724 HANDBOOK OF PHOTOGRAPHY 



which rises 1000 ft. above the mean datum plane, the scale of the picture at the top of 

 this mountain will be 1 in. = 900 ft. 



By the use of the last formula, a picture may be taken into the field and the scale 

 determined at a given ground elevation. Then, if the field operator wishes to know 

 the scale of the picture at some other elevation, he uses the difference in the elevation 

 and the focal length in this formula, which gives him the desired scale of the picture 

 at the new ground elevation. 



It should be kept in mind that this difference in scale results in a radial displace- 

 ment of the image. Based upon this fundamental geometric relationship, methods 

 have been derived for map assembly which are known by the general classification of 

 "radial control." 



Compiling the Mosaic Map. — Rough mosaic maps to give approximate picture 

 information only are frequently assembled with complete disregard of the radial 

 displacements described in the last section. A map so assembled may be a reasonably 

 good general picture, but, in putting it together, sections of country will be entirely 

 lost where the ground is higher than datum and double images of terrain will occur 

 where the ground is lower than datum. In other words, on the high ground where the 

 scale of the picture is too large, the image on two adjacent pictures is too large to fit 

 and some must be cut away and entirely sacrificed in joining the pictures up. Where 

 the ground is too low, the images are small and fail to meet, with the result that at the 

 juncture double images will appear. Thus, if two prints were being joined in a rough 

 mosaic and the pictures contained sloping ground, part of which was above datum and 

 part below datum, the two pictures would join up perfectly at the datum elevation. 

 Above the datum elevation, part of the image would be lost, the amount lost increasing 

 as the elevation of the ground becomes higher. Below datum, double images would 

 appear, getting farther and farther apart as the ground elevation decreases. Under 

 these circumstances, a skillful mosaicer can make a reasonably acceptable picture bj^ 

 carefully choosing the places to cut the pictures together where the loss of image or 

 double image is not going to be apparent. However, the user should be warned that 

 maps compiled by this method are highly inaccurate whether the error can be easily 

 seen or not. 



For many purposes these rough mosaics, in spite of their errors, are good enough. 

 One satisfactory way of assembling such a rough mosaic is to take a good base map, 

 such as a U. S. Geological Survey sheet, enlarge this to the scale at which the mosaic 

 is to be assembled and paste this enlarged bromide print of the Geological Survey map 

 on a piece of composition board. Then the pictures, comprising the mosaic, can be 

 pasted down in their approximate positions by making the major detail of the pictures 

 match to the Geological Survey map. This method may be quite successful if the 

 differences in the elevation of the ground are slight and if the flying has been skillfully 

 done and a uniform altitude maintained. 



In laying a precise mosaic, the first essential is adequate control. This control 

 may be the U. S. Geological Survey map enlarged to scale as outlined in the previous 

 paragraph. Frequently, however, such maps are not available, or if available were 

 made many years ago, when the art of topographic mapping had not progressed to a 

 point where the resulting maps were accurate. 



If the operator is so fortunate as to have a modern Geological Survey map avail- 

 able, he may approach the problem of making a precision map by measuring the 

 distance between identifiable objects on the U. S. Geological Survey map and com- 

 paring this measurement with the corresponding objects on the contact print. A 

 ratio can thereby be established for the enlargement or reduction of the contact print 

 to fit the particular points measured on the Geological Survey map. The print 

 which is made to such data is known as a ratio print; it may be pasted down over the 



