AERIAL PHOTOGRAPHY 721 



Pictures which are to be used purely for visual observation, such as obliques, or 

 pictures which are to be reproduced by half-tone cut should be ferrotyped. Prints 

 for all other purposes should not be ferrotyped as this process distorts the image. 



Prints which are to be used for any sort of precision work should be made on one 

 of the several types of low-shrink paper, such as Air Map Special or Positype. If 

 the prints are not to be used for the utmost precision, ordinary grades of contact paper 

 may be used. 



Prints which are put through a dnun drier are appreciably distorted. For 

 precision work, prints should be dried face down on cloth in the ordinary atmosphere 

 of the room without the use of an electric fan. 



Indexing. — There are a variety of ways of indexing an aerial survey. Sometimes 

 the area covered by each photographic print is drafted on a map such as a U. S. 

 Geological Survey map, but the preferred way with most customers is to assemble 

 a set of contact prints in their proper relationship and either draft thereon large 

 numbers or paste stick-up numbers on the prints and copy the assembly on a large 

 negative. 



Prior to the making of any prints, the question of numbering the negatives should 

 be considered. A number which shows in the photographic image detracts from the 

 artistic value of an oblique picture, so that for this type of negative it is most desirable 

 to number the film in the margin and write this number on the back of the print at 

 the time of printing. It is advantageous to have the number show on the face of 

 prints which are to be used for various mapping purposes, and most vertical negatives 

 are numbered with celluloid ink either freehand in a corner on the celluloid side of the 

 film or are stamped on with a numbering machine. 



Many contracts require that the final prints bear consecutive numbers. This 

 requires that the film immediately upon development be assigned temporary numbers 

 which are written in the corner of the film with an ink that can be washed off. After 

 all the fUghts have been completed, rejected film is eliminated and the final index map 

 compiled and numbered. A list can now be prepared bj' which the temporary 

 numbers may be removed from the film and replaced with the permanent numbers. 



Compilation of Maps. — Everything pertaining to an aerial survey should be 

 planned with the ultimate purpose for which the survey is made in mind. If the 

 pictures are purely for pictorial purposes, the matter of a small amount of lens dis- 

 tortion is generally negligible. If, on the other hand, the pictures are to be used for 

 the making of precise mosaics or are for the purpose of determining land area or if 

 they are to be ultimately used in the compilation of planimetric or contour maps, 

 the characteristics of the lens become of paramount importance. For precision work 

 a lens which is substantially distortion free must be used. For this purpose the 

 symmetrical type of lens is generally suitable. If the operator proposes to use any 

 lens which has not been especially designed for aerial photography, he will be wise to 

 send the lens to the National Bureau of Standards, Washington, D. C, to have its 

 characteristics determined. 



It should be kept in mind that, while the definition of a lens can be improved by 

 stopping it down, its distortion characteristics remain substantially the same at all 

 stops. 



Lenses for oblique purposes may have an absolute distortion of image of as much 

 as 0.05 in. without seriously impairing the appearance of the picture. At the other 

 extreme of precision, a lens which is to be used for contour mapping should ordinarily 

 have no distortion greater than 0.0005 focal length. 



All precision mapping by aerial photography is contingent upon the picture having 

 been taken with the lens axis vertical. Any departure from a vertical lens axis is 

 known as tilt. Actually the term vertical picture is generally used, whereas geometri- 



