Aerial Photographs - Orienting of photograph
87. ORIENTING OF PHOTOGRAPH
Orienting the photograph is important because it is of very little value as a map supplement or substitute if its location and direction are not known by the user.
a. If a map of the same area as the photograph is available, the photograph is oriented to the map by comparing features common to both and then transferring a direction line from the map to the photograph.
b. If no map is available, the shadows on a photograph may be used to get an approximate truenorth line. This method is not recommended in the torrid zone
(1) North temperate zone. The sun moves from the east in the morning through south at noon to west in the afternoon. Conversely, shadow fall varies from west through north to east. Before noon, therefore, north is to the right of the direction of shadow fall; at noon, north is the direction of shadow fall; and after noon, north is to the left of shadow fall. On an average, the amount of variation in shadow fall per hour is 15 degrees. From marginal information, determine the number of hours from noon that the photo was taken and multiply that number by 15°. With a protractor, measure an angle of that amount in the proper direction (right to left) from a clear, distinct shadow, and north is obtained. For photographs taken within three hours of noon, a reasonable accurate north direction can be obtained. Beyond these limits, the 15° must be corrected, depending on time of year and latitude.
(2) South temperate zone. The sun moves from east through north at noon to west. Shadows then vary from west through south to east. Before noon, south is to the left of shadow fall; at noon, south is shadow fall; and after noon, south is to the right of shadow fall. Proceed as in (1) above to determine the direction of south.
c. On a photograph that can be oriented to the surrounding ground features by inspection, a magnetic north line can be established using a compass.
(1) Orient the photograph by inspection.
(2) Open the compass and place it on the photograph.
(3) Without moving the photograph, rotate the compass until the north arrow is under the stationary black line.
(4) Draw a line along the straight edge of the compass. This is a magneticnorth line.