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I am having an issue with the CRS of a given Aster DEM. I have downloaded a Aster DEM from www.oziexplorer3.com which comes with the EPSG:4326 - WGS 84 as the CRS.

This is a DEM from Gabon, and all the other data I have for the project I am working on has the CRS defined as EPSG:32632 - WGS 84 / UTM zone 32N.

I have to create some contours from the DEM in order to use them in another software for the elevation definition of a water network. When I open the DEM and SHPs I have in QGis they perfectly overlap (although they have different CRS) and so do the contours I extract from the DEM.

But then, when I import the contours into the other software, because they have different CRS, they do not overlap and so I can't use the newly created contours in order to define the water network's elevations... and if I change the CRS in QGis (for the DEM or for the contours) they do not overlap even in QGIS...

Is there a solution for this problem?

PS - please know that I am quite inexperienced with GIS softwares, but you might already noticed that...

  • Is on-the-fly transformation enabled in QGIS? You can check that in Settings->options->crs – neogeomat Jun 23 '14 at 8:27
  • Yes it is on... I guess this is why I can see the overlapped shp and DEM even though they have not the same CRS... – Rui Prata Jun 23 '14 at 8:44
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If you are going to do calculations I would go to the trouble of reprojecting your data. Since the majority of your data are in EPSG:32632, I would use the Raster->Projections->Warp tool to create a new version of your DEM in EPSG:32632. The tool is built on the GDAL tool of the same name whose documentation is here. Don't let the documentation put you off as the plugin GUI is fairly self explanatory.

  • Ok, thank you very much for your reply, i'll try to do as you tell me... – Rui Prata Jun 23 '14 at 8:45
  • Dear Mappagnosis I have used the Warp tool and it worked perfectly. I just have one question, do you recommend any specific resampling method? – Rui Prata Jun 23 '14 at 9:32
  • Your choice can depend on what your data represent as well on the speed/quality of processing you require. there are a number of discussion on that question on this board (e.g. here) – MappaGnosis Jun 23 '14 at 9:40
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I guess your mistake is in and if I change the CRS in QGis.

If you do this with Set CRS for Layer, your data gets corrupt. You change the CRS, but the coordiantes won't get reprojected.

To do it right, set the CRS back to the original value, then rightclick -> Save As ... to another file name, and the target CRS. If you check add result to canvas it will be placed same as the original. But when you turn `On -the-fly-reprojection OFF, the reprojected layer should still stay in the right place.

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