GPS Questions yet again

peregrina2000 · Mar 15, 2019

I am loading the Olvidado tracks for my upcoming walk, and I always try to compare what's out there. Well, as I was looking for tracks for the new mountain stage from Boñar to Vegacervera, I saw two that looked pretty identical. One was posted by Ender, one by Ray y Rosa, but to me they look the same. Yet total elevation gain is very different:

685 m elevation gain -- https://www.wikiloc.com/hiking-trails/etapa-13b-camino-olvidado-bonar-vegacervera-24882201

1058 m elevation gain -- https://www.wikiloc.com/hiking-trails/etapa-13-b-bonar-vegacervera-camino-olvidado-15178298

I am not looking for precision, but that is quite a difference, almost 400 m. Not that it really matters, since I assume that no matter which one I download, I will have to walk the same ups and downs.

But 1085 m is more like a SJPP to Roncesvalles day, whereas 685 is more like a Pola de Allande to a Puerto de Palo day.

Just curious if anyone knows what could explain it, especially since they look to be the exact same tracks!

dougfitz · Mar 15, 2019

Elevation is the least accurate of the dimensions measured by a GPS, but I doubt that alone would explain the differences you are seeing.

If you are a Google Earth user, you could import both tracks and compare the elevation gain for each. Google Earth uses a different algorithm for these calculations, and uses seperate elevation data, not the elevation data stored in the .gpx file. If the tracks are sufficiently similar, the elevation gains should be as well, although that could be a different result again from the two you already have

.

I would have a crack at this myself, but I am travelling, and don't have access to Google Earth right now.

Deleted member 67185 · Mar 15, 2019

Hi, Laurie. . .

In comparing both, they are differing elevation profiles. I do not know why one is different from the other, but there is enough difference to explain the discrepancy.

peregrina2000 · Mar 16, 2019

davebugg said:
Hi, Laurie. . .

In comparing both, they are differing elevation profiles. I do not know why one is different from the other, but there is enough difference to explain the discrepancy.

Maybe one of the walkers is taller than the other.

Rick of Rick and Peg · Mar 16, 2019

It may be that one track recorded the elevation and the other did not but used a tool to insert the elevation into the track. This might be from satellites using radar.

For kicks use the profile tool at gpsvisualizer.com and tell it to get the elevations for the track from someplace other than the track.

Jeff Crawley · Mar 16, 2019

There seems to be a glitch with the first one (rayyrosa). When I imported it into Google Earth it looked like this:

whereas the second one (enderjace) looks like this

I downloaded it again to be sure and got the same results. I then tried importing it into Garmin BaseCamp and got the same results.

GPS problem? Or maybe it's me

peregrina2000 · Mar 16, 2019

Wow, I do not even understand the last two posts. You guys are way way ahead of me. I will download Ender’s tracks, after all he is the man of the Olvidado.

Jeff Crawley · Mar 16, 2019

peregrina2000 said:
Wow, I do not even understand the last two posts. You guys are way way ahead of me. I will download Ender’s tracks, after all he is the man of the Olvidado.

Sorry, didn't mean to confuse

- I think that's the one to go for.

Off to bed now!

islandwalker · Mar 16, 2019

What an interesting puzzle. The idea dougfitz mentions that "Elevation is the least accurate of the dimensions measured by a GPS," fits with what happened to us on our last trip to Spain. We got some wildly inaccurate elevation profiles for Wikiloc tracks we recorded when we were walking in deep canyons - things like lines falling off at 90 degree angles, etc. I'm guessing the satellites can't get a read on accurate elevations when the walker's exact perch on a mountainside is obscured by the steepness of the gorge. The wikiloc summary figures for elevation gain and loss for those days were way off also.

dougfitz · Mar 16, 2019

Jeff Crawley said:
There seems to be a glitch with the first one (rayyrosa). When I imported it into Google Earth it looked like this:

View attachment 53261

whereas the second one (enderjace) looks like this

View attachment 53262

I downloaded it again to be sure and got the same results. I then tried importing it into Garmin BaseCamp and got the same results.

View attachment 53263

GPS problem? Or maybe it's me

Laurie, If there are rogue values in the .gpx file, these can be removed using the standard Garmin desktop tools. I cannot remember which one you use, but both can do it.

Jeff Crawley · Mar 16, 2019

islandwalker said:
What an interesting puzzle. The idea dougfitz mentions that "Elevation is the least accurate of the dimensions measured by a GPS," fits with what happened to us on our last trip to Spain. We got some wildly inaccurate elevation profiles for Wikiloc tracks we recorded when we were walking in deep canyons - things like lines falling off at 90 degree angles, etc. I'm guessing the satellites can't get a read on accurate elevations when the walker's exact perch on a mountainside is obscured by the steepness of the gorge. The wikiloc summary figures for elevation gain and loss for those days were way off also.

There's some chat about elevation on the Wikiloc forum HERE - if you understand it you're better than me!

willydp · Mar 16, 2019

What I can make up of all the explanations and comparisons is that you can not trust Wikilocs to be accurate in elevation.
Last example shows two almost identical and the one from Wikilocs...not correct.
Measuring with a cellphone GPS is not as correct as real GPS equipment.
You need to measure with different equipment at the same moment because GPS satellites are moving all the time.
Buen Camino

Jeff Crawley · Mar 16, 2019

willydp said:
What I can make up of all the explanations and comparisons is that you can not trust Wikilocs to be accurate in elevation.
Last example shows two almost identical and the one from Wikilocs...not correct.
Measuring with a cellphone GPS is not as correct as real GPS equipment.
You need to measure with different equipment at the same moment because GPS satellites are moving all the time.
Buen Camino

You need a signal from 4 satellites to get a 3D fix - if you're in a mountainous or hilly region (which this track is) the GPSr will not be as accurate as say on the Mesata - but then, on the Mesata, who cares?

islandwalker · Mar 16, 2019

Jeff Crawley said:
There's some chat about elevation on the Wikiloc forum HERE - if you understand it you're better than me!

Jeff, thanks for the link to that discussion. Very interesting! Also, your comment about needing 4 satellites to get a 3-D fix explains why our track in the photo below (in the yellow circle) has us starting at 55 meters below sea level and ascending straight up more than a thousand meters! We were in the depths of the Poqueria Gorge and our GPS must have been "hidden" to satellites that were not directly overheard. I've been puzzling over that for awhile and am glad to have an answer.

Jeff Crawley · Mar 16, 2019

islandwalker said:
Jeff, thanks for the link to that discussion. Very interesting! Also, your comment about needing 4 satellites to get a 3-D fix explains why our track in the photo below (in the yellow circle) has us starting at 55 meters below sea level and ascending straight up more than a thousand meters! We were in the depths of the Poqueria Gorge and our GPS must have been "hidden" to satellites that were not directly overheard. I've been puzzling over that for awhile and am glad to have an answer.View attachment 53329

Rocket man!