REL SE = 4
i
ACCURACY IMPROVEMENT OF DIGITAL MATCHING FOR
EVALUATION OF DIGITAL TERRAIN MODELS
Dan Rosenholm
Royal Institute of Technology, S-100 44 Stockholm, Sweden
ABSTRACT
Accuracy aspects of matching algorithms based on the least square
method are investigated. Parallaxes computed automatically with
matching methods have been compared to manual measurements made in a
stereocomparator. Examples have been taken from large scale and small
scale aerial photographs and from close-range photogrammetry. Different
linear geometric and radiometric parameters have been used Other
changes in the algorithm have also been investigated such as the effect
of using different window sizes. The precision obtainable were between
10-15 um, expressed as root mean square (r.m.s.) deviation. The results
show that window sizes below 20X20 pixels should not be used, that
affine parameters should not be used when the window size is lower than
30X30 pixels, all for reasons of reliability.
A multi-point matching method using finite elements has been formula-
ted. Parallaxes of points in a grid connected with bilinear elements
are computed simultaniously directly from image data using the least
square technique. Constraints, such as minimizing of curvature, are
implemented as ficticious observations. Preliminary tests of the multi-
point matching method have been performed.
INTRODUCTION
One of the research areas currently of most interrest in photogrammetry
is to find methods for automatic measurement of parallaxes and co-ordi-
nates in digital stereo images. In a paper entitled "On the Geometric
Precision of Digital Correlation" Fórstner (1982) presented a method
for grey level matching of images based on the least square method. For
photogrammetric purposes the least square matching method has great
advantages due to its flexibility. The least square method may be used
in connection with a number of well known mathematical tools of which
some will be used in this investigation.
In another important paper "Quality Assessment of Object Location and
Point Transfer Using Digital Image Correlation Techniques" Forstner
(1984) made a valuable rewiew of earlier results using matching techni-
ques for parallax measurements. He expresses the results as:
"There seems to be a tendency of the empirical results towards subpixel
accuracy namely 0.1 pixel or better, though already in 1973 Bernstein
and independently in 1975 Cafforio and Rocca reached this figures. They
seem to be realistically obtainable under production conditions and
come very close to the coinciding results from computer simulations and
theoretical developments."
Pertl has in 1984 and 1985 reported practical results from the use of
the least square method (16X16 pixels window and affine parameters) in
an analytical plotter equipped with CCD-cameras. Results from relative
orientation performed using this method show a standard deviation of
= 573 *
