EuroSDR Projects - Host Ireland

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road cluster is refined by removing big open areas, i.e., buildings, parking lots, fields, etc., again
by means of a fuzzy classification based on a shape descriptor using the Angular Texture
Signature (Zhang and Couloigner 2005a, 2006b). Road segments are found from the refined
clusters via a localized and iterative Radon transform with window size 31 by 31 pixels with
improved peak selection for thick lines (Zhang and Couloigner 2005b, 2005c). The segments are
grouped bridging gaps smaller than five pixels and forming intersections. Finally, only segments
longer than twenty pixels are retained.
The results of the evaluation are summarized in Table 1. It is ordered in the first instance according to
the test areas (from analog to digital aerial to satellite data) and in the second instance alphabetically
according to the group and possibly its approaches. For each test area where more than one result is
available, the best result in terms of the geometric mean of completeness and correctness is marked in
bold. In addition, all values for completeness or correctness, which are beyond a value of 0.6 or 0.75
respectively, are marked in bold. These numbers can be seen as a lowest needed limit so that the
results become practically useful. The value for correctness was set to a higher value as experience
shows that it is much harder to manually improve given faulty results than to acquire roads from
scratch. To be of real practical importance, in many cases both values probably need to be even
higher, e.g., for correctness around 0.85 and for completeness around 0.7, but we have chosen the
lower values, to distinguish ‘the probably useful’ for the obtained results from the rest.
In Figures 1 to 10 we give one result for every image, besides the details from Ikonos3. Only for the
latter have we obtained a larger number of results in the range beyond 0.85 for correctness and 0.7 for
completeness deemed suitable for practical applications. Also because of that, we show some of the
results for Ikonos3 and its details in Figures 5, 7, and 10 using a larger size. As the result for Beumier
for Ikonos1_Sub1 is very interesting and the result of Gerke_WB for Aerial3 is definitely above 0.6
and 0.75 for completeness and correctness, we also present them larger in size in Figures 6 and 2.
Figure 1: Aerial1 – Zhang (left), Aerial2 – Gerke_WB (right); correctly extracted roads are
given in green, incorrectly extracted roads in blue, and red marks the part of the ground truth
for roads which could not be extracted.