[Qgis-user] Wrong values if DEM in grid format is reprojected

[Nicolas Cadieux]

Hi, could be a bug or it could be simply the tool you are using to get the values.  How are you getting the values?

Below, you are comparing 2 things. A) project  reprojections on the fly vs b) file reprojecting.

Project on the fly reprojection  works for the screen but is not the best way to get the values.

To make sure you have no mistakes using tools like the value tool, make sure the project is in the same CRS as the raster layer.  Then, the tools and values should be good.

If you use gdal warp, remember that all reprojection lead to changes in raster values so some change will happen.  That's why you can use various algorithms to do that. 

To see if you have a bug with the file format,  save the Esri grid in .tif using  UTM31N on ETRS89 datum (EPSG:25831). 

Then change the project CRS  and reproject on the fly and see if you have the same problem as above.

If you have the same problem, the problem is in the "on the fly reprojection" and how the tools works with it.  If you don't have the same problem, then the only thing that changed is the file format therefore you have a bug.

Good luck.

Nicolas Cadieux M.Sc.
Les Entreprises Archéotec inc. 
8548, rue Saint-Denis Montréal H2P 2H2
Téléphone: 514.381.5112  Fax: 514.381.4995

Le 2014-12-09 05:10, "Agustin Lobo [via OSGeo.org]" <ml-node+s1560n5176993h44 at n6.nabble.com> a écrit :
>
> I'm using a DEM in esri grid format 
> (see http://www.icc.cat/eng/Home-ICC/Geoinformacio-digital/Sobre-la-geoinformacio-ICGC/Models-Digitals-d-Elevacions) 
> which consisits on a txt file with the data and an xml file with the metadata. 
> The projection is UTM31N on ETRS89 datum (EPSG:25831) 
>
> If I display the layer on a project with the same EPSG:25831 there is 
> no problem, 
> but if I set the project projection to Geographic coordinates in WGS84 
> (EPSG:4326), 
> while the grey shades look coorrect and the coordinates are correct, the 
> actual values are totally wrong: values such as 7.999140e-136. 
>
> Is this a general problem with this format? Is there any fix? Is this 
> a bug to be reported? 
>
> Reprojecting to a gtiff using Raster/Projections/Warp works fine. 
>
> Thanks 
>
> Agus 
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