The photographs are apt to be made up of intense blacks and whites and to have so much contrast that the detail is poor. Considerable training and experience are required. The chief disadvantage of the parallax method is that it requires more training and much more experience than the shadow method.
Its development should provide a guide to city and county administrators and photogrammetric engineers alike in planning surveys based on aerial photography and should encourage the more exten sive use of aerial survey methods in day-to-dayadministration. For most purposes, a simple lens type stereoscope (Figure 13) is preferable, as it is portable, is easy to use, and provides adequate magnification.
It is rather a picture distorted among other causes by: (1) topographic displacement, (2) tilt of the airplane and the camera at the moment of photography, and (3) film and paper shrinkage. Species may be identified and their images measured with a surprising degree of accuracy. The appearance of these standard types of black and white photography is illustrated by three photographs taken during the summer of a portion of the Harvard Forest in central Massachusetts. Search for other works by this author on: Photogrammetry (also called metric photogrammetry) is concerned with obtaining exceptionally precise quantitative measurements from aerial photographs, whereas photographic interpretation (or interpretive photogrammetry) focuses more on the recognition, identification, and significance of features on photographs (Film-based photographs may be converted into digital format through scanning (Relationship between scanner resolution and ground resolution for multiple scales of aerial photography. The most useful type of photograph for land-use purposes currently known is the modified infrared. In Figure 5, the panchromatic photograph, the detail is clearly defined and the distinctions between open and forested land readily apparent. In using aerial photographs to control field inventory work, it is important to remember that the photograph is not an accurate map. They contain a wealth of information not capable of being reduced to a map, words, or any other form of record. square and are taken with the camera pointed vertically from an airplane flying parallel strips in a cardinal direction. All were taken at a scale of 1:12,000. The amount of displacement, though, is so small that the practical application of this method is limited to photographs taken with a wide-angle lens at a relatively low altitude and to conical trees, such as spruce and balsam fir, where both the tip and base of the tree can be seen and identified precisely on the displaced image. Within this range, the photographs are at a sufficiently large scale that the individual trees and shrubs can be clearly seen and studied, and yet at a sufficiently small scale that each photograph covers about 1,000 ha. Occasionally a single one of these pictorial elements will serve to identify an object but more often several elements must be taken into consideration.
For example, consider that there is a dark area on an infrared photograph.
Characteristic differences in shape, too, can be recognized. Another, the Multiscope, is a combination of the mirror stereoscope and the camera lucida in a highly flexible form (Figure 11). With these, the image of a single photograph can be superimposed on the radial line network and photographic detail can be traced directly. Along the outer margins of the photograph, the tip of the tree will be displaced outward from the center more than the base and the tree will appear to be lying down, from which condition the actual displacement may be measured. Because of the variable conditions during photograph collection (wind, turbulence, etc. Photographic contrast, or the range of values within the photograph, is a product of the film's emulsion type, the degree of exposure to light, and the film development conditions (Of fundamental importance to the quality of aerial photographs is the camera used to obtain the images. Possibly of greater use in land-use and land-use planning studies is the information which can be obtained by the stereoscopic study of the photographs without recourse to mapping techniques. By the use of a micrometer wedge, or any other precise device for measuring horizontal distances on the aerial photographs, measurements of crown diameter can quickly and accurately be made. Such a photograph is a compromise between the normal panchromatic and normal infrared photographs, and it combines much of the good detail of the panchromatic with much of the good tone variation of the infrared. Finally, a small portion of the east-central part of Figure 1 is portrayed at a scale of 1:300, or 250 ft. per inch in Figure 4.