Finding Aerial Photography for Orienteering Mapping

Finding aerial photography in the United States is a process that relies upon equal parts luck and hard work. But first you need to know what you're supposed to be looking for. Here's some qualifications that photos for basemapping should meet:

1. Negatives are 9-inch (23 cm) square format, 78 to 155 mm focal length (3 to 6 inches, 6 inches is standard).
2. Diapositives (these are positive prints on FILM, contact-printed from the original negatives). Enlargements or paper prints will not work.
3. Standard parameters for STEREO MAPPING (60% end-lap, 20% side-lap, vertical). If the negatives are color, get color diapositives, else black and white is fine, even preferable.
4. Photographs should be taken at a time of year when the ground is most visible through the trees. For most areas, this is late winter/early spring before the leaves have begun to emerge (even swelling buds can cut down on the ability to see the ground).
5. The photography should be a recent as possible.
6. The scale of the photography should roughly match the scale of the finished mapping, +/- 50%. For typical 1:15,000 orienteering mapping anything between 1:10,000 and 1:20,000 is OK. Note that most American aerial mapping firms refer to their photo scale in feet per inch, e.g. "1000 scale" would likely really mean 1:12,000.

Now, how do you find the best photography of the area you're interested in mapping? While the USGS does have photography covering the entire country, it is mostly pretty dated, and usually at too small a scale to be really desirable. Other government agencies like the Soil Conservation Service usually like to see the vegetation with leaves on, so they're not generally very useful, at least for eastern terrain. Nine times out of ten, you'll find the best photography with a private aerial mapping company. So how do you find it? A very useful resource that is virtually free is something called the Aerial Photography Summary Records System (ASPRS). It is available from USGS (1-800-USA-MAPS), and you can order microfiche at $2 per state. You can also get the entire country on CD-ROM for $50 (last time I checked). It's a voluntary system where all owners of aerial photography are invited to list their photography. It's updated about once a year, so it's reasonably up-to-date. In addition to the actual photography listings, you also get some other useful information, including a list of all the contributors with their phone and fax numbers.

Once you've identified a potential source, it's time to rev up the fax machine. Identify yourself as a member of a tax-exempt organization (if that applies). Include in your letter a list of the photography requirements (as above) and a copy of the portion of USGS map outlining the EXACT area that you are interested in acquiring coverage of. Make sure you write somewhere the name of the USGS quad. Finally, you ask that if they do not have coverage to please suggest a company who they think might have coverage of this area.

That last part is the key to this process. You could very possibly strike out with a half dozen or more companies, but if they can keep suggesting new ones to try, then the search is still alive. The aerial mapping business, is a pretty tight-knit community, and they usually do not hesitate to send business to one-another as long as it's not cutting into their own business. They can often provide contact persons to ask for, and that can save a lot of time.

Typical prices for black and white diapositives run around $25-30 each, but it's extremely variable. Some outright refuse to sell aerial photography, others insist that you pay a "library fee". This sort of behavior has become less common over the past few years. Make sure that you explain that you are making an orienteering map, and not using the photography for engineering purposes! Most aerial photography is performed as a sideline to businesses whose primary interest is engineering mapping. They don't generally make money on the photography per-se, instead they make their money by mapping. So, you might have to overcome this natural defensiveness. If you're lucky, they will have dealt with orienteers before, and be relaxed about it. So... when you get your bill - PAY IT!