Geoprocessing - Desktop GIS: Mapping the Planet with Open Source Tools - page 144

no_defs : defined
-PROJ_UNITS------------------------------------------------
unit : meter

units : meters

meters : 1

To project the DEM, we start GRASS in the target location (in this case
Alaska Albers) and use the r.proj command:

r.proj input=ancc6_dem location=world_lat_lon_nad27 output=ancc6_dem

Notice we specify the source location where the geographic version of
the DEM resides. Now we have the DEM ready to use in our analysis.

Doing the Line-of-Sight Analysis
Now that the data is in order, we can get down to doing some analy-
sis. To use r.los , we need to now where the observer is located in map
coordinates, as well as the maximum distance we want the analysis to
consider. We can also apply a height to the observer if we desire. For
now, we'll do a simple analysis using a point located on a gravel bar in
the river bottom to determine what we can see. To get the coordinates
for the observer (that's us standing on the gravel bar), we just used
QGIS or GRASS to get the location of the mouse cursor in map units.
With that, we can run r.los :

r.los input=ancc6_dem output=los_river coordinate=259315,1307037 max_dist=3000

This gives us a line-of-site analysis extending 3,000 meters from our
location. Depending on your version of GRASS, r.los may set any cell
outside the maximum distance to a value of zero, which means that
when we overlay the results on the DEM or other background layers,
our underlying layers are not visible. 4 Fortunately, there are a couple

of ways to fix this problem.

The GRASS r.mapcalc command allows you to do arithmetic operations
on the cells in raster layers. A full range of operators and functions
is supported. In our case, we want to do something pretty simple—set
the cells outside our analysis area to null values. This will allow our
background layers to show through. To do this, we will create a new
map from the results of r.los . The command is simply as follows:

r.mapcalc 'los_river_nulls=if(los_river==0,null(),los_river)'

As of this writing, GRASS 6.3 release candidate 3 does not have this issue. 4.