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movement of the images without delay. 
Digital compilation 
Most data for earth observation from satellites is recorded in digital 
form and it is therefore sensible to use it in that form but bridging 
the gap between the stereoscopic line following capabilities of systems 
using photographic imates and those of digital systems presents a great. 
challenge to the system designer. 
  
When considering the requirements for a suitable instrument the 
differences in scale and coverage between aircraft and satellite data 
must be noted. Aircraft photography is usually on a format of 230 x 
230mm at a scale of less than 1:100 000; data from a satellite will not 
be defined by format and scale but by the number and size of pixels in a 
scene of fixed width but arbitary length. In order to determine the 
elements of exterior orientation the area considered should be large but 
the plotting area need be only a part of the full scene, enough to cover 
a map sheet for example. There are therefore two requirements; first to 
be able to identify control points over a wide area necessitating the 
capability to rapidly scan a complete scene but to look in detail at 
only small parts of it and, second to scan and plot a limited area. 
The operator needs to view a display in which pixel edges are not visible 
and which covers a convenient area. If a pixel size of 10m is assumed and 
a map scale of 1:50 000 then a field of view of 512 x 512 pixels with the 
ability to zoom to greater magnification should be adequate. A 512 x 512 
view could be accessed in two ways; it could be part of a larger scene over 
which the operator is roaming with a fixed cursor or could be a sub scene 
over which the cursor can be guided. The former could be a whole scene 
at reduced resolution, that is for example only every 5th pixel displayed, 
or it could be a sub scene at full resolution, the latter could be a patch 
within which a control point is located. 
The system can therefore be specified in the following manner: a complete 
scene is to be displayed and scanned at reduced resolution, patches of that 
scene to be accessed and viewed at full resolution or a subscene to be 
specified and displayed at full resolution in a mode from which line data 
can be extracted. Considering the quantity of data involved and the 
access time a scene from SPOT can be taken as an example. Two scenes 60km 
x 60km will provide stereoscopic overlap, each scene contains 6000 x 6000 
pixels and one full scene could be contained within 1024 x 1024 pixels at 
one sixth resolution. Control point patches could be contained within 
256 x 256 pixels and a convenient sub scene for plotting, covering 10km x 
10km would contain 1000 x 1000 pixels. A scheme for such a system is given 
in figure 1. 
The requirements set out in the previous paragraph can be met by existing 
technology. An I?S digital image processor for example with 12 frame stores 
could contain all the necessary data with real time access. If the 
operation was considered in two stages, orientation and plotting, then with 
12 frame stores, a one sixth resolution image and 32 control point patches 
Soul be stored and subsequently be replaced by a 15 x 20km sub scene for 
plotting. 
Case (1982) sets out a specification for an instrument for digital plotting 
from aerial photographs, he assumes a digitisation level of 10um over a 
230 x 230mm photograph and a scanning speed of 2.5mm sec” !, this is a