# Full text: Sharing and cooperation in geo-information technology

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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
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