Aerial Photographic Survey

photographic surveying of a locality from the air by means of a special-purpose aerial camera mounted on an airplane, helicopter, dirigible, artificial earth satellite, or rocket. The plane of the aerial camera can take any specified horizontal position (the plane-type aerial camera is most common) or inclined position (perspective aerial photographic surveying). In certain cases the photography is carried out on a cylindrical surface or with a rotating objective (panoramic scan aerial surveying). Aerial photographic surveying is customarily performed with a single-lens aerial camera, but multilens cameras are sometimes utilized when the area photographed on a single plate is to be enlarged. The photographic scanning can be carried out using individual aerial photos in a definite direction (strip survey) or over an area (carpet-pattern survey).

When a photographic strip is laid out, part of the locality photographed on a single plate must also be photographed on another plate. The ratio of the area photographed on two adjacent plates to the area represented in each separate plate (expressed in percent) is termed the longitudinal overlap. This overlap is specified to conform with the requirements of subsequent photogrammetric reduction of the data (longitudinal overlap is usually 60 percent). When a region of considerable breadth is to be surveyed aerially, the area is photographed in a series of parallel strips with some transverse overlap (usually 30 percent). The height of flight relative to the locality to be surveyed, the focal length of the surveying camera, the season and time of day, and the sequencing of the photographic strips are specified beforehand in aerial photographic survey missions.

At each instant during the photographic operation, the center of projection and the plane of the aerial survey plate occupy an arbitrary position because of the mobility of the base. The quantities defining the spatial position of the plate relative to the system of coordinates adopted—called the elements of the external orientation of the plate—include three linear coordinates of the center of projection x₈ , y₈, and z₈, (see Figure 1) and three angles defining the rotation of the plate about the three coordinate axes (these are indicated in Figure 1).

Figure 1. Elements of the external orientation of a photographic plate

In order to determine the spatial coordinates of the points photographed from the aerial survey plates, the elements of external orientation of the plates must be found first; that involves finding the coordinates of certain geodetically specified points that are shown distinctly on the plates. A statoscope (which records the change in flight altitude by changes in air pressure), a radioaltimeter (which determines the height at which the photographic record was taken with respect to the locality surveyed), and radiogeodetic stations (for finding the distance from the surveying aircraft to the stations located on the earth’s surface at points having known geodetic coordinates) are utilized to establish the elements of the external orientation of the aerial survey in flight. These data make it possible to compute the map coordinates of the center of projection. The readings of the vertical gyroscope make it possible to find the angles of inclination of the photographic plate; these can also be determined by processing the plates bearing images of the starry sky, the position of the sun, or the horizon line.

Aerial photographic lenses of high resolving power and low optical distortion and aerial film with high resistance to buckling or deformation are used to improve the quality and precision of the aerial plates in aerial photographic surveying work. The instrumental drop of light over the field of vision must be minimized, the shutter must provide very short exposures (to 1/1,000 sec) in order to minimize blur, and the aerial film must be kept rigorously level in the film plane when the picture is shot. Films commonly used in aerial photographic surveying are black-and-white panchromatic, black-and-white infrachromatic, color, and spectrally zoned film on which the image is produced with shifted color response so that distinct objects stand out more clearly.