AERIAL PHOTOGRAPHY AXD FOREST MAPPING 521 



maps, sketched the requirements of National Forest surveys and recon- 

 naissance and present methods of meeting the principal of these require- 

 ments, we may proceed to discuss the possible value of aerial photogra- 

 phy as an adjunct or supplement to these methods. 



Neither cadastral nor engineering surveys can be made by photogra- 

 phy. They demand the setting of monuments and also a degree of pre- 

 cision which cannot be obtained by photographic methods, especially in 

 regions of bold relief. Hence photography can never do away with this 

 kind of work, but each may supplement the other in the making of a 

 fine Forest map. Every corner set is an additional control point. Ca- 

 dastral plats are weak in detail, which is difficult to secure by present 

 methods. An aerial mosaic of the area could readily be tied in with the 

 monuments, and would furnish any desired amount of detail which 

 could be drawn in on the properly controlled base. Thus every road 

 or trail, creek, river, fence, or building could be accurately located with 

 the minimum of effort and expense in the field. 



In intensive timber reconnaissance, it is obvious that aerial photogra- 

 phy as now developed cannot replace present methods, since neither the 

 species nor the dimensions of the timber could be determined. Further, 

 aerial photography has not been developed to the point wdiere it is possi- 

 ble to make topographic maps with a degree of accuracy approaching 

 that secured by the standard Forest Service method employed in con- 

 jimction with intensive timber reconnaissance. An added difficulty is 

 encountered in forested areas, in that the aerial photograph shows only 

 the tops of the trees and not the ground underneath. However, an 

 aerial mosaic would be a valuable supplement to an intensive recon- 

 naissance in several ways. Possibly it would be of greatest assistance 

 in furnishing an excellent check on the percentage of forested area and 

 parks. The accuracy of the reconnaissance depends, of course, upon 

 the accuracy with which this percentage is determined. In areas which 

 are completely forested, the value of the mosaic for this purpose be- 

 comes very small. In any event, a mosaic would be of general assistance 

 in working up the data into map form and w^ould present a picture of 

 the area in much greater detail than the standard topographic maps. 



The usefulness of aerial maps in connection with grazing reconnais- 

 sance is more apparent than in the case of intensive timber reconnais- 

 sance. While ordinarily topographic maps form the basis of grazing 

 reconnaissance, it frequently happens that such maps are not available, 

 and it becomes necessary to make surveys before the reconnaissance can 

 proceed. It is probable that in many cases of this sort aerial mosaics 

 could be used as base maps. They would have the added advantage of 



