518 JOURNAIv OF FORESTRY 



The trace of the optical axis of the lens on the plate may be readily 

 determined by means of cross-hairs or similar intersection. 



In order to secure the proper interval between exposures, so that 

 condition 3 may be met, cameras are made either semi-automatic or 

 fully automatic, and provision made to time the interval between ex- 

 posures to accord with the speed and altitude of the flight, the focal 

 length of the lens, and other conditions. 



In order that the exact height of the camera above the ground (con- 

 dition 4) may be determined, recourse will in general need to be had to 

 some type of preliminary survey. There are several ways in which the 

 desired object may be attained. Thus, if the relative height and hori- 

 zontal distance between two points appearing in two consecutive nega- 

 tives be known, it is possible, by geometric projection, to locate the posi- 

 tion of the camera relative to these points at the instants these negatives 

 were taken, and similarly determine the relative heights and horizontal 

 distances of all other points in the area covered by both negatives. 

 Further, if the exact geographic positions of the base points be known, 

 the geographic position of each point in the area can be determined. 

 It is not necessary to know the geographic positions of the individual 

 base points, however, since the whole survey can be based upon suitable 

 benchmarks at the beginning and tied in with others at the end. The 

 accuracy desired and other conditions will, of course, determine the 

 exact method of procedure as regards control. In any event, it is evi- 

 dent that any kind of photographic surveying must be supplemented 

 by a certain amount of terrestrial instrument work, and, as in all sur- 

 veying, the more control introduced the better the map. 



While the method just outlined is perfectly workable, its accuracy 

 depends upon many more or less uncontrollable factors, and it has not 

 been developed to a point where it is possible to say just what accuracy 

 may be obtained. One is quite safe in saying, however, that great diffi- 

 culty would be encountered in making a 50-foot or 100-foot contour map 

 of rough country by any method so far developed. Some of the reasons 

 for this statement are as follows : 



1. Even very slight divergence from the vertical in making an ex- 

 posure would make a reasonably large error in the finished map. A 

 30-minute error from a height of a mile would displace the entire pic- 

 ture 45 feet and cause distortion on the entire plate. 



2. Any shrinkage in the film or any lack of flatness at the time of 

 exposure will produce highly magnified errors in the finished map. 



3. Any errors in the lens will be magnified in the map. 



4. The scale of the negative is relatively small, and the determina- 



