2 = 1/2 1! x (k) * dy^(k)) d ... mean discrepancy of the x-,
kz3 y-coordinates in position
Thus a function d = f(c) which gives a connection between the
principal distance and the mean discrepancy of the points between the
three models can be defined (Fig. 2). If the results of the three
combinations are identical, d is 0. In this case the correct value
for the principal distance has been found.
Fig. 2: Definition of the function d = f(e)
The zero-point, d = 0 = f(c ), can be calculated by using two values,
Cys C; for the principal distance: C, «QU « boy. The mean
discrepancy di of c4 gets a negative sign vhile d,? remaíns positive.
The intersection point c. which has a smaller discrepancy (d4) than c,
may be used instead of e, now (Fig. 2). This algorithm is to bé
continued until d; becomes smaller than a threshold for the mean
discrepancy in position.
In order to get a good accuracy of the rectified points, it is very
important that the three images are taken from positions surrounding
the object. This method cannot be applied if the location of the
principal point is not identical with the image-center.
3 INTERIOR ORIENTATION FROM PERPENDICULAR VECTORS AT THE OBJECT
These algorithms are derived from graphical methods of descriptive
geometry, where they are used to rectify photographs of buildings.
Because of the high accuracy of metric cameras, these algorithms have
been of no importance for analytical photogrammetry.
3.1 Geometric Elements
Three vanishing points (F, ,F,,F,) of the edges of a Square block are
necessary to determine both the principal point and the principal
distance from one photograph /4/ (Fig. 3). The principal point H can
be found as the orthocenter of the triangle FT. F,. A cut along I
and F, gives the length of €; * HO' (0' ... pojection center turned
into the image plane).
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