AERIAL PHOTOGRAPHY AND FOREST MAPPING 519 



tion of the location of points by the method of intersections is inaccu- 

 rate on this account and on account of the acute angles of the inter- 

 secting lines. 



5. Errors are liable to creep in as a result of inaccurate determina- 

 tion of the same point on dififerent negatives, owing to change of its 

 aspect from the different angles of view. 



It is probable that scale maps could be drafted from aerial photo- 

 graphs with reasonable speed. 



One manufacturer states that in experiments conducted by him it has 

 been possible for one draftsman to map ten square miles per day, with 

 700-foot contours. He believes that, with improved transposing instru- 

 ments, 25 square miles per day could be mapped. 



It is probable that the sketching of contours and the determination of 

 various topographic features could be assisted materially by the use of 

 stereoscopic views and a stereo-comparator, an instrument used to de- 

 termine relative depths in stereoscopic views. Oblique views, properly 

 chosen, would also be of assistance in determining topography. 



National Forest maps are made and used for many purposes, and their 

 character varies with the use to which they are to be put, ranging from 

 the crudest sketch to contour maps of high accuracy. In many cases 

 these maps are compiled from all the best surveys available, and re- 

 course is had, not only to Forest Service data, but also to maps and 

 surveys of the General Land Office, the U. S. Geological Survey, the 

 U. S. Coast and Geodetic Survey, and infrequently to the work of cor- 

 porations or local surveyors. On some National Forests the amount and 

 grade of existing surveys are much below the needs. Where the de- 

 sired surveys and maps do not exist, it becomes necessary for the For- 

 est Service to make the proper survey. This is especially the case when 

 topographic maps are needed in the appraisal of timber and forage. 



For the purpose of this article. Forest Service surveys may be classed 

 as follows, the principal uses to which each is put being shown as sub- 

 heads : 



1. Cadastral Surveys: 



Boundaries and acquisition. 

 Entry surveys. 

 Administrative mapping. 



2. Topographic Surveys : 



Timber reconnaissance. 

 Grazing reconnaissance. 

 Soil reconnaissance. 



3. Engineering Surveys : 



Road construction. 



