the medium used for presentation required a photographic process limited it’s real use within 
most organizations. The concept of having these type of presentations available in digital form 
was always outside of reality until software became available to analytically resolve the 
correction process. 
In simple terms a DOI consists of a digital representation of an aerial photograph, corrected to 
account for the distortions in the original image due to camera calibrations, aircraft movements 
and terrain elevation. By removing these distortions, (also the objective of traditional 
photogrammetric mapping systems) it is possible to provide a digital image which is true to scale 
in all directions and which can be used in every way like a map. 
THE DOI PRODUCTION PROCESS 
The process required to produce a DOI consists of four main components, the source imagery, 
aerial triangulation, the digital elevation model and the rectification. 
rce Im 
Usually the source imagery consists of aerial photography taken with conventional camera 
systems, although any camera for which the calibration characteristics are known could be used. 
For best results the camera system should be equipped with forward motion compensation to 
minimize the affect of image motion (the movement of the plane during the instant of exposure). 
Also exposure conditions should be optimized to enhance the photographic quality of the 
imagery. Obviously the imagery must be free from imperfections, clouds and other marks which 
might lessen the visual quality of the resulting data set. The scale of the source imagery must 
commensurate with the intended result and this should be decided prior to the acquisition of the 
photography if possible. Traditional enlargement relationships between source image scale and 
final map scale (usually 4 or 5 times) do not generally apply to DOI because the whole 
rectification process is digital and therefore not subject to the same sources of error as found in 
traditional photogrammetric procedures. Accordingly, the final product must be defined in terms 
of pixel resolution rather than map scale. This relationship will determine how the source 
imagery is to be scanned at the initial stages of production. See Figure 1 for a comparison of 
scanning resolutions and resultant pixel dimensions at map and ground scale. With a high 
resolution graphics board on a PC it is possible to obtain 10 times enlargement of 2 m pixel 
resolution data. 
rial Triangwlation 
Aerial triangulation is a process used in conventional photogrammetry to develop the relationship 
between the measurements made in the aerial photography and the ground coordinate system. 
Aerial triangulation is a well known process and is used in virtually all photogrammetric projects 
and provides a rigorous reference framework for DOI as well. 
A new development which is beginning to become accepted as an addition to the aerial 
triangulation process is the integration airborne GPS with the camera system. This provides, 
after post processing, perspective centre coordinates in the ground coordinate system with 
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