Well, I think it wasn't that hard anyway. Here is how I did it. First, you
should calculate the angle of each point with respect to the center of the
circle (the observer's position). Then you can select those points which
reside in the wanted range.
grass.mapcalc("${temp} = if(isnull(${temp1}), null(), if(
atan(x()-${i0},y()-${i1}) < ${low} || atan(x()-${i0},y()-${i1}) > ${high},
null(), ${temp}))",
temp = map,
low = low_angle_range,
high = high_angle_range,
i0 = areawest+position[0],
i1 = areasouth+position[1])
--
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