388 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



up-to-date throughout at the instant that the exposure is made. 

 They are also free from gross errors in position of objects about 

 which information is desirable. 



The vertical photograph, even though it should happen to be 

 taken with the camera truly vertical, will not give an absolutely true 

 plan view of what is in front of the lens unless that subject happens 

 to be absolutely level and without relief. The relief causes distor- 

 tion in the photographs, which can be accurately computed if the 

 difference in elevation between the various parts of the photograph 

 is known. This distortion, due to relief, makes possible the visualiz- 

 ing of relief when two nonidentical photographs of the same subject 

 are viewed stereoscopically. This is made use of in stereoscopic map- 

 ping machines, and also in the mathematical method of determining 

 relief from distortion in overlapping vertical photographs. How- 

 ever, in a single photograph or a mosaic map, this distortion exists 

 as an error in the location of the object. Also the single photograph 

 fails to fulfill all the requirements of a map in that it does not accu- 

 rately show relief. A person who has studied vertical photographs 

 knows that relief is indicated in other ways. For instance, we know 

 that streams follow the lowlands and that their tributaries become 

 smaller and finally disappear as they approach divides. Also, if a 

 photograph is held in such a way that the shadows on the photo- 

 graph fall toward the observer and the observer then stands facing 

 the light, a visualization of relief of a kind is realized. 



MOSAIC MAPS 



Mosaics are two or more overlapping vertical photographs so cut 

 and fitted together that they form one composite picture of the area 

 they cover. In addition to the errors in each individual picture, a 

 mosaic contains errors due to fitting the photographic prints 

 together. 



The making of large mosaics and of conventional planimetric 

 maps from photographs is the work of civil engineers, and although 

 it requires some changes from the methods used in making a survey 

 on the ground, the principles are in general the same, and the work 

 is carried on in a manner very similar to that used in plane-table 

 surveying. 



CONVENTIONAL MAPS 



In order accurately to show relief on maps, the use of contours 

 has been generally adopted. These contours or lines of constant ele- 

 vation can be plotted from photographs so taken that every point 

 shows on at least two overlapping pictures. Of the methods devel- 

 oped, the only practical ones are those making use of stereoscopic 

 vision. This requires that the two eyes each see the same object but 



