GPS and basecamp discrepancies

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GPS and basecamp discrepancies

Postby polas » Mon 09 Mar, 2020 8:48 am

Hi,
I noticed that my gps (garmin gpsmap64s) live stats are quite different from the stats reported by mapsource from the same exact track. In the past, I thought it was user error but recently I paid a bit more attention to it. As you can see in the attached picture none of the stats match. It doesn't bother me much but I'm curious why this is happening. The difference is quite significant and I cannot think of any good explanation. The only thing that could explain it is that the GPS is calculating the statistics more often and does it even in the places with very bad coverage when the position jumps all over the place. The recorded track is more sparse and contains less of the errors and is then shorter. However, I would expect garmin software to be smarter than that and be more accurate than the sparser resulting track at the end. Does anyone know or have any better theories?
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Re: GPS and basecamp discrepancies

Postby neilmny » Mon 09 Mar, 2020 10:14 am

Have a look at your basecamp settings under device transfer. Some options there will trim data off your recorded track.
Elevation measurement on a GPS is not precise. Check your specs for the expected accuracy.
For a GPS with barometric elevation measurement it is a good idea to calibrate your elevation to a known elevation before you start the walk.

From a google search -

"The general rule of the thumb is that vertical error is three times the horizontal error. If a decent signal reception is available, a modern GPS receiver should be able to give elevation data accurate to a range of 10 to 20 meters"

I would think this error potentially applies to each recorded elevation.

From Garmin -
https://support.garmin.com/en-AU/?faq=Q ... yoxITW2vZ6
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Re: GPS and basecamp discrepancies

Postby polas » Mon 09 Mar, 2020 11:31 am

the elevation is inaccurate but it is recorded in the track. The distance, however, should be fairly accurate assuming decent GPS signal. 7km map source vs 8.58km on the GPS is a huge difference - roughly 20%. Again this is recorded in the track so even if the measurements are inaccurate then they should match.
I checked the basecamp settings too and the tracks are not being altered (simplified) by it.
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Re: GPS and basecamp discrepancies

Postby neilmny » Mon 09 Mar, 2020 11:47 am

It's a bit of a mystery then. I do get variations between divices and I use Mapsource not Basecamp. But not as large as you have there.
Have you downloaded the track to Basecamp or just viewing it on the GPS? Download a copy to Basecamp so as not to stuff up the original.
Which topo map are you using? I use OSM maps that I make myself but I am unable to set the track to the elevation with them.
If I want to do that I change the map in Basecamp to Topo Australia & New Zealand (a Garmin product) I can then set the downloaded track to
map elevation. If you can do this, it would be interesting to see if this changes the track length to nearer the GPS distance.
Not sure where that leads to but it's a start.
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Re: GPS and basecamp discrepancies

Postby tastrax » Mon 09 Mar, 2020 2:21 pm

It may well be that the device does regular calculations 'on the fly" whereas the download just uses the extremities (start/finish/highest/lowest) for the calculations. Also I have no idea if each system uses cartesian or ellipsoid calculations.

Cartesian distance is as measured on a flat Earth. Ellipsoidal distance is as measured on a spherical shape (or ellipsoid). To understand why the latter is longer, draw a circle and then draw a square inside it whose corners touch the circle. Now you can quickly see that the distance between any two adjacent corners is less if you follow a path along the edge of the square, than if you follow a path around the circle.

Also check the settings to make sure you are getting the best points in the gps logs as the device will do some point removals to save space. Set a recording interval (3 metres) versus the Auto setting and see how things change.
Cheers - Phil

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Re: GPS and basecamp discrepancies

Postby polas » Mon 09 Mar, 2020 5:43 pm

tastrax wrote:It may well be that the device does regular calculations 'on the fly" whereas the download just uses the extremities (start/finish/highest/lowest) for the calculations. Also I have no idea if each system uses cartesian or ellipsoid calculations.

Cartesian distance is as measured on a flat Earth. Ellipsoidal distance is as measured on a spherical shape (or ellipsoid). To understand why the latter is longer, draw a circle and then draw a square inside it whose corners touch the circle. Now you can quickly see that the distance between any two adjacent corners is less if you follow a path along the edge of the square, than if you follow a path around the circle.

Also check the settings to make sure you are getting the best points in the gps logs as the device will do some point removals to save space. Set a recording interval (3 metres) versus the Auto setting and see how things change.


good call I will make it record more points and then compare again.
I don't think cartesian/ellipsoidal distance in such small distances matter. I see 1.5km difference on 8 km track. I'm simply curious if this is normal and if there is a simple explanation.
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Re: GPS and basecamp discrepancies

Postby tom_brennan » Tue 17 Mar, 2020 1:06 pm

If it's not sensitive, you could post the track for others to examine.
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