Aerial photographs that are the subject of this paper are defined as vertical photos, or those that are taken looking straight down from an aircraft. These photos usually require a special camera mounting apparatus as well as sophisticated aiming and triggering mechanisms. Photos taken out of an airplane window or the door of a helicopter are oblique aerials, and they are taken at such a severe angle to the ground that distortions and perspective effects make the resulting images difficult or impossible to use in a reconstruction environment. Interestingly, even the most carefully held "vertical" or banking shot out of a helicopter is still far less accurate than a controlled vertical using a camera mount designed for this purpose.
To take and use meaningful vertical aerial photographs, "targets" must be added to the scene. These markings will assist the pilot, aerial photographer and photo lab technician in producing an accurate print. Targets usually consist of "X" designs, squares, triangles or other geometric shapes to help the pilot find the site and establish the photo image boundaries desired. A distance of 100' between targets is useful, especially when the final photo product will be scaled at 1"=10' on the print. In these cases the darkroom technician simply enlarges the image until the targets are 10" apart; the resulting print will be at ten-scale. Placing these targets with a measuring wheel will generally result in accuracy to within a few inches.
Technical Aspects
The camera mount, mentioned previously, is an important element in establishing a vertical angle to the ground. It also isolates vibrations and allows the camera to be rotated (in the mount) so that proper site orientation may be accommodated. It is probable that wind conditions will force the aircraft to fly over the site on a specific heading. To keep roadways aligned with the rectangular shape of the photo, the camera must be rotated so the street pattern aligns with the rectangular photo frame. This is more of a cosmetic enhancement than a technical requirement, but a photo enlargement aligned to a roadway grid is easier for most people to view and understand.
Some types of aircraft are particularly well suited for aerial photography. Among one of the best is a Cessna 177. This aircraft has a strutless wing for an unobstructed view from the cabin. It also offers an excellent compromise between low speed flight for vertical aerials and respectable cruise speeds.
The heart of this or any aerial system is the camera, its mount and monitoring
equipment. Pictured left is one solution to the problem. This photo shows the cameras
mounted in front of the right seat of the aircraft. Note the rudder pedals in
the background. Two cameras are shown in black on the right side of the photo. One
camera is loaded with film and actually takes the vertical. The second camera sends
its video output to the monitor so that the pilot-photographer can be sure of framing
and timing. Slow flying is a relative term. Putting the aircraft in the right place
at the right altitude with the proper heading takes a concentrated effort. Several
of these precision photo-passes are required to be sure critical evidence is not
obscured by traffic.
Film type, camera format, lens, altitude, scene size and final desired product are several of the interrelated variables that must be considered next. An example of how these factors influence the photographic outcome follows.