2.5 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 Unfortunately, freely available versions of DOQs tend to be out of date (frequently 8 to 10 years old), and recent DOQs are not only expensive but often aren't even available.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.