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QGIS From DTM / DSM to 3D Topography
This tutorial will illustrate the process of importing and transforming Digital Terrain Models (DTM) or Digital Surface Models (DSM) into a .xyz or .csv file for further manipulation. DTMs and DSMs are commonly provided by municipalities as base data for planning tasks. However they usually come in the format GeoTIFF, a geo-located picture format, that is incompatible with most CAD programs. To transform the file into a usable three-dimensional topography model, several transformation steps have to be undertaken in QGIS. These steps will be illustrated in the following.
Content
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- Importing DTM / DSM to QGIS
- Merging Raster Layers
- Correcting the Raster Values
- Clipping the Raster Layer
- From Raster to Point
- Exporting Points as CSV
- Final Adjustments
Importing DTM or DSM to QGIS
Download a set of GeoTIFFs from a website of your choice. This tutorial will focus on the Klopstockwiese in Zurich. (Zurich GIS) Since larger datasets are usually divided into smaller tiles the download of the selected area as shown in the picture below will likely contain several tiles. In this case your download will include four .tif files.
After downloading the different tiles they have to be unpacked and afterwards combined into one consistent raster file. For this step we are only interested in the .tif file from the download folder. Drag and drop those files into your QGIS workspace to load them. They are currently not connected and have their own colour range indication the height of that raster point. The breaks between the different sets are clearly visible.
Merging Raster Layers
To combine the different tiles go to Raster > Miscellaneous > Merge
Select the raster sets you would like to combine by clicking on the three dots in the top right corner (see image below).
In this case you can select all of them. Press “Run” after selecting the raster sets.
Correcting the Raster Values
After merging you might notice that the values of the new file are not the same as in the original files (compare the maximum and minimum raster values). This inconvenience can be solved by adjusting the layer properties. Select the merged raster layer, right-click on it and navigate to “Layer Properties”.
Within the properties window navigate to “Symbology” and expand the “Min / Max Value Settings” tab by clicking on it (see image below).
Within the Min / Max Value Settings change the accuracy from “Estimate (faster)” to “Actual (slower)”. Click apply and close the properties window. Now the values should the same as your input layers. This is a purely cosmetic adjustment. The true values are still saved in the raster - QGIS just estimates them for display purposes.
Clipping the Raster Layer
To limit the raster layer to the area we are interested in select the merged raster layer and go to Raster > Extraction > Clip Raster by Extend. Make sure the merged raster layer is selected at the top. For the field “Lcuping Extend” choose “Draw on Canvas”. Alternatively you could also use a preset rectangular shapefile as a clipping boundary. Click “Run” and close the window.
After disabling the merged layer the clipped extend might look somewhat like this:
(Please make sure to chose a rather small extend - the bigger the extend the more computational power is needed in the next steps)
From Raster to Point
To turn our raster data into a point set which we can import and open in Rhinoceros 3D we have to use the Processing Toolbox. To open the toolbox panel go to Processing > Toolbox. The toolbox offers a wide range of geoprocessing tools and has a surprisingly good search function.
The tool we need is called “Raster values to points”
Within the tool click on the three dots in the to right to select the desired raster layer for the transformation.
Select the “Clipped (extent)” layer and press okay. (If you have renamed the layer your layer setup might look a bit different)
For the “type” field choose “(0) nodes” and press “Run. The following calculation might take a while. The result of this calcualtion should be a layer called “Shapes”, containing points.
Right-click on the layer “Shapes” and select “Attribute Table” to have a look at the data each point contains. The table should look as following:
Exporting Points as CSV
Now we are ready for exporting.
Navigate to the “Shapes” layer, right-click and select “Export” > “Save Features As”. Within the exporting panel perform the following actions:
- Select CSV (Comma Separated Values) as a file format.
- Select a place to save the file and give it a name
- Untick the box “ID”.
Click “OK” to start the export.
Final Adjustments
If you open the exported file in a text editor it will look somewhat like the screenshor below. To make this code readable for the Docofossor plugin we need to delete the first line (marked in the screenshot) and save the file again. If your file is too heavy to open in a text editor it is likely to be too heavy for any of the next steps. Crop the raster extend a bit more and try again.
