2392 HYDROGRAPHIC MANUAL PaGE 100 



map containing the control points. Radial lines to other objects on the photographs 

 may then be used as cuts, intersecting other lines through identical points on overlapping 

 photographs to give true map positions. 



In the radial-line method of mapping, the radial lines are traced on celluloid 

 templets. These templets are then adjusted to the ground control stations on a base 

 sheet containing the usual poly conic projection, the radial lines forming a graphic 

 triangulation. Important detail such as control stations for the hydrographic survey, 

 buildings, and road crossings are located by the intersections of their respective radials 

 in the plot. Less important detail is adjusted to position between the intersected 

 points by simple proportion, as when transferring detail between map projections 

 which differ slightly in scale. Care is taken to locate by intersection a sufficient num- 

 ber of points at the changes in slope^ so that the correct position of the less important 

 detail will be a simple proportion of the distances between the positions on the map 

 obtained by radial intersection and the distances between the corresponding points 

 on the photograph. 



Probably about 80 percent of the photographs taken for mapping fall within the 

 limits of tilt and relief to permit the use of radial lines for graphic plotting without 

 correction. For shoreline and for stations only a short distance above it, the radial 

 lines will be graphically correct for probably 98 percent of mapping photographs, 

 even for tilts of the camera axis up to 3°. An excessively tilted photograph may be 

 recognized by the displacement of images away from its principal point on one side 

 and toward it on the opposite side, as shown by a radial plot or comparison with over- 

 lapping photographs. Space is not available in this Manual for a discussion of correc- 

 tions to radial lines for tilt and relief, or the plotting from oblique photographs. A 

 good text on photogrammetry should be consulted for the method of making such 

 corrections and for descriptions of the many ingenious machines and methods of 

 mapping from photographs (see 9533). 



2392. Planimetric Maps 



Most of the air photographic surveying of the Coast and Geodetic Survey has 

 been done in coastal areas of low relief wherp simple graphic radial-line methods are 

 efficient. The maps compiled have been planimetric only, that is without heights or 

 contours determined from the photographs. The maps show the shoreline and objects 

 visible from the water in full detail, usually on the same scale as the hydrographic 

 surveys of the adjacent waters. Roads, railroads, important public buildings, and 

 other detail for a short distance inshore are also shown. The compilations are made 

 on cellulose acetate at the average §cale of the photographs, and are then reproduced at 

 standard scale and printed by photolithography. 



When practicable to make the field inspection of the photographs and compile the 

 maps in advance of the hydrography, special prints of the maps are made for the hydro- 

 graphic party to use for boat sheets and smooth sheets (see 733). Identifiable objects 

 suitable for hydrographic signals and identifiable locations at which hydrographic 

 signals may be built are marked on the photographs during the field inspection and 

 located by intersection in the radial plot. The positions so obtained are indicated by 

 black circles (2.5 mm in diameter) on the printed maps furnished to the hydrographic 

 party. The limits of shoals and channels which appear in the photographs are indi- 

 cated by fine dash lines. Much that is irrelevant to the hydrographic survey, such 

 as names and notes in water areas on the printed maps, is deleted during reproduction 

 of the copies for hydrographic use. Generally the boat and smooth sheets made from 



