[closed] wrong value calculated when placing waypoints beyond sea level?

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GK-AC-Ger

[closed] wrong value calculated when placing waypoints beyond sea level?

Post by GK-AC-Ger » Thu Apr 08, 2021 8:32 am

Hello,

Last month I had to work in an opencast lignite mine in Western Germany. This included taking pictures at some places inside the mine. The placement of the waypoints worked fine and the values of the coordinates were correct, except for some altitude values. For example for one waypoint the altitude is displayed as +108m but the real value hast to be about -330m. I also mapped a track of my route through the mine. But in the dynamic profile the altitude is calculated correctly, as you see in the screenshot. Besides this waypoint I placed some others. I noticed that for the waypoints above -100 the altitude value was displayed almost correctly: -93m at -100m and -14m at -4m. But for waypoints beyond -100m the displayed values were wrong: +85m at -310m and ~-320m and +80m at -110.
Now I wonder why the displayed altitude values beyond -100m are wrong.

Best regards and thanks for your time.
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Psyberia-Support
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Re: wrong value calculated when placing waypoints beyond sea level?

Post by Psyberia-Support » Thu Apr 08, 2021 9:22 am

Hi,
First, how do you track and get the elevation inside the mine?
Then, when the GPS elevation is available, the application will use it. If the GPS elevation is not available, then it will use the DEM elevations (if available). Those elevations are the surface elevations.
When you're underground, you should turn off DEM elevations because they make no sense there, so at least you'll be sure the app will not use the surface elevations.
Then, be sure to place your waypoints at your exact GPS location, so the app will also record the GPS elevation. Otherwise it doesn't know the elevation and will leave it empty.
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GK-AC-Ger

Re: wrong value calculated when placing waypoints beyond sea level?

Post by GK-AC-Ger » Thu Apr 08, 2021 10:17 am

Hello,

thanks for the quick answer.
It's an opencast mine, so you can see it as a pit in the ground - a very vast pit several kilometers (approx 8km x 7,5 km) wide and about 400 meters deep up to 340 m beyond sea level.
This means I've got a free view to the open sky and to the satelites of the GPS. So the elevation should be available the whole time. And as you can see in the track I recorded alpinequest gets this information to create the dynamic profile. But when I placed the waypoint, it took the false elevation value for the altitude of the point if it was beyond -100.
Maybe you're right and alpinequest took the old DEM value of the surface before the mine had been there. But while the GPS elevation was available for the track and the dynamic profile alpinequest took the nevertheless the DEM based values for the stand alone waypoints.
Unfortunately I don't know, if I will work there again. But I'll try to turn off DEM elevation if I'll be there again.
Is there any possibility that the app takes the DEM elevation if the GPS coordinates differ too much from the DEM elevation? (In my case about -400 meters to the former ground.)

Psyberia-Support
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Posts: 4895
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Re: wrong value calculated when placing waypoints beyond sea level?

Post by Psyberia-Support » Tue Apr 27, 2021 1:28 pm

So yes (sorry for the delay), I think this is the explanation. The DEM elevations does not take into account the mine (SRTM data are quite old now, around 20 years) so when you create an item not over the current GPS location, the application does not use the GPS elevation but the DEM elevation, which is the correct behavior but does not work well in your particular case...
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