[GRASS-user] query values within different angles

[Hamish]

Tim wrote:
> I need to query raster values around a city for different
> angles. The result I am aiming at should be the amount of
> landcover surrounding the city within 10 degrees angles and
> different distances to the city.
> 
> So far I tried r.buffer and v.buffer but that are not
> providing what I am lookin for. Using r.mapcalc and
> addressing each pixel separately would be an option but I
> hope that GRASS combined with perhaps PGSQL and Postgis
> functions provides an easier way to achieve that.
> 
> I would highly appreciate any hints how to derive buffers
> for specific angles only.

Hi,

in years past I had a version of r.los for GRASS 5.0 where instead
of reporting the vertical angle to the source point I had it
report horizontal compass heading. I'm sure it's on an old hard
drive somewhere, but in hindsight using r.mapcalc and some simple
trig would be a heck of a lot easier and more efficient than
anything else. No other tools are needed beyond that one amazing
module.


#spearfish
g.region -d
g.region -c
  center easting:  599490.000000
  center northing: 4920855.000000

Ce=599490
Cn=4920855
 
r.mapcalc "angle_to_pt.cartesian = atan(x() - $Ce, y() - $Cn)"
r.mapcalc "angle_to_pt.compass.1 = 90 - angle_to_pt.cartesian"
r.mapcalc "angle_to_pt.compass = if(angle_to_pt.compass.1 < 0, \
   angle_to_pt.compass.1 + 360, angle_to_pt.compass.1)"
g.remove angle_to_pt.compass.1


MIN_ANGLE=0
MAX_ANGLE=10

r.mapcalc "MASK = if(angle_to_pt.compass >= $MIN_ANGLE \
   && angle_to_pt.compass < $MAX_ANGLE, 1, null() )"

d.redraw




Hamish