[GRASS-user] Set raster value for specific cell

[Glynn Clements]

Paulo van Breugel wrote:

> You can use r.mapcalc. If  mymap is your raster layer and you want to 
> change the raster cell at coordinates 52deg and 10deg, use
> 
> r.mapcalc "mymap = if(x()==52.0 && y()==10.00, 1, mymap)" --replace

Comparing floating-point values with == is seldom a good idea.
Instead:

r.mapcalc "newmap = if(abs(x()-52.0)<0.001 && abs(y()-10.00)<0.001, 1, oldmap)"

The above assumes that you already know the cell's centre coordinates
to within the desired accuracy.

If you want to set the cell containing a specific point, and be sure
of setting exactly one cell when the point is on the boundary between
cells:

eval `g.region -g`
r.mapcalc "newmap = if(row()-1 == int(($n-10.0)/$nsres) && col()-1 == int((52.0-$w)/$ewres), 1, oldmap)"

This calculates the integer row/column indices for the desired cell
then compares the current row/column to these.

-- 
Glynn Clements <glynn at gclements.plus.com>