13 
Mpst current orthophoto printers implement only the locational transformation and 
a change of the image scale. Locational transformation is applied either to streams 
of points along the axes of bands (for continuous printing), or to discrete points 
representing the centres (or a grid) of patches. 
im age transformation may be more or less involved. The simplest version is a 
change of scale only, whereas one of the most complex is matrix-wise transformation. 
Between these limiting cases are e. g. polynomial transformations of different orders. 
First order polynomial transformation is applied most commonly. This transformatic 
corrects for the effect of terrain slope. 
Fig, 4 shows a coarse flow chart of generating the image transformation data for be 
the optical and the electronic printers. 
Fig. 4 
Generating of data for image transformation 
Generation of control data is identical for the on-line and off-line operation. On-line 
systems require interface equipment, whereas off-line systems must be equipped with 
a fast recording unit and a control unit for the printer. 
Printing is carried out automatically. A printer comprises three basic units (fig. 5): 
the controljunit_, uiiit ( for the original photograph), and the output^unlt (for 
the orthophotograph). In fig. 5 the vertical lines represent the flow of control data for 
both the locations and the image transformations. The horizontal lines indicate the 
flow of pictorial information.