Tutorial: Calculate time of concentration (Kirpich)

5. Sample elevation of the first and last node

For the Kirpich equation we also need the elevation difference between the first and the last node of the river.
In this section we're going to sample the elevation at nodes from the DEM.

1.  In the Processing Toolbox choose Raster analysis | Sample raster values

2. In the Sample Raster Values dialogue choose firstlastnode as Input Point Layer, DEM as Raster Layer to sample. Expand the Advanced Parameters section and replace the Output column prefix with Z. Save the Sampled Points to the Kirpich_Data GeoPackage with the layer name firstlastz. Click Run.
Sample raster values dialogue

3. Click Close after processing

4. Click right on the firstlastz layer and choose Open Attribute Layer.

Now we have the distance and the elevation at the first and last node.

Now we can fill in the Kirpich equation. In the next section we're going to develop a PyQGIS script that can do the calculation.