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[RC] GPS accuracy - Sisu West Ranch

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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