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But from an interpreters point of view an image should present, apart 
from optimum contrast, also: 
- clarity of patters; 
- relief, or no effects of relief, depending on application; 
- information from different seasons; 
- correct or specifically coded radiometric densities; 
- correct geometry, etc. 
Pre-processing may aim at optimizing a given image so that these require- 
ments for interpretation are met. The interpreter can then work qualitatively 
with images, but many encounter difficulties in various types of quantitative 
Work: The eye can differentiate many tens of different gray of color tones 
on & gray or color wedge; but this may reduce to only 1o - 20 when the gray- 
tones or colours are chaotically mixed, as is the case in remote sensing 
images. Digital recording and analysis offers abundant capabilities in this 
respect: a system with 8 bits storage per picture element (pixal) permits 
to operate quantitatively with 256 gray (color) tones. 
An entire new perspective opens up in the area of radiometric and geo- 
metric manipulations. Digital approaches improve the flexibility of geome- 
tric corrections that have been previously possible, e.g. by photogrammetric 
rectification and orthophoto techniques, however, with limitations by opti- 
cal and mechanical designs. Radiometric corrections, on the other hand, 
were hardly possible at all until the advent of image processing methods. 
These, however, have led to impressive capabilities to combine data from 
different seasons (Blanc et al., 1977), different sources (Anuta et al., 
1976; Dailey et al., 1978) and permit to control perturbating effects of 
the sensing systems (Soha et al., 1977),and of the relief (Kahle et Ale) 
1977). 
Data compression of multiple images is another important image pre-pro- 
cessing step. Digital techniques offer flexibility of applying mathematical . 
and statistical methods for this task. 
Visual image interpretation is seen as opposed to automatic analysis. 
In fact, however, the two may complement or overlap each other in an inter- 
active type of operation so that automation would only support some of the 
visual interpretation. Such approaches are presently emerging in a wide 
variety of interactive hardware and generally involve a digital component. 
Digital pre-processing of aerial photography is very uncommon. Experienced 
photo-interpreters argue that there is no increase in the diagnostic value 
of photography and that apart from dodging no pre-processing techniques are 
needed. These considerations, and the effort required for digitization as 
well as the potential loss of resolution that could occur, have so far pre- 
vented the use of existing image processing capabilities for this purpose. 
3. PRE-PROCESSING OF SINGLE IMAGES 
According to Anuta (1977) image pre-processing serves to "rectify sen- 
sor problems, placing the data in the form desired for analysis, and to 
combine, transform or transmit the data". One may groups and discuss these 
pre-processing techniques in seven categories, resulting from a sub-division 
of categories as proposed by Goetz et al. (1975) and Pratt (1978) (compare 
section 1.):