The Program PALM for Automatic Line and Surface Measurement 
Using Image Matching Techniques 
H. Schewe and W. Fórstner 
Institute for Photogrammetry 
Stuttgart University 
0.Introduction 
During the last 10 years several algorithms for automatic 
Stereopsy have been developed. Theoretical and empirical 
investigations have proved that high precision measurements 
can be performed with digital image matching techniques. 
Though these procedures are not foolproof in an arbitrary 
environment they work satisfactory if certain conditions are 
met. The smoothness of the surface is the most important 
one. A severe restriction also results from the limited size 
of digital cameras with usually 512 x 512 picture elements 
(max. 2048 x 2048 today). This reduces the relative accuracy 
to 10-4 if one assumes a location accuracy of 31/20 of “a 
pixel. 
Both disadvantages can be circumvented by integrating 
digital cameras into theodolites or analytical plotters and 
by using the interpretation capability of a human operator 
in a semiautomatic system. By exploiting the high relative 
accuracy of classical surveying instruments being in the 
order of 10-5 to 10-8 as a reference the algorithms for 
computational stereo then can be used to replace parts of 
the measuring procedure. This essentially results in a 
two-level hierarchy of the measuring procedure, the upper, 
an operator and hardware level controlling the orientation 
or the position of the cameras and the lower, a software 
level performing the necessary object recognition and 
matching algorithms. This way the human operator still keeps 
control of the whole procedure, by at least measuring in 
difficult situations. The measuring procedure then possesses 
the same accuracy as the reference frame of the surveying 
instrument. A smooth interaction between automatic and 
manual measurements can always be guaranteed by proper cali- 
bration. 
This paper wants to present the concept for an automatic 
surface measurement procedure as it is realized on a photo- 
grammetric measuring device. 
The task is to determine the surface of an object at or 
in the vicinity of the intersection with a set of predefined 
planes of arbitrary orientation. This includes covering, 
esp. topographic surfaces in bands with arbitrary or regu- 
larily distributed points, the automatic detection of break 
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