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The invention of digital camera stimulates photogrammetrists
to pursue anautomatic system of photogrammetry. In
close-range photogrammetry,automatically detecting,
identifying and positioning targets are primary goalsto be
reached in recent years. Currently proposed methods utilize
targets ofregular shape, such as a round-shape target, which can
be detected and locatedeasily in a digital image. However,
a target like this kind can not beidentified among a number of
others in an image. It is therefore difficult tofind the
conjugate targets in some overlapped images. In this paper, two
roundshape targets and two digits are composed to be a
distinguishable target fromothers.By using the proposed image-
processing techniques, a compound target isnot only detectable
and locatable but also distinguishable by interpreting thetwo-
digit subimage to obtain its identification number.
There are 4 steps in the proposed method. First, a
circular-targetdetector is used to search all possible
targets, and locates the centersprecisely by using the least-
squares matching technique. Second, the scale ofthe target is
calculated by measuring the length between the centers of thetwo
round-shape targets, so that the boundaries of the two
digits can bedefined. Third, subimages of the digits are
extracted and resampled to be thesame orientation and
resolution as the provided templates of standard digits.Finally,
the subimages of the digits are compared with the templates of
thestandard digits to solve the identification number.
Three experiments were conducted to show the feasibility of
the proposedwhite color background. All the targets were
successfully and correctlyidentified in this experiment. The
second experiment is designed to findtargets in different
scales, so that several sets of targets attached in thevertical
sides of a stairs were imaged and tested. Targets tend
to bemisinterpreted when their scales are getting small. The
third experiment isdesigned to identify targets in an oblique
image. This experiment shows thatcorrect results can be
obtained when the image angle is smaller than 30degrees.
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