CMRT09: Object Extraction for 3D City Models, Road Databases and Traffic Monitoring - Concepts, Algorithms, and Evaluation 
Through the use of shape description parameters such as 
curvature computed from the snake vertices, another force can 
be added to the GVF force field. This is the so-called balloon 
force, which lets the contour have a more dynamic behaviour 
(Cohen, 1991), thereby addressing the two described problems. 
This new force, which makes the contour act like a balloon, 
applies an inflating effect to the contour to localize the concave 
part of the roundabout outline: 
F = k\n(s) (16) 
where n(s) is the normal unitary vector of the curve at point 
F(.v) and k\ is the amplitude of the force. The combination of 
the GVF force field and the balloon force modifies Eq. 15 to the 
form 
V [,] = (K + r /)"' * ir V l '-' ] - K G(u,v) v | iH] n(s)) (17) 
The balloon force is activated when the snake’s passive and 
active parts are approximately straight, i.e. their overall 
curvature, which is defined as the sum of the absolute 
curvatures along the curve, is below a threshold. It is applied 
only on the passive part of the curve. This is regarded as lying 
outside the roundabout’s border, whereas the snake at the active 
parts is assumed to be on the right track. The direction in which 
the balloon force is applied is towards the roundabout central 
area. However, in order to be able to delineate the roundabout 
outline, the balloon force has to be applied in two different 
directions, central island inwards and outwards (Fig. 9a). 
The answer to the question of when and in which direction the 
balloon force needs to be applied differs for different samples. 
As a result, several parameters need to be tuned on an ad hoc 
basis to address this question, which is not a desirable 
requirement. To resolve this, the external force field of the 
snake approach described so far is modified based on the shape 
of the central island. As the shape of the roundabout outline 
corresponds to the shape of the enlarged central island, the 
island is enlarged to an extent depending on the width of the 
circulating roadway (Fig. 9b). Subsequently the snake external 
force field is modified based on the enlarged central island. The 
external force field in the enlarged central island is replaced 
with the GVF of an intensity-step image (Fig. 9c) whose main 
characteristic is that its external force points directly from the 
centre outwards so that snakes situated in this area are drawn 
toward the outline of the roundabout. 
The intensity-step image is generated from a signed distance 
function. To generate this function, the border of the enlarged 
central island is taken as the reference (Fig. 9b). Successive 
concentric layers at a specific distance interval from the 
reference to the centre point are then defined. Conversely, 
proportional to the distance of each layer to the reference, an 
intensity value is calculated and assigned to the respective 
layer, i.e. layers closer to the reference curve are brighter and 
vice versa. 
The obtained intensity-step image has a gradual increase of 
intensity values from the centre point towards the reference 
curve. Consequently, its GVF field points directly outward. The 
modified force field pulls the snakes toward the outline even if 
the initialization is far away from true borders. Furthermore, 
with this modified force field, problems created by the presence 
of various kinds of disturbances such as trees and vehicles 
within and outside the central island are overcome. An example 
illustrating the improved result using the proposed modified 
force field is shown in Fig. 10. The complete reconstruction of a 
roundabout using the proposed modified snake model is shown 
in Fig. 11, along with intermediate results. 
(a) (b) (c) 
Figure 9. (a) Two directions in which the balloon force is 
applied; (b) reference for the signed distance function (white 
curve) computation and concentric regions (black curves); (c) 
intensity-step image from the signed distance function. 
(a) (b) (c) 
Figure 10. The effect of the modified external force field: (a) 
intersection lines (black) from initial snakes, (b) results from 
unmodified GVF, and (c) improved results with modified GVF. 
4. RESULTS AND EVALUATION 
The proposed approach was tested using 0.1m GSD 
panchromatic aerial orthoimagery covering rural and suburban 
areas. The Authoritative Topographic Cartographic Information 
System of Germany (ATKIS), which nominally corresponds to 
a mapping scale of 1:25,000, was used as the source of external 
vector data. Roads are modelled as linear objects in ATKIS. 
Tests were conducted on 10 roundabouts. Sample results that 
highlight the capabilities of the proposed approach are shown in 
Fig. 12, where is can be seen that the method can deal with a 
variety of disturbances inside and outside the central island. 
Also, most of the roundabout borders were captured correctly. 
However, in areas where the curvature of the outline was too 
high, as is the case in the top-left example (lower border) and 
top-right image (right border), roundabout borders were 
extracted with some deviation. 
In order to evaluate the performance of the approach, the 
extracted roundabout areas were compared to the manually 
plotted roundabouts used as reference data. The comparison 
was carried out by matching the extracted road borders 
resulting from the connection of the roundabout to its associated 
road arms to the reference data using the so-called buffer 
method (Heipke et al. 1998). Although the buffer width can be 
defined using the required accuracy of ATKIS, which for a road 
object is defined as 3m, it was decided to set the buffer width 
within the range of 0.5 m to 3 m, i.e. 5 pixels to 30 pixels, in 
concert with the image resolution of 0.1 m. This allowed 
assessment of the relevance of the approach for practical 
applications that demand varying degrees of accuracy. 
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