prudent person makes sure that things are backed up.  The cell phone is 
backed up by letting somebody know where they are going and when they will 
come back.  The GPS is backed up by a paper map.  The electronic compass is 
backed up by a mechanical one.  The mechanical compass is backed up by 
knowing how to find direction by stars and sun.  The paper map is backed up 
by pre-trip studying and observation as the trip progresses.


This February we took a trip to Phoenix.  We stayed next to the Estrella 
Mountain Park.  In preparation for the trip, I downloaded detailed maps of 
the trails in the park.  I then used a mapping program to calibrate the 
maps.  From these calibrated maps I generated a bunch of waypoints which I 
downloaded onto my GPS.  Later I compared the observed positions with the 
calculated waypoints.  My friend also made some observations with her GPS (a 
newer, but inexpensive one).  Careful comparison of all of these 
measurements showed that they all were right on, in the practical sense. 
Admittedly, in the desert we had good reception.

The base map for these park maps was the USGS quad covering the park.  I 
suspect that the trails were transferred to the map from GPS measurements 
taken with a high quality unit.  In MN I compared "official park maps" with 
USGS quads and field observations.  It was not uncommon to find that trails 
were drawn hundreds of meters from where they actually are located.  Before 
the availability of GPS, many park systems apparently just sketched the 
trails where they thought they were.  In my local area, the location of 
irrigation diversions is very approximate at best.  A hundred, or more, 
years ago surveyors were not hired to find out where the rancher had 
diverted part of the creek for irrigation.  The rancher just went into the 
courthouse, and pointed to a map.  That point was translated into the 
quarter, quarter, quarter, quarter section.  One of the diversions I manage, 
is actually a 100 m or so from where recorded, this puts into the next 
section.


Moral:  For ordinary land navigation the GPS works fine.  I still do not 
completely trust distance measurements, you likely will find the measured 
value to be low by up to 10%.

Now if  I were calling in explosive ordinance on my position, I would 
undoubtedly check the GPS against my map, and still run like hell.   If  I 
were landing an airplane, I would be worried if the elevation data on my map 
were to be based on NAD27 or if  I was depending on the GPS calculated 
elevation.  On horseback, I really don't care if my GPS says I am at 1200 m 
elevation and I am really at 1000 m elevation.  If my horse is actually 200 
m above  the ground level, I have more immediate serious problems than 
getting an accurate GPS reading.

Ed
Ed & Wendy Hauser
2994 Mittower Road
Victor, MT 59875

(406) 642-9640

ranch(at)sisuwest(dot)us 



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