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Im trying to create topographic surveys using georeferenced images from a drone and a DEM/orthomosaic which has been created using Photoscan. I've managed to create contours and label them with elevations but for the purposes of architectural design i need to show height levels at individual points which im hoping to chose myself. Below is a screenshot of my DEM, a screenshot of what i've managed to do so far and then an example of what im looking to achieve. Im fairly new to QGIS so my skills are limited (to say the least!) but can anyone tell me if this is possible using QGIS? I'd like to use open source or freeware software on Mac as i don't earn any money from the topos that i do. Finally i'd like to export the product as a .DXF for import into AutoCAD.

DEM Contours Topo Example

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I would do it the following way:

  1. create a new point feature (Layer -> New Shapefile Feature) with an attribute for the elevation (Type: Decimal number). Make sure your create the point feature in the same coordinate system as your DEM.

    new point feature

  2. set the points where you want them (Toggle Editing)

  3. install the plugin Point sampling tool (Plugins -> Manage and Install Plugins...)

  4. run the Sampling tool (Plugins -> Analyses -> Point sampling tool) sampling tool. Select your manual set of points (eg. elevations_points) and the layer where to get the values from (eg. DEM). The Output point vector layer will contain your elevation values. At the tab Field you can see the band. If you generate an empty attribute table, doublecheck the coordinate system of your DEM and Point layer. They have to be the same!

    point sampling tool settings enter image description here

  5. Check your attribute table from new created file (eg. elevation_points_with_elevation.shp) display the labels (and or export the file as DXF)

    example attribute table DEMelevation point example

Hope that helps.

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  • Thanks for your advice. I've followed the steps and managed to insert the points however i have to manually enter the elevations into the labels. Is there a way i can use the elevations at each point automatically? The DEM is a geotiff and should have all the elevation information embedded. Additionally (and im not sure this is possible) i'd like to set the lowest point as 0 and then all other points will be in reference to that point in +metres. – Jamie Jan 10 '16 at 15:15
  • Yes the plugin Point sampling tool does that automatically. In the tab Fields you chose which band/value is used. The output field contains the elevations values. By the way with this method you don't need to create the attribute elevation as I said before. – wittich Jan 10 '16 at 17:05
  • I added some more details. If you want to correct the elevation relatively, use the Field Calculator and correct the values in a new column by addition or subtraction. – wittich Jan 10 '16 at 17:20
  • Not sure where i'm going wrong but i've uploaded my steps as an image to this link :i1282.photobucket.com/albums/a523/paime/…. I've also uploaded the DEM to dropbox (dropbox.com/s/dfa4p0f7lg0ak5r/DEM32-DroneMapper.tif?dl=0) in case you'd like to have a look at it. When i finish all the steps the labels don't show on the points i've created and the attributes table is empty. – Jamie Jan 10 '16 at 18:58
  • Hmm you selected in the Point Sampling Tool both, your point feature as well as the DEM. Try it only by selecting the DEM... (see i.stack.imgur.com/dHUtY.png) – wittich Jan 10 '16 at 19:08

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