The science of photogrammetry relies on some kind of a photographic record as the source of data. Such a record represents a two-dimensional registration of, in general, a three dimensional object. After certain reduction, each photograph may be regarded as a central perspective projection of the object. This reduction is warranted because the original light ray from an object point, which is represented by a straight line in the perspective theory, is in fact a curve. In addition, the position of the latent image on the photographic emulsion undergoes shifts due to variation in environment from exposure to processing and measurement. All such effects are termed "systematic" since they can be modelled mathematically. They include: 1. Atmospheric refraction 2. Earth curvature (due to mapping a spheroidal surface on a plane, that of the photograph). 3. Lens distortion, both radial and tangential 4. Image deformation due to emulsion and base instabilities. 5. Systematic errors due to characteristics of imaging and measuring equipment.