Topographical Mapping 101
As you
roll out wireless equipment, you'll find yourself
looking at your environment in a different way. Air conditioning
ducts, pipes, microwave ovens, power lines, and other sources of
nastiness start leaping into the foreground as you walk around. By
the time you've set up a couple of nodes, you will
most likely be familiar with every source of noise or reflection in
the area you're trying to cover. But what if you
want to extend your range, as in a several mile point-to-point link?
Is there a better way to survey the outlying environment other than
walking the entire route of your link? Maybe.
Topographical surveys have been made (and are constantly being
revised) by the USGS in every region of the United
States. Topo (short for topographical) maps are
available on both paper and CD-ROM from a variety of sources. If you
want to know how the land lies between two points, the USGS topos are
a good starting point.
The paper topo maps are a great resource for getting an overview of
the surrounding terrain in your local area. You can use a ruler to
quickly gauge the approximate distance between two points, and to
determine whether there are any obvious obstructions in the path.
While they're a great place to start assessing a
long link, topographical maps don't provide some
critical information: namely, tree and building data. The land may
appear to cooperate on paper, but if there's a
forest or several tall buildings between your two points,
there's not much hope for a direct shot.
The USGS also provides DOQs (or Digital Orthophoto
Quadrangles) of actual aerial photography. Unfortunately, freely
available versions of DOQs tend to be out of date (frequently 8 to 10
years old), and recent DOQs are not only expensive but often
aren't even available. If you absolutely must have
the latest aerial photographs of your local area, the USGS will let
you download them for $30 per order and $7.50-$15 per file. You will
probably find it cheaper and easier to make an initial estimate with
topo maps and then simply go out and try the link.
Interestingly enough, MapQuest
(http://www.mapquest.com/) has recently started
providing color aerial photos (in addition to their regular street
maps) from GlobeXplorer
(http://www.globexplorer.com/). While
there's little indication as to how recent their
data is, it may be a good place to get a quick (and free) aerial
overview of your local area. Another popular software package is
EarthViewer3D
by Keyhole Software (http://www.earthviewer.com/). It incorporates
satellite image data, aerial photography, GIS data, and business
databases into an interactive overview map.
You can buy paper maps from most camping supply stores, or browse
them online for free at http://www.topozone.com/. If
you're interested in DOQs, go to the
USGS directly at http://earthexplorer.usgs.gov/.
We'll take a look at some nifty things you can do
with topo maps on CD-ROM and your GPS in Chapter 6.
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