Practical Manual - Malareo

Active Layers in the TOC
Mathematical Operators
Expression Area
In order to perform a calculation we need to setup the calculation as an expression. This is done in the
Raster Calculation Expression Area. To build this expression we use the layers from the Raster Bands box
and the mathematical operators.
We are going to answer the question: "Where in eThekwini is the elevation higher than 500 meters"
The raster calculation expression for this is: "elevation_UTM" > 500
If we had wanted to include the 500m height, we would have used the >= sign.
To start the expression by selecting the layer. To do this double click the elevation_UTM layer in
the Raster Bands box. This will add it to the expression box.
Then single click the "greater than" operator (>)
Then type in 500.
You will notice a note in the bottom left hand corner of the Raster Calculator. This note tells you
whether or not your expression is valid.
Make sure you give the Output layer a name and check the Add Result to Project box.
Select GeoTIFF as the output format.
Display the resulting image as a
Psuedocolor map.
The areas in Red are those that are
higher than 500m.
QGIS for Malaria Page 55