CMRT09

Stilla, Uwe

Bibliographic data

Monograph

Persistent identifier:: 856955019

Author:: Stilla, Uwe

Title:: CMRT09

Sub title:: object extraction for 3D city models, road databases, and traffic monitoring ; concepts, algorithms and evaluation ; Paris, France, September 3 - 4, 2009 ; [joint conference of ISPRS working groups III/4 and III/5]

Scope:: X, 234 Seiten

Year of publication:: 2009

Place of publication:: Lemmer

Publisher of the original:: GITC

Identifier (digital):: 856955019

Illustration:: Illustrationen, Diagramme, Karten

Language:: English

Usage licence:: Attribution 4.0 International (CC BY 4.0)

Publisher of the digital copy:: Technische Informationsbibliothek Hannover

Place of publication of the digital copy:: Hannover

Year of publication of the original:: 2016

Document type:: Monograph

Collection:: Earth sciences

Chapter

Title:: ROAD ROUNDABOUT EXTRACTION FROM VERY HIGH RESOLUTION AERIAL IMAGERY M. Ravenbakhsh, C. S. Fraser

Document type:: Monograph

Structure type:: Chapter

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. 24

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