4 REPORT OF COMMISSION VII 
ground photographed on each exposure, combined with the need for 60% over- 
lap between successive exposures, imposes the requirement for a much more rapid 
recycling than can be achieved with conventional aerial cameras. One recently- 
developed aerial camera, having been designed especially for low-altitude, high- 
speed reconnaissance, has a maximum recycling speed of ten frames per second, 
which is approximately 30 times at fast as that of conventional cameras cur- 
rently being used for civil purposes. While continuous stereo strip cameras also 
provide a solution to this problem, they do not provide the photo-interpreter 
with single point image perspective on a frame-by-frame basis. The consequent 
limitations imposed by this condition have already been mentioned. 
Obviously, not all of the interpreter's photographic requirements are satis- 
fied by large scale photography flown at very low altitudes. For example, he 
may have other requirements for large scale photography flown at very high 
altitudes. Requirements of this type can be met only through the development 
of aerial cameras having very long focal lengths. The requirements for cameras 
having extremely long focal lengths (perhaps of 400 inches or more) were men- 
tioned at the 1952 annual meeting of the American Society of Photogrammetry. 
The combination of photographic characteristics afforded by cameras of 
long focal-length (namely large-scale, high-resolution, large format size, relatively 
large ground area covered per exposure, and slight relief displacement) is of 
great interest to the interpreter who makes extensive use of photo mosaics in 
areas of high relief. In. such areas, mosaics constructed from conventional 
photography by means of ratioed and rectified prints still leave much to be 
desired by the interpreter who, when working in the field, would like to measure 
areas, distances, and directions to a high order of accuracy directly on the mo- 
saic. 
Because of the slight relief displacement exhibited by long focal-length 
(narrow angle) photography, there is very little apparent “lean’”’ of trees or 
other tall objects radially outward from the photograph nadir. In this respect, 
such photography is decidedly superior to low altitude photography (Rogers, 
1952). As only a minimum of ground detail is obscured on long focal-length 
photography, the interpretation and delineation of such planimetric and topo- 
graphic features as roads, trails and contours is greatly facilitated. Figure 2 
provides an example of the clarity of details obtainable from photography 
flown at a relatively high altitude (45,000 feet) with a camera of long focal 
length. Although much detail has been lost in the process of photolithographic 
reproduction, some of the interpreters who examined the original photography 
claimed they could see individual golf balls on the green. However, theoretical 
calculations indicate that from this distance of nearly nine miles, no object small- 
er than a tennis ball would be discernible on the photography. Even this, 
however, is eloquent testimony of some of the recent amazing progress in photo 
reconnaissance as a result of which the photographic interpreter is being pro- 
vided with better and better images for interpretation purposes. | 
B. PHOTOGRAPHIC FILMS AND FILTERS: 
Several experimental aerial photographic tests have been conducted re- 
cently for the purpose of determining the most suitable film-filter combina- 
tion for a given type of photo-interpretation (Spurr 1949: Jensen & Colwell 
1949; Savage 1950; Schulte 1951). Also several classified projects have been 
undertaken in this field by various military organizations. 
Probably. the two most commonly used film-filter combinations are infra-red 
minus-blue and panchromatic minus-blue. Photo-interpreters have found that 
infra-red minus-blue photography usually is superior to panchromatic minus- 
  
  
   
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