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Monday, July 26, 2010

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.

Data required

For the purpose of this exercise. All the data must be have the same spatial projection.

gr.asc

an ESRI ASCII grid. This could also be an ArcGIS binary grid if you know how to use RGDAL. That perhaps will be another post.

pt.shp

a point shapefile.

You also need the maptools and sp packages in R.

The Code

library(maptools) # autoloads sp gr <- readAsciiGrid("www.unc.edu/~toddjobe/data/gr.asc") pt <- readShapePoints("www.unc.edu/~toddjobe/ndata/pt.shp") overlay(gr, pt)

           coordinates     gr.asc
1497 (569292, 1224170)  6094.6080
539  (567718, 1227840)  7964.5331
1023 (564565, 1225810)  -293.6599
663  (562462, 1227260) -5351.7297
69   (563675, 1229710) -2923.9394
716  (563062, 1227180) -4241.8255
339  (567636, 1228780)  4781.6488
509  (561722, 1227870) -3958.7312
805  (560981, 1226690)   139.9091
155  (560884, 1229220) -2719.7353

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.

Data Required

• gr : A GRASS grid
• pt : A GRASS point dataset

The Code

The basic flow of this is that you create an empty column in the point dataset with the right data type (i.e. varchar(10) string of length 10, double precision floating point numbers, int integers). Then, fill the column with the raster values.

v.db.addcol map=pt columns="grval double precision"
v.what.rast vector=pt raster=gr column=grval