In: Stilla U, Rottensteiner F, Paparoditis N (Eds) CMRT09. IAPRS, Voi. XXXVIII, Part 3/W4 — Paris, France, 3-4 September, 2009
SEMI-AUTOMATIC CITY MODEL EXTRACTION FROM TRI-STEREOSCOPIC VHR
SATELLITE IMAGERY
F. Tack a- *, R. Goossens a , G. Buyuksalih b
d Dept, of Geography, Ghent University, Krijgslaan 281, 9000 Ghent, Belgium - (f.tack, rudi.goossens)@ugent.be
b IMP-Bimtas, 34430 Beyoglu, Istanbul, Turkey - gbuyuksalih@yahoo.com
KEY WORDS: Photogrammetry, City DSM generation, Tri-stereoscopy, Ikonos, Urban
ABSTRACT:
In this paper a methodology and results of semi-automatic city DSM extraction from an Ikonos triplet, is introduced. Built-up areas
are known as being complex for photogrammetric purposes, mainly because of the steep changes in elevation caused by buildings
and urban features. To make surface model extraction more robust and to cope with the specific problems of height displacement,
concealed areas and shadow, a multi-image based approach is followed. For the VHR tri-stereoscopic study an area extending from
the centre of Istanbul to the urban fringe is chosen. Research concentrates on the development of methods to optimize the extraction
of a surface model from the bundled Ikonos triplet over an urban area, without manual plotting of buildings. Optimal methods need
to be found to improve the radiometry and geometric alignment of the multi-temporal imagery, to optimize the semi-automatical
derivation of DSMs from an urban environment and to enhance the quality of the resulting surface model and especially to reduce
smoothing effects by applying spatial filters.
1. INTRODUCTION
The high level of detail and geometric accuracy of very high
resolution satellite data such as Ikonos imagery, has made this
kind of imagery suitable for DSM generation at feature level of
urban environments. Due to the photogrammetric complexity of
urban areas, quite some research is done to cope with the
specific problems of urban surface model generation from
standard stereopairs. As a multi-image based approach can
make the 3D modelling more robust, a methodology and results
of semi-automatic DSM production from an Ikonos triplet over
an urban area, is highlighted in this paper. From a theoretical
point of view the redundancy of a third image should lead to a
more reliable photogrammetric processing. Only a few
investigations have been published dealing with the concerning
subject. Research published in (Baltsavias et al., 2006) and
(Raggam, 2006) can be referred to.
Research is conducted within the framework of the MAMUD
project (Measuring And Modelling of Urban Dynamics) funded
by the STEREO (Support to The Exploitation and Research of
Earth Observation data) program of Belgian Science Policy.
The objectives of the MAMUD research project is to investigate
the possibilities of earth observation for a better monitoring,
modelling and understanding of urban growth and land-use
change. Urban change processes are affecting the human and
natural environment in a not unimportant way. This enlarges the
need for more effective urban management approaches based on
sustainable development. A sustainable urban management
needs sufficiently detailed and reliable base information on the
urban environment and its dynamics. Satellite imagery has
proven to be an important data source to monitor and describe
urban areas and its changes. Hereby, detailed information on the
vertical structure is vital to label urban features, to describe
urban morphology and to generate spatial metrics. If the
subsequent approach is proved to be successful, it will increase
the flexibility of producing semi-automatic 3D city models from
high resolution satellite imagery.
The complexity of an urban environment for photogrammetric
purposes will be highlighted in section 2. In section 3 the image
dataset and work area will be outlined. The different phases of
the photogrammetric processing of the Ikonos triplet are stated
in section 4. In following section, spatial filtering is applied on
the height values of the surface model to improve the quality
and reduce smoothing effects. Geometric accuracy analysis is
discussed in section 6. Finally, in section 7 experiences and
conclusions are summarized.
2. COMPLEXITY OF URBAN AREAS
A Digital Surface Model is a digital representation of the terrain
and topographic object height in a grid structure. Interpolation
of the discrete height values is needed to approximate the
continuity of the ground surface. Urban environments are
experienced as complex for 3D modelling purposes because of
the steep changes in elevation and the discrepancy between the
smoothness of the ground surface and abrupt discontinuities
caused by buildings and other urban features. Without manual
plotting or spatial filter techniques it is difficult to reconstruct
vertical walls out of VHR satellite imagery. An interpolation
technique creates a smoothed surface and causes individual
buildings will have a shape of a bell instead of the rectangular
geometry (Jacobsen, 2006). A second consequence of steep
changes in elevation is the occurrence of shadow and concealed
areas. Due to the convergent viewing angle of VHR sensors like
Ikonos, terrain features with certain height above the surface are
geometrically displaced in the imagery, leading to
dissimilarities between the stereo images.
By this distortion of its true position, parts of the ground surface
can be hidden in the satellite image. These are so-called
occluded areas. Shadow areas, which have poor contrast, and
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