83 
For quick applications using normal and wide angle cameras, 
the baseline to (the nearest) object distance ratio should be 
roughly 2:10. 
Further topics for research in basic stereoscopy are : 
tF the generation of a lost stereo mate : 
If the object is still accessible, the missing stereo mate 
preferable should be taken from the real object. If at least a 
model of the object is available, the single picture can be 
projected onto the model and the stereomate can be taken. 
Further research is necessary to solve this task by computer. 
• the geometric potential of (available) stereo imagery ; 
considering the effect of the so called stereoscopic 
exaggeration : 
A first idea of solving this problem can be obtained from 
dealing with stereo imagery, based on normal case 
photography, taken with the unknown baselength b, and an 
unknown focal length c. In case of a measured object depth 
Ahl and the corresponding x-parallax Apxl, such imagery 
allows to calculate Ahi from Apxi for different features 
according to the parallax formula 
Ahi = F x Apxi 
with F^Ahl / Apxi = (h/c) x (h/b) 
Of course, strongly this formula is limited to differential model 
areas. 
A devinition of an assumed object depth Ahl has a constant 
influence on the scale factor F. It influences the scale of the 
object depths Ahi as calculated from horizontal parallaxes Apxi 
in the same manner. 
Additional topics for research in basic stereoscopy beside others 
are : 
• the complete history of stereo imagery, including the 
stereoplasticon; 
• the benefit of combining day-and night stereomates; 
• the benefit of stereo panorama imagery; 
• the effect of different pixelsizes for the stereomates; 
• the presentation of terrestrial stereoimage blocks; 
• the extention of the optical sharpness within a model, if 
different focal lengths for the stereomates have been 
applied; 
• the simulation of the perception of the “natural” 
appearance of the object dimensions; 
• the market potential for stereo imagery; 
3.3 Basic Education in Imagerectification 
Of similar importance for undergraduate education in 
Fotomatics is the rectification of single images of plane objects, 
based on 4 controlpoints. This 2 dimensional plane to plane 
transformation, nowadays digitally based on PC, can be 
achieved independent of the focal length. 
Videoclips, clearly indicating the target, successfully have been 
used, to replace sketches and signals for controlpoints for 
rectification and triangulation purposes !. 
3.4. Basic Education in Image-Triangulation 
However, the need to consider the focal length for 3dimensional 
photogrammetric purposes is not enough understood in practice. 
The general idea of this requirement for the focal length for 3D 
Photogrammetry can be devired from a forward intersection 
sketch of at least 2 images . In a photogrammetric normal case 
situation the triangle to determine the object point P is defined 
by the given base line and the 2 adjacent angles a and p, see 
Fig. 1. According to the sketch, the triangle O’O” P is not 
defined, if the focal length c is unknown, because the pure 
knowledge of the image coordinates x’ and x” for the image 
points P’ and P” in both images is not sufficient, to define the 2 
adjacent angles a and P; i.e., as if a conventional space 
intersection lacks the adjacent angles to be measured with a 
theodolite. 
This is the reason, why snapshots of hobby Photogrammetrists, 
without recording the focal length in general hinder a 3D 
evaluation. This statement is also valid for more sophisticated 
photogrammetric situations. 
Looking on the importance of the focal length, it is difficult to 
understand, why only a view Photogrammetric cameras 
available show the focal length used on the image frame. 
Of course, if a sufficient amount of object coordinates is known, 
the unknown focal length subsequenltty can be calculated. 
image 1 image II 
Fig. 1: Need of focal length for forward intersection purposes 
(plane view) 
For practical applications there is no need to know the base 
length a priori. In the photogrammetric normal case the base 
length can be derived only from 2 given object points shown in 
both images. 
In addition the image triangulation task can be visualized from 
multiple forward intersection, with unknown base lengths. 
Consequently the relation between these base lengths can be 
derived from multiple point determination. 
The golden photogrammetric rule for object documentation is to 
verify complete object coverage with known focal length by 
★ stereo synoptic photography for overview and possible 
triangulation purposes, 
★ single imagery for triangulation purposes led by the idea of 
multiple forward intersection and 
★ stereo detail photography for interpretation and geometric 
purposes. 
★ in addition object features, in particular object coordinates, 
should be determined for geometric purposes, if possible. 
Finally the forward intersection has been replaced by the bundle 
block adjustment, including multiple imagewise backward 
intersections, based upon collinearity equations. 
OUTLOOK 
Today digital image processing systems and GIS systems stay 
within the good tradition of Photogrammetric instrumens. There 
is no doubt, these modern tecnology in conjunction with GIS