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The calculator in the raster operations tools can be used to perform many different queries from a simple elevation query, to an NDVI query for multi-spectral satellite imagery. To see an example of the later, check out this how to video where a vegetation layer is extracted from satellite imagery.In this example we look at how to find all of the lowland areas across the UK in a single query.In this case all areas need to be defined between 0 and 10 meters above mean sea level.
1) Open table GB OST50 UK CLIP that can be downloaded from Exercise 1 if you have not already downloaded it.
2) Navigate to the Raster tab. Go to Raster Tools > Calculator.
3) Select the Expressions drop down, there are number of predefined expressions created for use. In this case the query needs to find all values from 0 to 20 meters. Consider if you want the result to have values including 0 and 20 or including 0 and 20.
4) Example Expressions:
5) Define an Expression as above. In the Variables tab click New, this will add a second variable allowing both 0 and 20 to be input as variables.
6) Click Compute. This process should take less than 30 seconds. The new grid will be opened in to the map window. Run the second variable option. Note the differences in results. One grid has the original values and one has been classified.This exercise contains OS data © Crown copyright and database right 2016. OS OpenData is free to use under the Open Government Licence (OGL). OS Terrain 50 user guide and technical specifications can be found here.