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I created a contour map layer on top of an ortophoto map using QGIS, and noticed that the two layers do not overlap perfectly and wonder if this is normal. What happens, is that the contour lines seem to be slighthly displaced to one side of the map with one side having the contour lines going off the map by some 4m. I created the map using the command Raster->Extracting->contours

If the way this layer was created is not correct how can I make the two layers coincide/overlap exactly?

Below is how they look like The first picture shows the side where the contours fall short of reaching the end of the ortophoto:

enter image description here

The second shows the opposite side where they fall outside:

enter image description here

Any help suggestion welcome.

Sinergy
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  • do the orthophotos and the DEM come from the same source (stereo images)? – radouxju May 29 '17 at 11:42
  • yes (the DEM is actually a Lidar image) but don't understand what do you mean by stereo images. – Sinergy May 29 '17 at 11:49
  • you can derive a DEM and an orthophoto from two overlapping images. In this case, there is a perfect match between ortho and DEM. If your DEM comes from LIDAR, it is another source of information than the aerial photo. Therefore there could be a mismatch due to processing issue of the orthos (residual parallax shift). So my newt question is: was your ortho created based on your LIDAR of from another DSM? – radouxju May 29 '17 at 13:04
  • To be honest, I wouldn't be able to tell. I took the ortho and the DEM-lidar from our national map service, and wasn't able to find out (as per your question) what you ask. May be I should ask them, but it will take a while before thet reply to me I am afraid. I wonder if I just move the contours so that they just fit the underlying ortophoto or is there any chance that the actual contours aren't well formed? – Sinergy May 29 '17 at 14:54
  • I double checked the products on offer on our national map service and the only product they offer for the coordinates in question with a DEM is a DEM-Lidar type, so not much of a choice a this time. – Sinergy May 30 '17 at 09:16

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