International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B1. Istanbul 2004 
  
A digital model of the elevation of the visible surface, known as 
a Digital Surface Model (DSM), can be generated from satellite 
images through the use of automatic image matching. 
However, a digital model of the elevation of the bare earth, or 
DEM, is what is usually requested. In some cases and 
depending upon the applications and accuracy requirements, a 
manual reduction of a DSM to a DEM is too time consuming, 
but this step can also be done automatically by qualified 
filtering (Jacobsen 2001). A sequence of geometric tests can be 
used to detect and remove the points located above the bare 
earth. The model is classified as flat, rolling or mountainous 
and the required tolerance limits are determined by data 
analysis (Figure 4). 
When doing automatic image matching with high resolution 
space images it is important to have images acquired under 
similar conditions — similar sun elevation and atmospheric 
conditions, and no change of the object. This is best achieved 
using images acquired during a single orbit. If this is not the 
case, problems with automatic image matching cannot be 
avoided, and matching may fail entirely. The DSM shown in 
figure 5 is based on IKONOS images acquired 12 seconds 
apart. Most of the points have been correlated with a correlation 
coefficient exceeding 0.95. On the other hand, matching of an 
IKONOS image pair acquired three months apart with a change 
of sun elevation from 67° to 42° failed nearly entirely. Some 
modestly accurate results were obtained in built-up areas. A 
time interval as short as 12 seconds is not usually necessary, so 
two QuickBird images acquired 10 days apart produced good 
results. 
6. CONCLUSIONS 
Mapping with high resolution space images is becoming more 
and more important. For several applications there is direct 
competition between aerial and space images, and the choice of 
which product to use is purely economic. Satellite triangulation 
is integrated in the Z/I ImageStation application suite, allowing 
individual images or blocks of images to be rectified. By 
processing blocks, the number of ground control points can be 
reduced and misalignments between neighboring images will 
be avoided. 
REFERENCES 
Büyüksalih, G., Oruc, M., Jacobsen, K., 2004. Precise 
Georeferencing of Rectified High Resolution Space Images, 
ISPRS Congress, Istanbul 2004 
Dowman, I, Dolloff, J.T., 2000. An Evaluation of Rational 
Functions for Photogrammetric Restitution, ISPRS Amsterdam 
2000, IAPRS, VOL XXXIII 
Fraser, C.S., Hanley, H.B. and T. Yamakawa, 2001. Sub- 
metre geopositioning with Ikonos GEO imagery. Proc. Joint 
ISPRS Workshop “High Resolution Mapping from Space 
2001”, Hanover, 19-21 September, 8p. (on CD ROM) 
Grodecki, J., 2001. Ikonos stereo feature extraction - RPC 
approach. Proc. ASPRS Annual Confereñce, St. Louis, 23-27 
April, 7 p. (on CD ROM) 
Jacobsen, K. 2001. New Developments in Digital Elevation 
Modeling. Geoinformatics, June 2001, pp 18 — 21 
152 
Madani, M, 1999. Direct Versus Indirect Methods of Sensor 
Orientation, workshop of ISPRS Commission III, Barcelona, 
November 25-26, 1999, (on CD-ROM) 
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