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Custom terrain coordinates do not align

Posted: Tue Feb 26, 2019 4:12 pm
by Levasco
Hi all,

new user here. I am importing my own DEM for Switzerland (from SwissTopo, coordinates should be 100% accurate). Yet when I draw a polygon in the app, the computed terrain model does quite not line up with what I drew on the map. Somehow the coordinates do not align perfectly. See photos:

https://drive.google.com/file/d/1TiW5XU ... aGSvRVanLI

https://drive.google.com/open?id=1dF6nc ... QWdtOU38cd


Any ideas what might cause this? I tried exporting the .asc file both through ArcMap and R (raster package), same error.

Thanks!

Re: Custom terrain coordinates do not align

Posted: Thu Mar 07, 2019 1:12 pm
by Levasco
Update: reprojecting the same rasters to WGS84 before importing solved the problem for me.

I suspect the issue might be in the CH1903 coordinates of DH maps, so you might want to look into it.

Re: Custom terrain coordinates do not align

Posted: Fri Mar 08, 2019 7:35 am
by Martin
Levasco wrote:
Tue Feb 26, 2019 4:12 pm
Hi all,

new user here. I am importing my own DEM for Switzerland (from SwissTopo, coordinates should be 100% accurate). Yet when I draw a polygon in the app, the computed terrain model does quite not line up with what I drew on the map. Somehow the coordinates do not align perfectly. See photos:

https://drive.google.com/file/d/1TiW5XU ... aGSvRVanLI

https://drive.google.com/open?id=1dF6nc ... QWdtOU38cd

Thanks!
Welcome to the forums! I did not fully follow the "misalignment" from your two pictures, as one is in 3D and one shows the google map with a 2D polygon on top. Did the corners not align exactly?
Levasco wrote:
Thu Mar 07, 2019 1:12 pm
Update: reprojecting the same rasters to WGS84 before importing solved the problem for me.

I suspect the issue might be in the CH1903 coordinates of DH maps, so you might want to look into it.
When you import the Esri ASCII grid DEM file you can select as File Type
  • Esri Ascii Grid WGS-84
  • Esri Ascii Grid Swiss local
Did you try with Swiss local before reprojecting? In the end, it all comes down to knowing how to interpret the data, i.e. what projection was used.

If misalignment still happens with both file types, please let me know.

Re: Custom terrain coordinates do not align

Posted: Mon Mar 11, 2019 7:47 am
by Levasco
Thanks! Indeed, look at the two pictures carefully. Fig 2 is the 3D model we obtain from polygon in Fig1. You can see how the lake that is right in the middle of the Fig 1 when drawing the polygon gets shifted to the side in the 3D model.

As I said, the problem is resolved (what I draw = what I get) when I first re-project the DEM and then import it as wgs84. This, plus the fact it comes from Swisstopo, leads me to believe the problem lies in the Swiss coordinates of your basemaps, rather than in the DEM itself.



(I believe that this is not a projection issue, as the shift is only some dozen meters. A wrong projection would result in the DEM being at a completely wrong location).