map algebra with ascii grid

Roger,

I think that having Arithmetic operations defined for 
SpatialGridDataFrame objects would be very useful.
May be I'm missing something, but the argument that "columns of the data 
frame (here a single column) may contain data of different classes" does 
not seem a valid one to me, as
this is the case for any dataframe object in R while arithmetic 
operations are
defined for them, just yielding the proper results and warnigs,i.e.:

 > d <- data.frame(cbind(x=1, y=1:10), fac=sample(L3, 10, repl=TRUE))

    x  y fac
1  1  1   C
2  1  2   A
3  1  3   A
4  1  4   A
5  1  5   A
6  1  6   A
7  1  7   A
8  1  8   C
9  1  9   A
10 1 10   C

 > d+3

    x  y fac
1  4  4  NA
2  4  5  NA
3  4  6  NA
4  4  7  NA
5  4  8  NA
6  4  9  NA
7  4 10  NA
8  4 11  NA
9  4 12  NA
10 4 13  NA
Warning message:
+ not meaningful for factors in: Ops.factor(left, right)

So I would be most happy with such a feature added to spatial objects.
The "solution" of using other tools (i.e., python, cf. message by 
Philip, but this can also be done with grass and many other GIS) is not 
optimal at all, as we keep having copies of the same information in 
different formats for different tools.

Agus


Roger Bivand escribi?:

On Sat, 14 Jul 2007, Michal Gallay wrote:

Dear R Users,

could you please advise me on doing map algebra with spatial grids? 
It's the first time I am using spatial objects in R. I have imported 
an ascii grid file and wanted to round the values in it or sum with a 
value, but it gives an error message:

require(maptools)
x <- readAsciiGrid(fname="xxxx.asc")
x+3

Error in x + 3 : non-numeric argument to binary operator

round(x,2)

Error in round(x, digits) : Non-numeric argument to mathematical 
function

Arithmetic operations are not defined for SpatialGridDataFrame 
operations, as the columns of the data frame (here a single column) may 
contain data of different classes. Do the operations on the columns 
directly:

names(x)

If there is a column called "band1", then

x$band1 + 3

will print "band1" + 3, and

x$band1 <- x$band1 + 3

will add 3 to the "band1" column. Think what would happen if band1 was 
categorical or logical to see why doing arithmetic directly isn't such 
a good idea.

Hope this helps,

Roger

Thank you for any suggestions.

Best wishes,

Michal