Aerial photographs can be a key piece of information when determining the history of a property. Aerial photographs are one piece of the puzzle, and when combined with historical city directories, Sanborn Fire Insurance Maps, historical topographic maps, and site owner/user interviews, can assist the assessor in determining the previous usages of the property that may contribute to environmental issues. Aerial photograph review is an element of the historical document review specified in the Phase I Environmental Site Assessment standard (ASTM E1527-13). In the standard, they are defined as photographs taken from an aerial platform with sufficient resolution to allow identification of development and activities of areas encompassing the property. They can be used to identify areas of interest both at the Subject Property and at neighboring facilities. Reviewing multiple sources of historical information allow for the researcher to fill in the gaps and make sure they can identify all former uses of the property.
History of Aerial Photography:
Since the introduction of Google Earth in 2001, the layperson is now aware of the existence of aerial photographs, but most people don’t know that aerial photography began with hot air balloons and kites. Some people even used pigeons. (http://professionalaerialphotographers.com/content.aspx?page_id=22&club_id=808138&module_id=158950)
Aerial photography really took off during World War I and was extensively used during the second World War. Here’s a photograph of an early camera used in aerial photograph, the Fairchild F-1 Camera:
During World War 2, the Army Corps of Engineers hid the Lockheed Burbank Aircraft Plant from aerial photography to protect it from Japanese air attack.
Before the cover was put in place . . .
And after the cover was put in place . . .
Here’s what it looked like underneath . . .
Photographs taken at an angle are called oblique photographs. If they are taken from a low angle, they are called low oblique and photographs taken from a high angle are called high or steep oblique. Oblique photographs are purposely taken with an angle between 3 and 90 degrees from the vertical. The advantages of oblique photographs is the ability to see a greater area, resistance to cloud cover obscuring your view, and ability to see beneath trees and other tall objects.
A 1939 photo of the Los Angeles Harbor looking south, with the Union Oil refinery in Wilmington in the foreground from the Robert Spence Collection which is housed at UCLA's Geography Dept's Benjamin and Gladys Thomas Air Photo Archives.
Vertical photographs are taken straight down. They are mainly used in photogrammetry and image interpretation. Pictures that will be used in photogrammetry are traditionally taken with special large format cameras with calibrated and documented geometric properties. Vertical photographs are particularly useful is measuring distances between features and they can be used to produce stereo pairs.
Here's a cartoon depicting the difference between oblique and vertical photography:
The next photo is a 1994 photo of the Los Angeles Air Force Base
As you can see in the cartoon below, a succession of photographs are taken from the airplane which overlap as the plane flies over the region of interest.
The overlapping portion can be used to create 3-D stereo image which shows areas of elevation and depression.
In the past, these images were viewed using a stereoscope but now they can be created digitally and viewed on a computer screen or television using 3-D glasses.
With photo editing software, stereo pair images can be combined into a single color anaglyph that can be viewed through colored glasses. The basic requirements are that they be taken at the same magnification, be the same size, and include as much common area of the specimen as possible. Stereo viewing produces information about the relative depth of features in the image when the viewer’s eyes rotate inward in their sockets to focus on an object.