[R-sig-Geo] Methodology to compare crop maps

[Rich Shepard]

On Thu, 2 Jun 2011, Thiago Veloso wrote:

> I am working with crops planted area maps from two distinct sources. One
> of the maps is based on a maximum NDVI composition, and the other map uses
> joint information from satellite and census to estimate the planted area.

> After downloading the datasets, I have performed a visual inpection, and
> they show reasonable agreement. However, I need a more robust comparison
> method. Could anybody point out a methodology which allows me to show the
> difference between both maps?

Thiago,

   Please accept my suggestion that you're not using the best tool for this
job in the positive way I intend. This is a spatial analytical question best
answered by an analytical GIS rather than statistical software.

   I've been using GRASS <http://grass.osgeo.org/> for a dozen or so years
and it will quickly answer your question as well as many others. Assuming
that your maps are grid- (or raster-) based a simple subtraction of one from
the other will show differences. You can also ask for differences in other
ways. The r.mapcalc() module is quite powerful. If your source data are in
vector format you can transform them to rasters for more powerful analysis.

HTH,

Rich