## Tutorial: Spatial Analysis of Accessibility of Wells using Map Algebra

### 5. Condition 3: Wells Less than 40 Meters Deep

For the last condition we need to identify the wells that are less than 40 m deep.

The gwlevel layer gives the absolute elevation of the groundwater level in the well in meters above sea level. In order to calculate the depth to the groundwater, we need to subtract this from the surface elevation given in the digital terrain model (DTM).

1. Open the Raster Calculator.

2. Subtract the absolute well depth from the DTM using this calculation: dtm@1 - gwlevel@1. Call the output layer welldepth.tif.

3. Style the welldepth layer with the appropriate renderer for this raster type.

4. Next, calculate in the Raster Calculator a Boolean map with wells less than 40 m deep. Call the output layer notdeep.tif.

5. Style the notdeep layer.