Mapping Hacks

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Chapter 6, Mapping on Your Desktop

#74 Plot Wireless Network Viewsheds with GRASS

HACK

In reality, of course, you can use any base map you like. If we want to dump

this map to a graphic file, so that we can share the good news with our

friends, we can export this set of layers to a PNG file

[Hack #75].

Caveats

Using GRASS to analyze line of sight for wireless networks still comes with a

few caveats. First, you may have noticed that we didn’t specify a receiver

height; that’s because r.los doesn’t allow us to do so, so we’re stuck with a

map that shows us line of sight to a receiver at ground level. In this particu-

lar case, it won’t make much difference, but one can imagine circumstances

where it might. Second, r.los is abysmally slow for anything but the shortest

distance plots, and even passing it a pattern map doesn’t really speed things

up much. One alternative that purports to solve these issues is r.cva,a

“cumulative viewshed analysis” tool that ships separately from GRASS and

must be built against a compiled GRASS source tree. The r.cva tool runs

faster and allows you to specify receiver height, but the calling conventions

to r.cva are a bit more complicated, so we’ll leave its use as an exercise for

the reader. You can get the source for r.cva from http://www.ucl.ac.uk/

~tcrnmar/GIS/r.cva.html.

Figure 6-44. Shaded line of sight