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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B2. Istanbul 2004 
hours on a Dell PC with Pentium 4, 2GHz CPU and 2GB RAM 
running Windows XP. Thus, using this not up-to-date computer 
configuration, all 1:25,000 map sheets of Switzerland could be 
processed in 36 days. Note that typical map sheets, excluding 
large urban centers, large lakes and the Alps, have about 2,500 
km of roads, with about 4595-5094 of them in rural areas. 
4.2 Geneva Site 
Our system achieves good results with the 50-cm orthoimage 
(Swissimage), similar to the ones in Thun (see Table 3). 
However, the performance (mainly the completeness) with the 
HRS data is poor, especially the 1-m IKONOS image. In this 
image, higher-class roads are usually extracted, while most 
narrow roads such as 4", 5? and 6" class roads are not, because 
the system prerequisite of 3 pixel wide roads is not fulfilled. 
The increased ground resolution in Quickbird makes more roads 
visible than in IKONOS, and also the road surface and road 
edges are clearer, resulting in a better performance. However, 
compared with the 60-cm aerial film orthoimage in Thun, the 
completeness is still rather low. 
  
  
  
  
  
  
  
  
  
  
  
  
Quality Aerial | 1K ONOS-PSM | Quickbird-PSM 
measures 50cm 100cm 70cm 
Completeness [90.89% [54.22% 72.68% 
Correctness 95.36% {81.22% 89.58% 
Length of 50.72 50.72 50.72 
reference (km) 
Length of 48.35 33.87 42.16 
extraction (km) 
RMS x 0.62 0.93 0.81 
error (m) y (0.56 0.82 0.75 
Mean x 10.07 -0.73 -0.44 
error (m) | y |-0.05 0.34 0.50 
Process time (s) |1510 992 924 
  
  
  
  
  
Table 3. Quality evaluation of the results in Geneva site. 
It is apparent that the definition quality of an object does not 
depend only on the pixel size but other image quality factors 
too, and that each object type can be favourably extracted 
within an object-specific image scale range. Critical factors 
influencing image quality, like atmospheric and illumination 
conditions, sensor and sun elevation and image sharpness are 
much less or not controllable with spaceborne sensors compared 
to airborne ones, resulting thus in inferior image quality and 
object definition with the former, even if the ground pixel size 
is similar. Both HRS images lead to accuracy (RMS) of less 
than 1m. The mean values are high, due to a systematic bias 
caused by probable errors in the transformation from the 
coordinate system of Canton Geneva to the Swiss coordinate 
system. Thus, in reality the road accuracy from the HRS images 
is similar or slightly better than that from Swissimage, if the 
HRS orthoimages are produced with a submeter accuracy 
DSM/DTM (as in this case) or the sensor elevation is high. Fig. 
8 shows several examples of extracted roads and road junctions 
from the Swissimage, IKONOS and Quickbird orthoimages. 
In the Geneva test site, no extraction is applied to the roads 
inside the villages since the sizes of the villages are large and 
are classified as urban area. The non-extracted roads are usually 
those in fields with very weak edges. An example is given in 
Fig. 9. False extraction in Swissimage occurs when a road in 
fields is neighbouring with road-like lines (Fig. 10a). Several 
false extractions are also because the actual road width differs 
from the width expected for the given road class. This was 
; © | ; 
noticed with several 5" and 6" class roads. An example is 
shown in Fig. 10b, where a ca. 6.6-m wide 5" class road is 
incorrectly extracted 
  
  
  
     
  
  
    
Figure 8. Examples of extracted roads and road junctions in the 
Geneva site orthoimages. The black lines are the 
results and the white lines are the VEC25 roads. Left: 
Swissimage, middle: IKONOS, right: Quickbird. 
  
s 
Figure 9. Road in field with weak edges can not be extracted 
from Swissimage. Black line: reference data. White 
line: VEC25 roads. 
Figure 10. Examples of false extraction from Swissimage. 
Black line: extraction results. White lines: reference 
data. (a) a road is incorrectly extracted due to the 
interference of many road-like features. (b) false 
extraction caused by assuming wrong road width for 
the given road class,