The Boy Surveyor
Plotting a Camera Survey
By Harold G. McGee
[The camera records pictures that can be taken in camp or on a vacation trip and kept until more leisure may be had in winter for plotting the ground.—Editor.]
A previously measured base triangle with "stations" at each corner is necessary for making a camera survey, just as it is for the plane-table survey. It is preferable to have each of the three sides measured independently, though if one side has been accurately chained, the other two may be less satisfactorily determined by the use of the plane table. If the camera has a fixed focus, it is possible to make an entire survey from the two ends of a single base line; but this method has no check and should be used only when and where the triangle method is impossible. With an adjustable focus, it will rarely give good results.
Two Fine Hair Lines must be Scratched on Each Plate Before It is Used to Plot From, or to Make Pictures from Which the Plotting is Done
Once the triangle has been laid out, the fieldwork is very simple. The camera is set up at one station, carefully leveled, and then a series of pictures is taken, each single plate overlapping the last so as to form a panorama of the area to be mapped. The focus of the lens must not be changed during a series, and plotting is facilitated by keeping the focus constant during all the exposures which make up a survey. To secure good depth of focus, a small stop is generally used, since it is necessary to use a tripod to keep the camera level. If contours are to be drawn, the height of the lens above the ground at the station should be measured and recorded. After a series has been taken at each station, the fieldwork is complete. It is an excellent plan to keep a record of the plate numbers, and the order in which and the station from which the exposures were made, so that the 10 or 12 plates which a small survey will comprise may not get hopelessly mixed up. If the camera is turned each time to the right, clockwise, and the plates are numbered A-1, A-2, B-4, etc., indicating by A-1, for example, the leftmost plate taken at Sta. A; by A-2, the plate just to the right of A-1, just as II is to the right of I on the clock dial, and by B-4, the fourth to the right taken at Sta. B, there ought to be no difficulty in identifying the plates after the exact details of the ground are forgotten.
