what causes a gps to be wrong?

[Khlash]

What are some causes of a gps being wrong? My Meridian Plat. seems to sometimes be off when I am backtracking over the same trail. sometimes it shows me up to 150 feet off the trail I am backtracking. Is this normal? There was some tree cover but I checked and it was tracking 6+ satellites fairly strongly. If I didnt know better and I looked at the tracks when I was done I would say I took a different trail. Any ideas? Finding caches seems to usually not be a problem. Could it have anything to do with the topo cd I have loaded?

[+briansnat]

Cloud cover, trees, buildings, canyon walls, ravines, rain, your body, boulders, et. al., although 150 feet seems to be an awful lot. Are you sure it's that far off?

 

"Au pays des aveugles, les borgnes sont rois"

[Khlash]

Its not consistently that far off. Its sporadic. But my backtrack trail is shifted. I mean It usually has a similar shape as the outgoing trail but its shifted to the side. Sometimes the two will meet, but like I said its sporadic. At least 100 feet off at times, maybe more.

[+Cornix]

quote:

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Originally posted by Khlash:

sometimes it shows me up to 150 feet off the trail I am backtracking. Is this normal?

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Yes, this is normal. It's due to interpolation between the stored track points.

 

Cornix

[Khlash]

Interpolation? can you give a bit more info?

[+The Cheeseheads]

Could be a bad satellite day. I've had days where there are 10-12 satellites scattered across the sky giving me great reception, then days where there's only 6-7 that are visible, or the bulk are all on one side of the sky. Then I walk behind a tree and cutt off 3/4 of the birds leaving me with terrible accuracy.

 

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[+Cornix]

I don't know the exact algorithm. Every 200 ft or 30 sec or something like this, the GPSr stores a track point. These track points are connected by a straight line. Your real track between these points is lost. On your backtrack different track points are stored and connected. So depending on the shape of your trail, you'll get a deviation.

[+Cache Hunter D]

If it's a short hike you can turn up the track detail on the MeriPlat and it will help, but it's still a problem. If I turn mine off, then drive somewhere else and turn it back on, I get a track going straight from where I last was to my current loc. Just one of those goofy things. Makes saving track to route kinda difficult.

[+Cornix]

quote:

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I get a track going straight from where I last was to my current loc. ... Makes saving track to route kinda difficult.

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You can edit your track with MapSend. In such cases I store the complete track twice and delete the second part of the track in the first file and the first part in the second file afterwards.

[+Cache Hunter D]

So it's not just me getting those? I thought it was just something being flaky with my MeriPlat.

 

I knew MapSend could edit the routes, but not tracks. Thanks for the tip.

[+Cornix]

quote:

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I knew MapSend could edit the routes, but not tracks.

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It's a nice feature. You have a table with the data of all your track points. When you click on a row, the corresponding trackpoint is highlighted on the map. By pressing the up and down keys you can follow your trail.

[bug and snake]

quote:

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Originally posted by Cornix:

I don't know the exact algorithm. Every 200 ft or 30 sec or something like this, the GPSr stores a track point. These track points are connected by a straight line. Your real track between these points is lost. On your backtrack different track points are stored and connected. So depending on the shape of your trail, you'll get a deviation.

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<wink>If it is 30 secs, (as opposed to distance)this is easy to solve then - Walk slowly!</wink>

 

Only nuts eat squirrels,

Snake

[+TMAN264]

Sometimes I wish I were only 150 feet off!

 

Make a sanity check.