  Surveillance and surveying are two important applications of empirical
research. A major part of terrain modelling is supported by photographic
surveys which are used for capturing expansive natural surfaces using a wide
range of sensors -- visual, infrared, ultrasonic, radio, etc. A natural surface
is non-smooth, unpredictable and fast-varying, and it is difficult to capture
all features and reconstruct them accurately. An orthographic image of a
surface provides a detailed holistic view capturing its relevant features. In a
perfect orthographic reconstruction, images must be captured normal to each
point on the surface which is practically impossible. In this paper, a detailed
analysis of the constraints on imaging distance is also provided. A novel
method is formulated to determine an approximate orthographic region on a
surface surrounding the point of focus and additionally, some methods for
approximating the orthographic boundary for faster computation is also
proposed. The approximation methods have been compared in terms of
computational efficiency and accuracy.
