e From the 186 perfect image triples 149 succeed
when submitted to the iterative registration procedure.
There are 37 constellations which yield a final number
of hits in the range of 10 to 45 which is again signifi-
cantly lower than the acceptance threshold. Note, that
a set of 45 correspondences implies that there are at
least 38 correct matches which were missed due to too
large residuals. Thus, the resulting orientation esti-
mate cannot be satisfying and, consequently, rejecting
a total match which does not yield the maximum num-
ber of 83 correspondences is appropriate and justified.
e The number of 149 successfully matched image triples
represents 7096 of all perfect constellations and 2296
of the total number of valid image triples. Thus, we
can conclude that on average we have to perform 4.5
random trials until the matching process signals suc-
cess. All inappropriate constellation matches are guar-
anteed to be rejected, while the choice of the accep-
tance threshold is highly uncritical.
4. SUMMARY AND CONCLUSION
We presented an approach for automatic exterior orientation
of aerial images which is based on the use of manhole cov-
ers as landmarks. For this purpose we developed a landmark
extraction scheme which enables us to automatically detect
a quarter up to a third of all manhole covers visible within
an image while the error rate is reasonably low. Using the
positions of the detected landmarks for estimating the ori-
entation parameters requires to identify each landmark, i.e.,
to find the correct correspondence with a landmark from the
cadastral database. This database includes the geodetic co-
ordinates of all manhole covers that exist within the observed
area. Thus, we have to match landmark constellations, where
the set of image landmarks is incomplete and distorted (un-
detectable covers and false detections, resp.). We introduced
a matching approach which is based on matches of triples of
landmarks. In a randomized selection process a triple match
is hypothesized; the match is verified by estimating the orien-
tation parameters and looking for additional correspondences,
both combined in an iterative process. Candidate selection is
efficiently implemented using indexing through geometric in-
variants. In our experiments the matching approach proofed
to be successful: It robustly yields the correct total match.
Based on a large number of correspondences, the final match
provides a reliable estimate of the exterior orientation param-
eters.
We conclude that by combining both, a robust and precise
landmark extraction scheme with a reliable matching mech-
anism, we are able to provide an effective approach to au-
tomatic exterior orientation of aerial images. Our approach
offers the opportunity to automate the orientation process
in urban environments, presuming that the approximate im-
age scale and the approximate location of acquisition can be
derived from the flight plan. It requires that the geodetic
coordinates of manhole covers are available from cadastral
databases in digital format. This might not be the typical
case today, but triggered by the transition from analog to
digital photogrammetry and by the rapidly spreading use of
GIS-technology, there is a strong trend towards digitalization
in many cadastral tasks.
152
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B3. Vienna 1996
SOURCES OF DATA
Aerial photographs and map material used in our experiments
have been kindly provided by the Vermessungsamt Hamburg.
For illustrations we used the image FIR 87-20-828 acquired
by the Gesellschaft fiir technische Photogrammetrie m.b.H,
Hamburg. Reproduction is done with the permission of the
Vermessungsamt Hamburg. Digital coordinates extracted
from the cadaster of the sewerage system of the city of Ham-
burg have been kindly provided by the Hamburger Stadtent-
wasserung, Anstalt des Offentlichen Rechts, Hamburg.
ACKNOWLEDGMENTS
We thank Rainer Sprengel for constructive suggestions with
respect to the landmark model, Joachim Rieger for suggest-
ing the use of invariants, and Carsten Schroder for support
in the implementation and for making valuable contribu-
tions to the revision of this manuscript. For providing the
data material we thank Mr. Rohwedder (Vermessungsamt),
Mr. Sachweh (Vermessungsamt), and Mr. Gleim (Stadtent-
wasserung). Parts of the work reported here have been done
in the LUKAS-project carried out at the Al-Laboratory at
the Computer Science Department, University of Hamburg,
in cooperation with the Ingenieurbüro Basedow and Tornow
GmbH, Hamburg.
REFERENCES
Drewniok, C. and K. Rohr, 1995. High-Precision Localiza-
tion of Circular Landmarks in Aerial Images. In Sagerer, G.,
S. Posch, and F. Kummert, editors, Mustererkennung 1995 —
17. DAGM-Symposium, Bielefeld, Germany, 13.-15. Septem-
ber 1995, Informatik aktuell, pages 594—601. Springer-Verlag,
Berlin — Heidelberg — New York, 1995.
Gülch, E., 1995. From Control Points to Control Structures
for Absolute Orientation and Aerial Triangulation in Digital
Photogrammetry. Zeitschrift f. Photogr. u. Fernerkundung
63 (3), 130-136, May 1995.
Lenz, R. and P. Meer, 1994. Point Configuration Invariants
Under Simultaneous Projective and Permutation Transforma-
tions. Pattern Recognition 27 (11), 1523-1532, 1994.
Meer, P., S. Ramakrishna, and R. Lenz, 1993. Correspon-
dence of Coplanar Features Through P?-Invariant Represen-
tations. In Mundy, J.L., A. Zisserman, and D.Forsyth, editors,
Applications of Invariance in Computer Vision, Ponta Del-
gada, Azores, Portugal, October 1993, volume 825 of Lecture
Notes in Computer Science, pages 473-492. Springer-Verlag,
Berlin — Heidelberg — New York, 1994.
Schickler, W., 1992. Feature Matching for Outer Orienta-
tion of Single Images Using 3-D Wireframe Controlpoints. In
Proc. XVII ISPRS Congress, Washington, D.C., 1992, volume
XXIX, part B3 of International Archives of Photogrammetry
and Remote Sensing, pages 591-598. American Society for
Photogrammetry and Remote Sensing, Bethesda, MD, 1992.
Schickler, W., 1995. Ein operationelles Verfahren zur au-
tomatischen inneren Orientierung von Luftbildern. Zeitschrift
f. Photogr. u. Fernerkundung 63 (3), 115-122, May 1995.
Vosselman, G. and N. Haala, 1992. Erkennung topographi-
scher PaBpunkte durch relationale Zuordnung. Zeitschrift
f. Photogr. u. Fernerkundung 60 (6), 170-176, Nov. 1992.
KEY WORF
ABSTRAC
Convention
coordinates
along the p
coordinate
control the
project is tl
Global Pos;
of man mac
different G
adjustment
Since the
satellites in
application
triangulatic
at the ir
determined
combined (
observatior
control poi
completed
signals are
and as long
Howevet, t
assisted ae
calibration
ambiguity
(Ackermar
After kinei
the antenn
frame. Mo
national c
between tl
formulas (!
available i
ellipsoid a
