I just discovered that historical aerial photography is now available for free download from the U.S. Geological Survey. In the interest of fair disclosure, I actually work for this agency (in a different capacity), but was unaware of this development until this evening while doing some online research. The USGS houses the national archive of a wide variety of geospatial information and has been working to get this info out to the public. A subset of the the aerial photography holdings are now available online and I suspect that more will be added in the future.
Now the search link isn't the easiest, but can be found at:
http://www.usgs.gov/pubprod/aerial.htmlSelect :Buy digital format on media: Online (but you don't have to pay)
This should lead you to an interface that lets you select your data type and coordinates
Select the aerial photo tab, and then Aerial Photo Single Frames (you can try other types as well).
Select your area and then hit search
The search results will have thumbnails and a download link for those that are available. Trying to find the best image quality is a bit of trial and error, but the smaller the geographical area in the thumbnail, the better the spatial resolution in the downloaded image.
Note the you will have to register to download images, but it is free and relatively painless.
I got results for my area of Milwaukee around the Schilitz Brewery going back to 1955. Here is subset example of the best resolution image that I could find from 23 August 1970. This is a very exciting result for me as I can see which buildings were around, how rail cars were spotted, etc... Unfortunately this was a Sunday so things weren't as busy as they would have been on other days. On the other hand if I choose to model this day I won't have to buy as many cars to fill the parking lots!
I hope that this resource is helpful to people....I am so excited I may not sleep tonight (man I need a life). I can try to help with questions if people are having trouble with this search.
Best wishes, Dave