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I'm trying to reduce the amount of physical field pickup for a feature survey plan by flying the property, tracing over the point-cloud and exporting the linework out as a DXF so I can edit it in my CAD software (12D model, not a huge following for it's drafting ability, very good for survey work though)

I've been able to do the following;

  • Create a custom CRS for the map projection that we use for the bulk of our survey work and what Photoscan also uses
  • Import the georeferenced orthometric photo as a raster layer
  • Import the georeferenced DEM as a raster layer and set transparency to 50% so I can see my "base map" underneath
  • Create and draw lines by Shapefile Layer (even if I don't fully get the fields list at this stage)
  • Export the Shapefile as a DXF, with correct CRS (I know it's right as when I view it on my CAD software it sits exactly as it should, 2D anyway) with Geometry type with several types (LineString/Polygon/GeometryCollection) with "Include z-dimension" ticked

I can import it in fine, 2D sits fine, but there is no height attached. I am, probably naively, thinking that since I'm tracing over the DEM then there should be z-values attached but obviously there isn't.

Is there something I'm missing out on?

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  • I think the step missing between 'create and draw lines...' and 'export...' is that you have to extract z values to that line. It is not automatically done by clicking 'include z-dimension' on export. There has to be a process for writing the elevation into the geometry of the lines. Unfortunately, I don't know how to do that with QGIS at the moment.
    – Tangnar
    Apr 25, 2017 at 20:26
  • Thanks Tangnar, at least that's something I can look into, cheers
    – W.C.A.I
    Apr 26, 2017 at 4:05

1 Answer 1

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I found the way to do this in QGIS. The tool is in GRASS commands, and is called v.drape. I tested it out, and when I view details about the geometry, it the output is a line that has Z values.

I also tested the output by calculating the z geometry into the attribute table, as described here. The calculated z values matched when I identify the raster cells at the same location.

line on a DEM v.drape in the toolbox

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  • Thank you very much Tangnar, I will try this tonight and report back but it looks exactly like what I am after :)
    – W.C.A.I
    Apr 28, 2017 at 3:20

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