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2. a drum plotter in which a light beam is successively projected on a rotating and 
stepwise advancing film drum (Bptronics Prakla); a Swedish manufacturer produced 
a drum plotter spraying ink under computer control (Lund). 
3. a system in which a digitally deflected (and mechanically shifted) light beam, 
cathode ray, electron beam or laser beam is moved over a film surface. 
Only the third type offers an output mode at varying deflections. The first two types are limited 
to rectangular pixel sizes. 
The use of digital grey level data of course also requires that the image is available in digital 
form. This is on the other hand the only means to assure transfer of the grey shade without 
information loss (untio the image is resampled to meet output requirements). Most scanners ofthe 
2 ; 
type used in satellites and in aircraft (Landsat, M'"S) already record in this manner. Analo 
tapes of other scanner types (Daedalus) or images (Reconofax, Radar, aerial photography) müst of 
course first be brought into digital form. 
The scanning of images can be performed by the following devices: 
1. a drum scanner in which a rotating film is scanned in a transparent (optronics) 
or reflected manner (F.I.M. Karlsruhe). 
2. a system in which a digitally deflected light beam, cathode ray, electron beam or 
laser beam is moved over a film surface. 
3. a flat bed mechanically controlled plotter in which sensor arrays consisting of 
charged coupled devices (Hirschberg /36/) are moved in profile mode. 
Of these devices only the second type offers arbitrary (usually limited) deflection possibilities. 
Types 1 and 3 generate a rectangular pixel image. 
An ideal output image is composed of rectangular pixels. Therefore it would be most desirable 
in a real tim pixel by pixel output to scan the image for grey level transfer in an irregular 
mode by a system of type 2. If this is not possible an off-line process must be adopted or a pro- 
cedure in which portions of the input image must be stored consecutively in a buffer. 
5.3. Mathematical Procedures for the Pixel Rectification 
  
The transfer of grey level information pixel by pixel may be accomplished in two modes (Anuta /2/, 
Baker & Mikhail /7/, Bernstein & Ferneyhough /8/, Konecny & Schuhr /50/, Schuhr /78/, Steiner /81/). 
i. Direct Method 
In the direct mode the image coordinate of the rectangular input pixel center position is used to 
calculate a point in the output space to which the pixel grey shade may be transferred.For this 
task the following equations may be chosen: 
a) Collinearity equations of the type (7b) and (7c) offer the strictest approach. 
  
For flat terrain zi is constant. Therefore the equations may directly be applied. For undulating 
terrain zi must be known for every image pixel according to (7d). This however requires a previous 
application of the indirect (inverse) method to transfer all D.T.M. points, for which elevations 
and positions are known into the image (see section 2). This information may then be interpolated 
for a zi assignment to each input pixel. Another more complicated approach might be to intersect 
the directed line of the image ray with the surfaces of the D.T.M. in an iterative manner until 
the intersection lies in the proper area element of the D.T.M. 
b) When simplified equations are used for flat terrain the image pixel center coordi- 
nates may be used to transfer the grey value to an output pixel with the coordinates x. and Y. S 
li T9