-23- 
The indirect method also offers the simple possibility to use other types of grey level assignments 
than by nearest neighbour. Two other types demonstrated in /9/ are: 
a. the bilinear interpolation in which the intemsity of the calculated image point x'y' 
  
is determined from the 4 neighbouring image pixels (u', v'); (u' 7 V'*1,v); (u'+1,v'+1) 
assigning weights p' proportional to 1/distance to the respective pixel center: 
G(x',y') - py d (u',v'+1) + p d(u',v') + P5 d (u'+1,v'+1) 
+p' d (u'+1, v') 
b. the cubic convolution in which 16 neighbouring pixels are used: 
d(x',y') = Py d(u' -1,-v!'-1) t... +P, gd (u'-2,v'-2) 
These filters constitue a form of image processing in which discontinuities are minimized. 
The advantage of digital rectification procedures in general is that they may be linked with 
image processing technigues. An example taken from a section of a Landsat - image. (left: elec- 
tron beam recorder bulk image ; right: digitally rectified and image processed CCT-printout) 
(see /50/ ) demonstrates the value of image processing as a means of image enhancement. 
Fig. 1 
Landsat Image of 21. 9. 1972 
of the lower Elbe Region 
bulk image imageprocessed, 
rectified 
C.C.T.printout 
For this reason digital rectification computer programs have been written as part of various 
image processing routines. The first known application of such rectification routines has been 
at Jet Propulsion Laboratories of NASA at Pasadena, California. here the "VICAR" - Image Pro- 
cessing Language contains direct and indirect geometric transformations. The basis for the geo- 
metric transfer are either low order correction polynomials or a weighted average calculation. 
Special routines have also been written for the problem of rotating an image by arbitrary angles 
(e.g. 90°) ; this is a special problem if handled in the normal manner it would exced available 
computer storage. 
Another geometric program package has been installed at the Laboratory for Applications of 
  
Remote Sensing at Purdue University (Anuta /2/). and a third at IBM in Gaithersburg, Md.