Combination of satellite-derived raster and vector data for 3D city modelling

Since the launch of satellites (IKONOS, QuickBird, the future Pleiades) that deliver images with a spatial resolution of more than one meter, the modeling of an urban scene became a new possibility of remote sensing. The high level of spatial detail allows identifying specific urban objects, by which an urban environment can be measured in its three spatial dimensions (X, Y and Z). The possibility of very high resolution sensors to capture a stereo pair of images in one orbital pass gives high quality stereo models that allow the generation of a digital surface model (DSM).

The strategy followed in the proposed research project is to combine 3D raster and 3D vector data in order to obtain a more reliable digital surface model of an urban environment. Vector data structures are very well suited to represent entity data models, in the case of this proposal that will be buildings. Accurate automatic vector extraction from satellite images has improved a lot over the last years but still has not reached the level of the human operator. For this reason, the vector extraction from a satellite stereo pair will be done interactively in the stereo model. Stereo measurement of an object, depicted in a stereo pair, is only visible if this object is clearly visible in both the images of the stereo pair. This means that for most buildings no stereo measurement is possible near to it at the ground level. Normally only points at rooftop level or at the level of the cornice can be measured. As a consequence, building volume cannot be directly derived from the 3D-measurement of the building outlines. Therefore we propose in this project to use a digital surface model (DSM) of the non-built area will be used to retrieve the ground level height value.

In this paper we demonstrate the use of the C-NAV 2050G Differential GPS (DGPS) to measure ground control points (GCP) for an IKONOS stereo scene over Cairo. The 2050G sensor consists of a 22-channel precision GPS sensor with two additional channels for receiving Satellite Based Augmentation System (SBAS) signals and an L-Band demodulator for reception of C-Nav StarFire Network correction service, for decimeter-level position accuracy.
The construction of the hybrid 3D city model is described. Processing the IKONOS stereo pair in epipolar projection brings us immediately to the step of absolute and relative orientation. The buildings are measured interactively in the stereo model. A digital surface model is derived through automatic image matching. The occluded areas that are due to the measured buildings are removed and interpolated from the surface model. Finally the 3D features (polygons) are the converted to a solid geometry, which allows calculating the volume of the buildings.