2.2 Orthoprojection process 
The orthophoto procedure begins normally with the preparation of 
the image material and with the adaptation of the camera focal 
length and the image size to the available orthoprojection system. 
In most cases copying processes for getting duplicates or enlarge- 
ments are necessary. This procedure can be done in special trans- 
formation printers and rectifiers. However, optical or electroni- 
cal dodging should be included in order to get even contrast on 
the photomap. 
Preparation also contains planning of the model scale and of the 
orthophoto scale, dependent on the ranges of the instruments. 
While the X and Y range for architectural work are mostly suffic- 
ient, it is difficult to fit the whole depth of the model into the 
Z range. So the model scale and the orthophoto scale in the arch- 
itectural field do not represent a value to be choosen independent- 
ly, but are determined by the instruments' limitations. Therefore, 
in order to obtain the final scale of the photomap, reproduction 
processes become necessary. 
The most important parameters for orthophotoproduction relate to 
the scanning procedure itself. 
Nearly all orthoprojectors which divide the model into parallel 
strips admit scanning only in the Y-direction, i.e. perpendicular 
to the base line. This direction seems to be suited to architec- 
tural work as well, because many objects are curved mainly in the 
X-direction. Therefore height must be changed only from strip 
to strip. 
For these reasons the scan width should be selected with care. In 
stripwise orthoprojection only those points will have exact hori- 
zontal position which are situated on the scanning profile. Points 
off the profile center change their position, depending on the 
height difference between the model and the approximating surface. 
So displacements such as gaps, double contours and mismatches 
appear in the orthophotoplan. These so-called system errors are 
greatest, if differential rectification to a horizontal plane 
(zeroth-order differential rectification) is applied, as in most 
orthoprojection systems. The amount of displacements can be 
reduced by using a smaller scan width, but scanning time and 
stress to the photogrammetric operator increase accordingly. 
Since particularly in aesthetical objects (art, architecture) 
errors in the final photograph displease the viewer, a refined 
model approximation is required. For this the orthoprojection 
system ZEISS GZ | for off-line operation seems to be exceptionally