Full text: XVIIth ISPRS Congress (Part B4)

  
  
4. CASESTUDY 
In this section we illustrate the presented concepts by 
an example. In particular we describe how explicit 
knowledge may guide the segmentation. 
We wish to verify if the roads in the road database are 
still present in the scanned aerial photograph. This is 
only a minor part of the procedure for automatized 
updating, but it illustrates very well the complexity of 
the problem. The strategy followed arises from the 
nature of the object (a network), and the nature of the 
task (updating). This task results in an extra know- 
ledge source: a road database. 
  
In fig. 1 the present state of the road database is 
shown, representing the "old" situation in object space. 
Fig. 2 shows an aerial image of the present situation in 
object space. The image is the blue part of a true 
colour aerial photograph, scanned in blue, green and 
red with pixel size 1.60 m.. The present situation 
shows a crossing instead of the T-junction in the old 
situation. It connects a new road to the network, which 
crosses the highway by a fly-over. In addition a new 
exit and a new slip-road are present. 
The high degree of complexity will be illustrated by 
considering just a single road segment, taken from the 
database. Its axis represents an arc of the road network 
in the database. The road segment is superimposed on 
the image (see fig. 2). 
Since the location and orientation of the road is 
approximately known, only a small region of interest 
(ROI) around the road segment can be examined. 
Using the real-world knowledge that roads are linear 
and have parallel boundaries, a region of interest is 
defined, such that the ROI rows are perpendicular to 
the local direction of the axis of the road. The road 
boundaries in the region of interest form straight lines 
parallel to the ROI columns. In this way the ROI can 
handle any type of curvature. The width of the ROI 
depends on the known road width and the distance to 
adjacent parallel roads, stored in the database. 
Fig. 3 shows the region of interest of the road segment 
in fig. 2. 
4.1 High level: hypotheses generation 
All possible changes that a road segment may 
undergo, should be defined to verify if the road 
segment is still present. With respect to the presence 
of the road segment, at least the following hypotheses 
should be tested: 
* The road segment is still present; 
* The road segment does not exist. 
If the road segment is still present, we should test 
whether: 
* Its properties have remained entirely unchanged; 
* The width of the road segment has changed; 
* The curvature of the road has changed; 
* The road is crossed by a new road. The new node 
is a level crossing; 
* The road is crossed by a new road. The new node 
is a fly-over, where the new road is at the high 
level; 
* The road crosses a new road. The new node is a 
fly-over, where the new road is at the low level. 
These hypotheses are not mutually exclusive. For 
example the width of the road may have changed, 
while the road is simultaneously covered by a fly-over. 
Hypotheses can even be highly correlated. For 
example, if the curvature of the road has changed, it is 
also likely that the width of the road has changed. 
  
  
  
Fig. 1 Part of the road database expressing the old 
situation. 
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