68o 



Yearbook of Agriculture 1949 



photographed from the air often 

 called "flown." But on at least half of 

 the area, the pictures are more than 8 

 years old. 



That is unsatisfactory, because for- 

 ests are changing all the time. Timber 

 stands decline in area and volume be- 

 cause of cutting, fire, wind, decay, in- 

 sects, and other losses. On the other 

 hand, they expand in area, size, and 

 volume through growth. Because of 

 the changing factors, the old photo- 

 graphs may not correctly show the 

 current condition of the forests. They 

 may, however, correctly represent old- 

 growth timber areas and topographic 

 features, such as streams and roads 

 that have not changed materially. It 

 is important to have up-to-date 

 photographs in timber surveys and pe- 

 riodically say every 5 or 10 years to 

 refly areas where the forest cover has 

 changed significantly. Although old 

 aerial photographs still have high 

 value for engineering purposes, new 

 pictures are preferred. 



Another problem is the scale. For- 

 esters require a larger scale for resource 

 studies than engineers need for their 

 work. So far, it has not been possible 

 to agree on one scale that would be best 

 for both kinds of work. Experienced 

 engineers and foresters feel that (con- 

 sidering costs and usability) the best 

 arrangement would be to have a 

 special kind and scale of photographs 

 for forestry and another for engineer- 

 ing purposes. Actually, that is only a 

 part of the problem; the other part is 

 the need for improvement in the tech- 

 nical aspects of taking photographs 

 ( such as the best kind of film, the scale, 

 season of the year, focal length of 

 camera), and concerted efforts to 

 adapt the pictures to the major use for 

 which they are being taken. 



The scale of usable photographs in 

 forestry work is somewhat restricted in 

 range from 1:12,000 to 1:22,000. A 

 scale of 1 : 15,840 is commonly pre- 

 ferred; the figure means that 4 inches 

 on the photograph covers 1 mile of 

 forest on the ground. Because the scale 

 is the factor that primarily controls the 



relative size of the objects that appear 

 on the photographs, its selection is im- 

 portant and must be adjusted as far as 

 possible to the purpose for which the 

 photographs are taken and the allow- 

 able cost. Some caution in using scales 

 to determine distances on photographs 

 is necessary, because changes in altitude 

 of the plane, its tip or tilt, and varia- 

 tions in elevation of the country being 

 photographed may introduce errors of, 

 say, 10 percent in area determinations 

 on individual photographs. 



The two general types of aerial pho- 

 tographs are verticals and obliques. 

 Vertical photographs taken with the 

 camera in as nearly a vertical position 

 as one can keep it in a fast-flying air- 

 plane are preferred for forest surveys, 

 topographic and planimetric mapping, 

 and on-the-ground forestry practices. 

 Oblique photographs are taken with 

 the camera intentionally inclined to 

 the vertical at a given angle. They 

 cover large areas at a low cost and are 

 sometimes used for rough mapping. 



The best type of film for forestry 

 photographs from the air is still to be 

 determined and perfected. Three types 

 now used are panchromatic, infrared 

 with various filters, and color films. 



Panchromatic is most common, but 

 fails to meet fully the foresters' needs 

 in differentiating between forest types 

 and species of trees. The infrared 

 film, with a minus blue filter, has pro- 

 duced photographs showing an im- 

 proved contrast between species and 

 forest types in summer pictures, but 

 needs further trial tests and experimen- 

 tation. Color film has not been tried 

 over a large area. In theory, it looks 

 good for identifying species of trees, 

 particularly for hardwoods in the fall 

 when seasonal coloring of the leaves is 

 at its height. 



Besides type of film, the season best 

 suited to bringing out forest character- 

 istics is important in interpreting the 

 aerial photographs in surveys or other 

 economic and management investiga- 

 tions. For forest-survey purposes, spring 

 and fall are believed to be the best 

 seasons for photographing forests. 



