Extracting Raster Values from Points in R and GRASS
A common task in GIS analysis is to extract the value of a remotely
sensed environmental variable at a point location. For instance we
may wish to extract the elevation of a field plot from a digital
elevation model. The elevation data is a raster (i.e. grid) and the
plots are a point shapefile (or a simple text file of X, Y
locations). The command for doing this in ArcGIS is
ExtractValuesToPoints available in the Spatial Analyst package.
Situations may arise where ArcGIS is not the most efficient way of
extracting these values. So, here, I provide a brief overview of how to
extract raster values to points in R and GRASS.
Extract Values to Points in R
This is strikingly easy is R. My work usually requires more statistical sophistication than is available in ArcGIS. As a result, I have completely switched to doing the extraction in R. I known I am going to end in R eventually, and it is easier to automate than writing a long python script in ArcGIS.
That is it. Fast, and easy.
Extracting Values in GRASS
Extracting raster values in GRASS is somewhat faster than in R, but it takes a little bit more planning in that you have to explicitly create the column that the raster values will go into.
gr: A GRASS grid
pt: A GRASS point dataset
The basic flow of this is that you create an empty column in the
point dataset with the right data type (i.e.
of length 10,
double precision floating point numbers,
integers). Then, fill the column with the raster values.