Map vs. Gps Mileage Discrepancy

[The Roaming Gnome]

I use a Delorme Inreach Explorer to track my mileage and waypoints. While mapping a section of the AT for a hike next week I noticed that the Inreach is showing a much shorter distance between waypoints than my old handy AT Pocket Profile. As an example, my Delorme shows 9.75 miles from Neel Gap to Low Gap Shelter, Ga. The AT Pocket Profile shows 11.5 miles.

Has anyone else experienced such discrepancies? I tend to believe the Delorme since it is GPS. I have noticed the same issue when hiking the AT. The signs read one thing and the Delorme reads another.

Thoughts?

Thanks,

Hiker63 Gary

http://www.hiker63.com
https://www.facebook.com/pages/Hiker63/1029737840375196?ref=hl

[bemental]

How frequently is your Delorme updating? If it's not actively tracking the entire time, there are straight-line distances in-between waypoints, and it's not tracking the trail exactly.

My other guess is differences in elevation. The ATPP might be calculating distances based on changes in elevation, the ups and downs do add up over time/distance.

[BirdBrain]

Maps of the AT are not infallible. In particular, scrutiny of elevation profiles will drive you foolish. There are areas where hills are created to marry data. There is a hill on the map just south of Wachipauka Pond that only exists on the map. It is very steep on the map. It is not there in real life. The map is a great guide. It represents extensive effort. It has mistakes.

[The Roaming Gnome]

How frequently is your Delorme updating? If it's not actively tracking the entire time, there are straight-line distances in-between waypoints, and it's not tracking the trail exactly.

My other guess is differences in elevation. The ATPP might be calculating distances based on changes in elevation, the ups and downs do add up over time/distance.

I have it tracking/updating every 10 minutes. The trip info which runs constantly is where I see the biggest difference.

[The Roaming Gnome]

I tend to believe the Delorme since it is GPS based. I'm curious as to how the mileage on the maps are decided. Where the signs placed there prior to GPS Mapping? Are the signs replaced when worn with out of date mileage?

[Deadeye]

Only your feet know the true mileage, Grasshopper.

[The Roaming Gnome]

Only your feet know the true mileage, Grasshopper.


LOL and my knees!

[bemental]

I have it tracking/updating every 10 minutes. The trip info which runs constantly is where I see the biggest difference.

If you're take a gps plot every 10 minutes, and connect those dots, your overall mileage is going to be shorter then actual, *especially* if the trail is windy or deviating from the straight and narrow.

Your GPS plots will be straight lines from each 10-minute plot, and will miss all of the nuances of the trail, including elevation.

An example (red is trail, pink are your 10-minute GPS plots):
http://images.tapatalk-cdn.com/15/05/15/81f812b65ec9f1ce102460d322d1d64e.jpg

[Offshore]

If you're take a gps plot every 10 minutes, and connect those dots, your overall mileage is going to be shorter then actual, *especially* if the trail is windy or deviating from the straight and narrow.

Your GPS plots will be straight lines from each 10-minute plot, and will miss all of the nuances of the trail, including elevation.

An example (red is trail, pink are your 10-minute GPS plots):
http://images.tapatalk-cdn.com/15/05/15/81f812b65ec9f1ce102460d322d1d64e.jpg

Excellent illustration. This also demonstrates the reason that some people think that their GPS devices (inReach, Spot, handheld) are "wrong" when the perceived inaccuracy is really just due to comparing high resolution data - the trail map with infinite points (i.e., a line) with a low resolution GPS track (1 point/10 minutes).

[Spirit Walker]

I have a gps watch I use when running. I've noticed that when trail running in the mountains, it is way off. Where there are switchbacks, the watch registers a straight line. Same when the trail winds in and out of drainages. Trees will sometimes block satellite reception so the watch will do a straight line from my last known point. For my needs, it isn't important. If I were writing a guidebook, it could be.

When we hiked the Great Divide Trail in Canada, it was obvious that the guidebook writer had either used a gps to gauge miles or simply wheeled the topo map because the very steep sections would register incredibly low and the road walks would be marked longer than they were. Fortunately, we were able to adjust expectations, especially after the first couple of times.

[RockDoc]

Many GPS devices are adjustable with regard to distance between points. The worst possible option is "battery saver" which records the minimum, maybe a point every couple hundred yards. The inaccuracy goes way up on curvy trails but makes no difference on pretty straight trails.

[blazercoach]

For those mathematically inclined, the problem that bemental illustrates is explored in detail by Mandelbrot in his paper "How Long Is the Coast Of Britain?"......more generically known as the Coastline Paradox. I suspect that is exactly where the difference comes from between the GPS and a hand-surveyed estimate of the distance.

[rocketsocks]

One way to solve this is to slow your stroll, and manually set points, most won't be willing to slow down. GPS will measure elevation quit well (the Hypotenuses or offset so to speak) but it requires additional satellite signal and patients to acquire in low signal areas.

[rocketsocks]

One way to solve this is to slow your stroll, and manually set points, most won't be willing to slow down. GPS will measure elevation quit well (the Hypotenuses or offset so to speak) but it requires additional satellite signal and patients to acquire in low signal areas.theses' a percentage of error there too.