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I'm just wondering, when there are discrepancies between your GPS unit and map, do you always default to your map? Today I did some trails in South Mountain, and the Beartooth waterproof map I had said the trails I did had a cumulative total of 9.5 miles. On the GPS for the same route, it showed 8.8 miles. This route is not going off trail or taking any shortcuts that differed from the Beartooth map.

This has happened before...and I'm not sure which to believe when it does.

I'd go with the GPS unless there is evidence the trail had been surveyed which would be unusual. I heard all 95 miles of the Wonderland Trail has been though.

just use whats called a "mountain man mile" when judging hiking distances, its really easy to do, since all hikes are exactly 1 mountain man mile long.

A mountain man mile is the distance from wherever you are to wherever your going.

I'd stick to the GPS unless there are tight switchbacks, dense canopy, or deep canyons where GPS signal may be lost (not the case at S. Mtn). Even a 1:24,000 topo map with a trail marked does not come close to the accuracy of your GPS (as long as it's getting a solid signal).

I've worked with lots of GPS data over the years coming into the site. Based on trails I know where data comes in from multiple members there's incredible variances. I find it's best to use TOPO! if you're serious about accuracy.

chumley wrote:I'd stick to the GPS unless there are tight switchbacks, dense canopy, or deep canyons where GPS signal may be lost (not the case at S. Mtn). Even a 1:24,000 topo map with a trail marked does not come close to the accuracy of your GPS (as long as it's getting a solid signal).

interesting ways of looking at it. my gps never lost its signal today, so i'll just trust it. surprises me there's that much of a difference...maybe the segments on the Beartooth maps are just rounded off too, which doesn't help...there were several segments on that map I had to add up, and it just rounded everything off to the tenth of a mile. That was at least part of the problem, I'm guessing...

I hiked with a guy who had been a Garmin Dealer. He had two Garmin GPS units and I had mine, we had 3 different distances for the same trail. We always get different distances between Garmin and Magellan with Garmin usually a little longer and Magellan a little shorter. I've gotten home and downloaded my track and found that when I took my pack off, the route would ping out 1/4 mile and back a 1/4 mile. When I was hiking it made me feel good that I had gone farther faster, but in reality it was an anomaly. If you have a Beartooth or Emmit Barks map I would trust them. They might have some rounding but they have been double checked. Anything else is an approximation.

Al_HikesAZ wrote:I hiked with a guy who had been a Garmin Dealer. He had two Garmin GPS units and I had mine, we had 3 different distances for the same trail. We always get different distances between Garmin and Magellan with Garmin usually a little longer and Magellan a little shorter. I've gotten home and downloaded my track and found that when I took my pack off, the route would ping out 1/4 mile and back a 1/4 mile. When I was hiking it made me feel good that I had gone farther faster, but in reality it was an anomaly. If you have a Beartooth or Emmit Barks map I would trust them. They might have some rounding but they have been double checked. Anything else is an approximation.

hmmm, interesting. how old were the gps units? are they recent models?

dshillis wrote:hmmm, interesting. how old were the gps units? are they recent models?

This was about 2 years ago, mine at the time was the Etrex Vista C, he had a ForeRunner on his wrist for training for the PF Chang marathon and an early model 60CS (I think). Good basic models and fairly reliable. Look at your track on the computer for any anomalies. On an out and back hike, look at your track. They should align but they don't because satellites move. The faster you move, the more variance you will get. I'm getting much less variance as I get older. How precise do you need to be? Hike the trail 10 times and take your average ;)

GPS units generally assume a piecewise linear track between sampling points, effectively cutting corners, and the distance between the points varies according to your settings. Even when set for pedestrian, the cumulative error can be considerable. I surveyed a couple running routes with a GPS and came up about 10% short of a distance wheel measurement that had been done several times. I brought the track up on MapSource and edited it to match the streets exactly, and the 10% shortfall was restored.

rally_toad wrote:just use whats called a "mountain man mile" when judging hiking distances, its really easy to do, since all hikes are exactly 1 mountain man mile long.

A mountain man mile is the distance from wherever you are to wherever your going.