GPS mileage versus actual

[masshysteria]

While taking a little stroll through the lovely Berkshire woods yesterday, I ran across an anomaly with my Garmin E-Trex Vista. Using both National Geographic'sTopo program, and Delorme's Topo 4.0, I mapped out a bushwacking rout. I positioned waypoints on the maps, and drew a track from point to point. I printed a map with this route, and an elevation profile. Both programs had a total distance of 7.1 miles. I downloaded the waypoints to the GPS, and was on my way.
I hit each waypoint along the way, and was very pleased with the accuracy. When I finished the hike, the GPS had a distance of 5.6 miles. This seemed a little odd, so I downloaded the track that I just walked, printed out a map of the route, with an elevation profile. The route matched perfectly my planned route, but the mileage from the Nat. Geo. and DeLorme programs showed it as being 7.13 miles.
So how far did I walk? Does the GPS only measure in linear distance, and the mapping programs measure actual distance? Or what?

Tom

 

[sardog1]

Here's my WAG:

You may not have actually printed out the track from the GPS but rather the route that you made using the waypoints you entered before heading out. A "track" is (sort of) a record of the actual path of the unit as it moves. A "route" on the other hand is a series of waypoints entered by the user.

Why "sort of"? When recording the track, the unit will fill in gaps from insufficient satellite coverage by drawing straight lines between the last fix and the next available one.

If this is what has happened, you should be able to see it by comparing the saved track on the GPS map screen with what's on your printed map.

If this is not what happened, then I'm full of you-know-what and eager to hear a better explanation. (The numbers are wrong for a metric/statute mile mixup.)

 

[SteveHiker]

Hey, don't feel to bad about it. I once found a pedometer about 2 miles from the base of the Owl's Head slide. After summiting Owl's Head and returning to the Lincoln Woods parking lot, I was surprised to find that I had walked something like 1.7 miles according to the pedometer. (and burned 37 calories!)

 

[bobandgeri]

What the GPS records is somewhat based on how often you have it record a track point. The closer the points are together, the more accurate the distance. If the points are far apart, you most likely didn't walk ina straight line between them which is what the GPS measures. I set mine based on teh time I think it will take me to get to the summit and divide the max number of track points (1024 for my GPS) by the number of minutes I expect to be hiking in order to record as many points as possible.

 

[DougPaul]

The GPS may not include the distance covered during any signal dropouts and the program may fill in the dropout regions with straight lines. The GPS might also have a speed threshold on the distance measurement.

The GPS track may also cut corners and thus give a shorter distance.

Doug

 

[Mark]

As they say, something doesn't add up.

Let me see if I have the facts. When you set up the waypoints in your Topo programs, you created a route. The total round distance of this route was reported to be 7.1 miles. You downloaded these waypoints into your GPS and hiked the route. When done, the GPS reported you had hiked 5.6 miles round trip. Do I have this right?

I have an eTrex Summit and also use DeLorme Topo. I've seen the opposite discrepancy where the GPS reports a total distance covered that is longer than the defined route (due to cumulative tracking errors), but never shorter. When you created the route in Topo, you asked for the trip distance waypoint to waypoint, not following any trail on the map, correct? I'm guessing that since this was a bush whack, there were no trails on the map.

Topo will report route length in linear distance and terrain distance. (I think those are the terms they use.) IIRC, linear distance is point to point as if walking along a perfectly flat route. Terrain distance will be greater than linear distance between two points if you are going uphill or downhill. (Think of it as hiking along the hypotenuse of a right triangle to get to a specific elevation.) I think the GPS just reports linear distance. However, for the difference between 5.6 and 7.1 miles, you would have had to hike a route with an average 38 degree slope (78% grade) which is probably unlikely.

I think the GPS calculates total trip distance by adding up all of the distances between the track points it collects as you walk along your path. Even if it loses a lock on the satellites, it will just add in the distance between the point where it reacquires a lock and the last known point. If you haven’t cleared the track from your GPS, you can download all of the track points it collected and examine the data to see if you can locate the source of the problem. (Note: If you download the track points into a spreadsheet, you can calculate the individual distances between points, but it’s not a straight forward calculation since the linear distance for one degree of latitude does not equal the linear distance for one degree of longitude.)

It is possible to reset the trip odometer, but I am assuming you didn’t do this. Did you turn off the unit or change batteries? I’m not sure if either of these will clear the trip odometer. I usually forget to reset it at the start of the trip.

One other thing to check is that if you created a route from the waypoints in your GPS, you can go to the Routes page on your GPS and look at the point to point distances calculated by the GPS for each leg of the trip (plus total route length) and see of they agree with the same leg distances and total route length from Topo.

 

[imarchant]

I also usually find the opposite true. I tend to use as few way points as possible because it make it easier to figure out when you will be at the next point of interest (viewpoint, lunch spot, summit, etc). The track follows every twist and turn. The amount of difference is a function of how many and how accurately placed the waypoints are placed on your route. The tracking error does not change the total distance much because the errors are random and tend to cancel each other out.

The GPS tracks seem to be slightly shorter than the published trail descriptions. I figure this is a combination of two things. I believe the GPS only calculates the 2-D map route. This is usually less than 1/2% error. The wheel used to measure trails measures slight more than you walk because you tend to step over some of the rocks that the wheel goes up and over, contributing a similar type error.

 

[Mark]

The tracking error does not change the total distance much because the errors are random and tend to cancel each other out.

We might be talking about two different things here, but my comment about tracking errors add to the total distance reported by the GPS came from closer examination of the track points recorded by my GPS. The tracking errors made it appear that I sometimes zig-zagged down the trail instead of walking a straight line which added to the total trip distance. The track points cluster around the actual path I hiked, so the errors cancel out in approximating my route, but the trip odometer appears to just add up all the track segment lengths making the tracking errors cumulative. Your GPS may be smart enough to toss out outlying points or average out your track points. If you are just looking at the route length vs the track length, you would also get a much shorter distance from your GPS.

I've seen similar problems with tracking error when viewing other trip data. After one hike, my maximum trip speed was reported to be over 50 MPH! (My brothers say I am a fast hiker, but I don't remember going that fast!) Obviously, the GPS calculated a current location that was incorrectly far away from the last know track point. Distance divided by time equals speed.

I wish there was a technical specification for the eTrex family of GPSs that documented how they process all of the collected data.

 

[Jean]

When I run into this type of problem I usually reroute power from the deflector dish to the sensor array and run a level 2 diagnostic using a tachyon beam.
Usually gives me a pretty accurate reading of the distance unless, of course, if you have superstring emissions from the rolling tachyon background.

 

[bobandgeri]

I wish there was a technical specification for the eTrex family of GPSs that documented how they process all of the collected data.

I have always found Garmin to very responsive. Try writing to them via:
http://www.garmin.com/contactUs/techSupport.jsp

 

[Orsonab]

Nothing's certain except taxes and death...

My GPS (Garmin Geko 201) never agrees with my Topo software which never agrees with my AMC White Mountains guide. Thus, when in doubt I tend to pick the longest distance ('cos it feels more manly) or divide my hikes into two categories: "not long enough" or "too bloody long."

 

[masshysteria]

For what it's worth, I thought Jeans answer was the best! Make it so #1....
OK, lets see if I can make sense of this, Mark. I made 7 waypoints on my Topo software, which I downloaded to the GPS as a waypoint only, no route. The Topo told me the walk would be 7.1 miles, if I walked in a straight line from point to point. I cleared the GPS of any old waypoints, tracks, data, everything, except the 7 waypoints. Being that I was interested in testing the effiiciency of the GPS and Topo programs to work together, I made the waypoints very obvious physical objects( trail intersections, mountain tops, bridges, geodetic survey markers). A couple of the spots were off by 20-30 ft, most were within 10-15 ft, and a railroad bridge was smack dab in the middle of it! I traveled in a bee line as close as possible. Now DougPaul suggests that any loss of reception might explain the shorter GPS mileage. I did lose reception 4 times when I was in a dense hemlock forest, but it was only for a few seconds each time. I did at one point stop and eat lunch for a half an hour, but the unit stayed on at all times.
Arriving home, I downloaded the track that the GPS compiled, and printed an elevation profile, which showed the distance as 7.13, while the mileage on the screen of the GPS was 5.6 miles.
Some thoughts I have; when i set Topo up to download the track from the GPS to the program, it asked if I wanted it to make a track showing each individual track point, or to make a continuous route of the tracks, which is what I did. I have since gone back to the program, re-downloaded the GPS. Except this time, I asked it to show each individual track point. Needless to say, the map it printed out was a garbled mess of trackpoints, the UTM coordinates for each point. All 538 of them. But the trackpoint log did show each trackpoint, its UTM coord., time of day it took the reading. The interesting thing was that there was no set time frame as to when the GPS made a trackpoint. Sometimes it was every 10 seconds, othertimes it was 30 seconds, and all ranges between the two. There were a few times when the time difference was a minute or so. But there was only 6 that lasted that long, and I'm going to write them off as when the GPS lost it's sattelite fix. I'll have to look in the 'Setup" menu to see if I can change how often the GPS makes a trackpoint.

I'm also wondering if there would be that much discrepancy if I were to use the Garmin Topo software. I'm going to borrow a copy and see what the scoop is. So what do you suppose the actual mileage was?
By the way, other than this discrepancy, I found everything else between the GPS and the mapping software to be fairly accurate. It does not eliminate the need to know how to use a map and compass.
And thanks to all of you for your input.

 

[bobandgeri]

The default setting for how often a track point should be saved is usually auto - it's records a track point when ever there is a change in direction. Change it to record one every 30 seconds, or less depending how long you will be hiking.