1

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

Community
  • 1
Jamie
  • 21
  • 1
  • 5

1 Answers1

3

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.

wittich
  • 2,356
  • 1
  • 16
  • 30
  • 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 :http://i1282.photobucket.com/albums/a523/paime/Topo%20explanation%20of%20steps_zpsnf28qgbg.jpg. I've also uploaded the DEM to dropbox (https://www.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 http://i.stack.imgur.com/dHUtY.png) – wittich Jan 10 '16 at 19:08
  • Still nothing coming through with the labels or in the attributes table :( – Jamie Jan 10 '16 at 19:24
  • Okay I tried with your DEM and run into the same issue. The problem is as so often with GIS that the DEM and the Point Layer need the same coordinate system! In your case WGS84/UTM 30N (EPSG:32630). So when you create the Point Layer, make sure you give it the same coordinate system. – wittich Jan 10 '16 at 21:39
  • Still no joy with the labels or attributes table. I changed the new vector layer's CRS to match the DEM's layer when i first created it and didn't add another attribute. Is there something im doing wrong with the point sampling? – Jamie Jan 10 '16 at 22:51
  • your DEM is EPSG:32630, your Point Feature is EPSG:32630? Maye chose also as project system (down in the right corner of QGIS) EPSG:32630... it worked for me! With your data! – wittich Jan 10 '16 at 22:57
  • The project system was still in the former CRS so i've started the project from scratch with the project system now in 32630 and still no joy. I can get the IDs to display but can't get any elevations to display, it's almost as if it's not finding any data in the DEM. – Jamie Jan 10 '16 at 23:06
  • Double check the metadata from the DEM and the point feature. Right click Layer Properties -> Metadata here it has to be under Properties / Extents / Layer Spatial Reference System the following parameter: +proj=utm +zone=30 +datum=WGS84 +units=m +no_defs... If that is okay I have no clue. Did you load my test files and compared them? – wittich Jan 10 '16 at 23:10
  • Check my Point Sampling Tool settings for your data: http://i.imgur.com/vLnu7kS.png – wittich Jan 10 '16 at 23:13
  • We seem to be matching up on the metadata and the Point Sampling Tool info. I downloaded your data and it works a treat, that's exactly what im trying to achieve! Are you adding the points but toggling edit and then clicking add feature? Here is the link to my shapefiles: https://www.dropbox.com/sh/kwygxhvg9irrkz6/AAAkZ-7ZqGjvNdU6jdSNEjlTa?dl=0 – Jamie Jan 10 '16 at 23:17
  • I'll check it tomorrow – wittich Jan 10 '16 at 23:42
  • Okay the example file you uploaded have no points inside. That s way the sample tool can't work! Maybe you should check again how to create a feature layer with points. Make sure that you finish the toggle edit after you added the points. Then before you execute the Point Sampling Tool make sure that in you QGIS project only 2 files are open, your DEM and your "new vector layer" file (with points!)... now run the tool! I just did it again without any trouble! – wittich Jan 11 '16 at 12:26
  • could you make it working? – wittich Jan 12 '16 at 09:08
  • It works! Thank you so much for help, it is hugely appreciated! – Jamie Jan 13 '16 at 11:41