Mapping Hacks

Why You Can’t Watch Broadcast TV #18

Chapter 2, Mapping Your Neighborhood

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HACK

splat.lrp. If you’re feeling particular, you can make a copy of splat.lrp with

the same name as the .qth file—in this case, kqed.lrp—and update the trans-

mission frequency. SPLAT! will look for this file and use it instead, if it

exists.

Drawing Maps with SPLAT!

Now, let’s make some maps! The following command tells SPLAT! to make

a coverage, or viewshed, map for the transmitter listed in kqed.qth, given

receivers with antennas 20 feet above ground level, which is probably a rea-

sonable height for a television antenna mounted on the roof of a house. The

map will be output in Portable Pixmap (PPM) format to a file called kqed.

ppm.

$ splat -t kqed -c 20.0 -o kqed.ppm

SPLAT! should report loading the .sdf files we created earlier and then

announce that it will compute line-of-site coverage for the given location.

Finally, after a great deal of computation, it should spit out the requested

map file. Most X11 graphics viewers can read PPM files with no trouble, but

the format itself is uncompressed, and the maps that SPLAT! produces can

be quite large. In the case of the KQED example, the generated PPM was

3600 pixels on a side and weighed in at over 38 megabytes! We can use the

fantastic netpbm tools that ship with most Linux distributions to massage

this file into something a little more manageable, like a PNG file:

$ ...