Photogrammetry is the art, science and technology of obtaining reliable information about physical objects and the environment through the process of recording, measuring and interpreting photographic images and patterns of recorded radiant electromagnetic energy and other phenomena. Data obtained from aerial photography via photogrammetric processing is often utilized with GIS.

Through the use of highly specialized equipment, extremely accurate three-dimensional data can be extracted from aerial photographs by overlapping photographic shots. This data can be used within GIS or 3D engineering software packages, such as AutoCad, or in the production of maps and plans. The height of the ground above sea level and the height of the geographic objects can be determined from these three-dimensional views.

View details of digital photogrammetry projects that Aruna Technology has carried out.

Aerial photography is an excellent resource to assess the visible features that exist on the surface of the Earth. It is often easier to interpret a photograph than it is to interpret symbols on other resources such as maps, plans or vector based GIS systems. Often more information can be visualized on a photograph than can be represented on one of the alternatives. Also, it is cheaper to take an aerial photograph than it is to capture large amounts of data into a GIS database or map. In the majority of cases, the information contained in a recent aerial photograph will be more up to date than information available from maps or in GIS systems.

Recent advances in technology have dramatically increased the utility of aerial photography. Until recently, the large file sizes of high-resolution aerial photography prevented their use in many applications, however modern hardware and software has removed the problem. In the area of GIS in particular, it is now common to use an orthorectified aerial photograph, with vector data overlaid, in order to promote the use of both types of data.

Photographs contain many distortions and displacements that affect the accuracy of measurements that can be made from them. The main distortions are:

• Relief displacement;
• Tilt displacement;
• Earth curvature;
• Lens distortion; and
• Film shrinkage.

A digital orthophotograph is a photo-quality digital image (a raster image) of surface features in their geometrical-corrected, true map position. Inaccuracies inherent in simple rectified photographs - owing to such factors as displacement, tilt and distortion - have been eliminated through a process known as differential rectification (conventional aerial photos are overlapped and distortions are eliminated), rendering surface features at their highest degree of positional accuracy. In essence, orthophotographs are 'photomaps'.

Orthophotos make excellent base maps for compiling data to be input to a GIS or overlaying and editing the data already incorporated into a GIS. Orthophotos increase the communication of spatial data, since data users can often relate better to an orthophoto than a conventional line and symbol map or display.