AERIAL PHOTOGRAPHY 725 



Geological Survey map and will be accurate between the points from which the 

 measurement was made. This method of map making may be carried to considerable 

 elaboration if the use to which it is being put justifies. For example, from the con- 

 tours on the Geological Survey map, the picture may be broken down into small 

 parts, each to be considered as a uniformly sloping plane. One print may then be 

 calculated for each such small part of a negative and made in a rectifying camera. 

 From each of the several prints representing a small part of the one negative, the part 

 for which the print is designed can be cut out and pasted in its proper position. In 

 this way, the large errors are reduced to many small errors, all of which may be brought 

 within the tolerance of a specification by proper care. 



Another method by which precise mosaics ai-e made on the basis of available 

 contour maps is called "pyramiding." By this method, the contours are transferred 

 from the Geological Survej'- map to the print, and depending upon the precision called 

 for in the specification, a separate print is made from the same negative for each zone 

 of elevation. If the map is to be very precise, one print may be made for every 100 ft. 

 If the specifications give greater tolerances, 

 a print may be made for every 500 ft. or 

 every 1000 ft. These varying prints are 

 then trimmed in accordance with the con- 

 tour line representing the elevation for 

 which the print was designed, and the prints 

 are built up one on top of the other with 

 their centers superimposed, with the largest 

 ratio print on the bottom and the smallest 

 one on the top. This is a very tedious and 

 very expensive method of mosaic compi- 

 lation but has been frequently used where 

 the resulting precision justified the cost. 



By far the most common practice for the 

 assembly of precise mosaics is by the radial- 

 control method. We learned earlier that Fig. 12. — Diagram illustrating that 

 differences in elevation resulted in a radial the image will lie along a radial line 

 displacement of image. Thus, regardless of ^'"'^^^ ^"""^^ ^^^ ^P*^^*^! ^'''^■ 

 variation in elevation, the image will lie some place along the radial which passes 

 through the position for the true point. In other words, the angle between the 

 radials passing through any two images is constant regardless of the elevation (Fig. 

 12). Utilizing the constancy of this central angle, we can now visualize radial control 

 as building up a net of graphic triangulation. 



Preparatory to making a radial-control layout, the boards upon which the mosaic 

 is to be assembled must have the known control plotted thereon. 



If the mosaic is of a large area, a "projection" must be laid out upon the mosaic 

 board. This projection generally constitutes drawing latitude and longitude lines in 

 their proper positions which take into consideration the fact that the curved surface 

 of the earth is to be compiled into a flat map. Thus on the polyconic projection, which 

 is the most usual form of aerial-map assembly, lines of latitude which run tnie north 

 and south on the surface of the earth will converge toward the north on the projection. 



Tables and instructions for laying out projections of this nature may be secured 

 from the U. S. Coast and Geodetic Survey or the U. S. Geological Survey at Washing- 

 ton, D. C. 



With the projection now plotted upon our mosaic board, we must next plot the 

 known control points. Perhaps these are points which have been established by the 

 government, which in many parts of the country has a very complete system of control 



