map algebra with ascii grid

Roger,

I think that having Arithmetic operations defined for 
SpatialGridDataFrame objects would be very useful.

The problem is that SpatialSomethingDataFrame objects don't inherit 
data.frame's behaviour (with respect to mathematical operators). Perhaps 
if R's OO implementation was a bit more conventional you'd get this for 
free by subclassing data.frame.

  But data.frame isn't subclassable properly. A SpatialPointsDataFrame 
is an S4 object with a data frame in the @data member. In order to 
implement data.frame-like behaviour, it is necessary to implement 
methods for SpatialPointsDataFrame that work on the @data object - for 
example, dim:

dim.SpatialPointsDataFrame = function(x) dim(x at data)

  It should be almost as equally trivial to implement the arithmetic 
operators - they would work on the @data member and leave the 
coordinates untouched. Although what happens to the coordinates if you 
add two spatial data frames with different coordinates (but the same 
shape of data frame)? I would be tempted to either make sdf1 + sdf2 
return a plain data frame - and if the user wants to slap coordinates on 
it they can take it from sdf1 or sdf2 - or always use the coords from 
the first argument.

Barry