to the particular transformation to be used is made only after a detailed 
examination of the input data and depends on the particular recognition 
task to be performed. 
The final functions to be performed by the preprocessor are 
geometric and radiometric corrections for the non-ideal characteristics 
of the sensor, sensor platform, and earth-platform relationship. An 
ideal image of a portion of the surface of the earth should be distortion 
free. The theory of map projections says this is impossible except 
as an approximation for small areas. Even if the effect of all other 
distortions could be removed the small geometrical errors due to map 
projection errors or earth curvature would still remain. Line scan 
imagery is taken over a time interval equal to the time it takes to 
travel over the ground being imaged making the geometry of line scan 
imagery complex due to platform motions as well as sensor distortions. 
Achievement of geometric fidelity and precision requires information on 
the navigational location and orientation of the platform. Compensation 
for sensor distortions and aircraft motions (roll and yaw) is routinely 
carried out but to date only incompletely and not entirely satisfactorily. 
Radiometric corrections are important not only for sensor induced errors 
but also such earth-platform related effects as changes of radiometric 
intensity with viewing angle. A discussion of these distortions for 
ERTS RBV and Skylab S-190 has been given by Steiner [32] which mentions 
ERTS MSS also. 
Potentially many problems may exist in each and every data set 
which, if not corrected, could significantly reduce the accuracy of 
recognition results for that data. Some of these potential problems 
are instrument-related while others are associated with the radiation 
environment and the scene being scanned. These problems include: 
1) level shifts and gain changes in the recorded data resulting from 
instabilities in system electronics and tape speed; 2) misregistration 
of data between spectral bands due to unequal resolution in all the 
bands, the lack of optical alignment (either by design or otherwise) of 
the detectors in all bands, or the imperfect alignment of the tape 
recorder record and playback heads; 3) noisy data resulting from a 
combination of insufficient radiation input and lack of detector 
sensitivity; 4) variations in signal levels as a function of scan angle 
due to nonuniform angular sensitivity of the scanner, the effects of 
atmospheric scattering, and bidirectional reflectance effects; and 
5) changes in the scene illumination level during the data collection 
mission. 
An illustrative example by Nalepka [33] will indicate one approach. 
Three segments in the 1971 Corn Blight Watch Experiment intensive study 
area of Western Indiana over which multispectral scanner data were being 
gathered on a biweekly basis were selected as potential sites. To 
support this investigation, ground information, in addition to that 
provided by the county agents, was gathered throughout much of the 
scanner data collection period [34]. This information included the 
location of fields, the crop planted therein, as well as the condition 
of the crop.