Air photo interpretation and photo-grammetry are two of the most effective and versatile tools available to the petroleum industry. The techniques have proven successful in providing solutions to many of the problems, encountered in the design and construction of oilfield projects.
While most technical personnel are aware that air photo interpretation and photogrammetry are useful in the selection of routes for roads and pipelines, few realize the true scope of the techniques or the wide variety of areas in which they are applicable.
This paper briefly describes the principles involved, and reviews a number Of typical applications of particularinterest to petroleum companies. Excessive detail is avoided, rather the emphasis is placed upon the wide spectrum of possible applications. It is stressed that on the ground examination generally is a necessary adjunct, and that there are certain limitations to the techniques discussed.
Properly applied however, air photo interpretation and photograrnmetry can offer significant advantages in terms of timeflexibility, secrecy and economy.
The principles of photo stereoscopy are based upon the fact that if an object is viewed from two different positions simultaneously, the visual effect is stereoscopic, or three dimensional.
Air photographs normally are taken along consecutive lines, with a 60 per cent forward overlap and 30% sidelap. Thus a permanent record of the ground is obtained at a certain scale and at a particular time. These photographs are projected through stereo plotting instruments, the lenses of which are similar to those of the aerial camera. When the projectors are positioned so that they exactly match the position of the taking camera at the moment of exposure, a three dimensional image is formed, and the original conditions during the film exposure are reproduced in model form.
If an object is viewed alternately with the left and right eye, its location will appear to shift from one position to another. This apparent displacement, caused by a change in the point of observation, is known as parallax, and it is this phenomenon which allows extremely accurate measurement of the elevations within the stereoscopic model. The apparent height of an object is therefore determinable from the difference in its image displacement on adjacent airphotos. The accuracy of measurement is directly related to the focal length of the camera lens and to the altitude above ground at which the photographs are taken. For example, photographs taken 18,000 feet above ground can yield accuracies of ± 1.8 feet; 1,200 foot altitude photography can result in measurements accurate to ±1.5 inches.
It is important to distinguish between stereo photos, which allow viewing of the terrain in three dimensions, and photo mosaics, which consist of the same aerial photos attached to a flat surface and forming a map visible in only two dimensions.
Photogranunetry is the technique of making horizontal and vertical measurements from air photos and producing maps which present these details.
Air photo analysis and interpretation are necessarily complementary to one another.
