A better way, to obtain corrections by improving the mathematical 
model, is currently under study for radar and scanners. 
b) Mapping of interpreted features 
can be carried out in a rectified photograph of plane terrain or 
in a stereo model formed from two overlapping images, if the terrain 
heights vary. The mapping of points and lines may be carried out by 
direct tracing (digitizing) or by graphical restitution in a 
stereoplotter suitable to take care of the appropriate geometry 
(an analytical plotter). Mapping of the first type is currently 
in operation at the Rijkswaterstaat in the Netherlands. The second 
type of restitution is being implemented (U.S. military organizations, 
Univ. of New Brunswick, Techn. Univ. Hannover). 
c) Image Gridding 
signifying precomputer ground coordinate values in the image has 
been implemented for imagery taken by Nimbus and by Mariner 
satellites. 
d) Differential Rectification of Images 
computer controlled electron beam deflection or generally via digital 
means according to an approximate mathematical model (bulk images) 
or according to an improved mathematical model using ground control 
to derive the correction function (precision images) constitutes the 
most effective means to rectify the imagery. If a (digital) terrain 
model is known from stereo model scanning or can be derived via 
plotted contours, then terrain displacements may also be appropriately 
corrected. 
Geometric correction programs for digital rectification are currently 
under development at various organizations (I.B.M., LARS, T.U. 
Hannover). 
e) Automatic Image Correlation 
has been suggested as a means to correlate spatially, spectrally and 
temporally different images. Experiences with electronic analog 
correlators have been gained in the attempt to derive height informa 
tion automatically for topographic mapping. Apart from the extreme 
cost these attempts have not been as successful as originally 
believed. The method of differential rectification to control 
therefore seems to be more promising. 
While at the present stage of development some results have only been 
presented for step a) at the ISP Commission III Symposium in Stuttgart, 
further progress as outlined is expected at the Helsinki Congress 1976.