On 14/07/08 15:14, Glynn Clements wrote:
Moritz Lennert wrote:
Well, this works:
r.mapcalc brol3=if(y()<=9521600 && x()<=427000,1,null())
but this doesn't:
r.mapcalc brol3=if(y()==9521600 && x()==427000,1,null())
which raises the issue of precision of the coordinates...
What are the region settings? Bear in mind that x() and y() return the
coordinates of the cell's centre, not a corner.
Just for completeness: It is a cell map, but resolution was aligned on
bounds, so something like 1.9876 and 2.0045
When I set resolution to 2 and align bounds to resolution and then use
r.mapcalc brol3=if(y()==9521601 && x()==427001,1,null())
it works
whereas
r.mapcalc brol3=if(y()==9521600 && x()==427000,1,null())
doesn't. So clearly an issue of falling right on the center of the pixel.
Also note that the calculation involves multiplying by the resolution.
You mean internally by mapcalc, or that I should include resolution into
the calculation ?
If the resolution cannot be exactly represented as both a binary
fraction and a decimal fraction, you will get rounding error.
That and/or hitting the cell center was probably the problem.
Thanks for your explanations.
Moritz
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