International Archives of Photogrammetry and Remote Sensing, Vol. 32, Part 7-4-3 W6, Valladolid, Spain, 3-4 June, 1999 
38 
from a vector database according to their attributes. An 
important aspect of registration is that all features do not need to 
be extracted, only enough to carry out the registration with some 
redundancy. 
Point features are not suitable for matching from very different 
types of data. A point which can be found by an interest 
operator in an optical scene will not necessarily have a 
conjugate in a SAR image (not least because of the speckle), nor 
in a vector database. Linear features, especially intersections can 
often be found in different data sets but in order to match them 
an initial registration is usually necessary. These are therefore 
used to refine the registration, if required, in our strategy. The 
use of polygons allows automatic initial registration and the 
possibility of refinement using points extracted from polygon 
boundaries. 
Reports on many of the issues discussed in this paper have been 
published in papers which describe two systems developed by a 
consortium led by University College London (UCL). A system 
has been developed for automatic registration of optical images 
as part of a Prototype Automatic Image Registration System 
(PAIRS) developed by Earth Observation Sciences Ltd., 
University College London, University of Stuttgart and the 
University of Oporto, for the Western European Union Satellite 
Centre (WEU) (Dowman et al. 1997). The work in the PAIRS 
project has been extended with all of the partners in the WEU 
consortium plus KTH Stockholm and the Swedish Space 
Corporation, to develop the automatic registration of images to 
maps. This is the ARCHANGEL project funded under the 
European Union Fourth Framework research programme 
(Dowman, 1998). 
The image to map process is illustrated with examples from the 
ARCHANGEL project. SPOT panchromatic data is matched to 
German ATKIS data. ATKIS is a digital database which is 
derived from the 1:5000 topographic maps. The OPTISEG 
algorithm was used to extract polygons from the SPOT data and 
the methods described by Hild and Fritsch (1998) were used to 
extract polygons from the ATKIS data. The matching was done 
using the match polygon algorithm developed for 
ARCHANGEL from Abbasi-Dezfouli and Freeman (1994) and 
a. ATKIS data 
the points extracted using the method on Newton et al. (1994). 
Figure 2 shows the ATKIS data with selected features (roads, 
forests and settlements) extracted and the original image with 
the polygons extracted and the road network overlain on the 
registered data. 
Work on extraction of intersecting features for the 
ARCHANGEL project can be found in Klang (1997 and 1998) 
and a full account of the ARCHANGEL project in 
ARCHANGEL (1999). 
The remainder of this paper will concentrate on new 
developments in polygon extraction and matching with 
examples of matching SAR data with SPOT data. 
3. POLYGON EXTRACTION 
Polygons may be extracted from imagery using three basic 
techniques: thresholding, identification of homogenous patches 
and segmentation. 
Thresholding is a simple technique but not considered to be very 
reliable. Up until now, the threshold has had to be chosen 
manually and contrast might change significantly over an image, 
making the choice of a global threshold impossible. In addition, 
there is always a lot of clutter after the thresholding. A method 
has been developed in which an image is tiled to reduce global 
differences and then to smooth the histogram within a tile to 
develop a bimodal histogram and then to automatically identify 
the threshold point. 
Homogeneous patch extraction (Abbasi-Dezfouli and Freeman, 
1994) is also straightforward but again is not consistent on all 
types of images. The algorithm works by scanning the image for 
homogeneous patches (i.e. regions with the same grey level) and 
separating them from the background. Since there is always 
some variation in patches, a tolerance value is set which 
specifies a range in which the grey level values must fall. 
b. SPOT image with extracted polygons (ATKIS motorway & 
road polygons overlain) 
Figure 2. Registration of a SPOT image with map data using ARCHANGEL software.