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

In. Stilla U, Rottensteiner F, Paparoditis N (Eds) CMRT09. IAPRS, Vol. XXXVIII, Part 3/W4 — Paris, France, 3-4 September, 2009 Figure 3. Schematic illustration of the relationship between roundabout geometric parameters. Vector data is in green. The resulting total energy function can now be defined: E(</>) = M P(<f>) + E m (<t>) (6) where /¿>0 controls the balance between the first and second term. The evolution equation of the level set function is then obtained via calculus of variation (Courant & Hilbert, 1953) and application of the steepest descent process for minimization of the energy functional equation (Li et al., 2005) as -77 = P[&<t> ~ div( 7777)] + ^(^)div(g -^77) + vg5(<f>) ot | V <p | | V ^ | (7) For all examples of central island detection, the same set of control parameters, 2=4, //=0.13, v=2 and the time step dt=2, were tuned for the evolution equation (Eq. 7). Since either evolution type alone, shrinking and expansion, has its own limitations, a hybrid evolution strategy is employed. For instance, in case of only shrinking curve evolution, vehicles on the circulating roadway, and in case of only expansion curve evolution, structures inside the central island, can block the motion of the curve toward the central island’s border. Using a hybrid evolution strategy overcomes various kinds of disturbances often present inside and outside the central island. Often before the curve evolution begins, a pre-processing step is necessary to remove some fine features that might hinder the curve motion. First, a morphological closing operator is applied in order to remove dark spots and subsequently the opening with the same structuring element (disk structuring element; size=2) is performed to eliminate small bright features followed by Gaussian smoothing (Fig. 4c). (a) (b) (c) Figure 4. Pre-processing sequence: (a) Original image, (b) cut out marked by the red box in (a), and (c) pre-processed result. Shown in Fig. 5 is the first sequence for island extraction, when shrinkage curve evolution is applied. After the vertices of the polygonal area identified as a roundabout object in the topographic database are located, the polygon is enlarged so that its increased area is one-tenth more than its initial area (Fig. 5a), thereby making sure that the new polygon is located outside the central island. Subsequently, shrinkage evolution begins through use of level sets. Among the obtained closed curves in Fig. 5b, the one with the largest area is selected as the initial guess for the island (Fig. 5c). This island candidate is subject to further processing. Next, the initial polygon obtained from vector data is made smaller so that its area is reduced by half (Fig. 5a). Subsequently, expansion curve evolution occurs (Fig. 6a). The largest resulting closed curve is most likely the desired solution due to the fact that the island is the largest object within the computational domain. This closed curve, however, can often not be regarded as the island because many disturbing features such as trees and various structures exist inside the island. This can block the motion of the evolving curve towards the island boundary. Leakages are therefore created at some points along the boundary of disturbing features where zero level curves have stopped in order to pass over them. (a) (b) (c) Figure 5. First sequence for island extraction: (a) Polygonal vector data (green) and its enlarged and reduced forms(red), (b) shrinking curve evolution result after 1335 iterations, and (c) the eventual result of shrinking evolution. With the assumption that disturbing objects inside the island do not contain smooth boundaries, cubic spline approximation is carried out to provide leakages (Fig. 6b). Subsequently, expansion evolution and spline approximation are repeatedly carried out (Fig. 6c) until no change in the position of the curve is reported. Again, the largest closed curve is regard as the island (Fig. 6d). Now that the results of island detection from the iterative expansion and shrinkage curve evolution have been obtained, the image positions of the resulting curves are compared and those points which are close to each other are selected, thereby eliminating curve positions that are not located on the island boundaries. The selection of points is based on their closeness in such a way that points having a distance below a certain threshold are selected. The final result is obtained when an ellipse is fitted to the selected points. When a roundabout appears as a point object in the topographic database (Fig. lb), the same hybrid evolution strategy is used but with a different initialization because the diameter of the inscribed circle is known to be below a given threshold, but how small it is is unknown. This brings some limitations for the shrinkage curve evolution. In order to apply the shrinkage evolution, the initial zero level curve must be placed outside the island. Since the approximate diameter of the inscribed circle is unknown, three successive circles are defined (Fig. 7a), on each of which the shrinkage curve evolution is carried out separately. The diameter of the circle interior to the central island is 10m and the diameters of exterior circles have an interval of 3 m. The results of shrinkage evolution on each initial curve from the largest to the smallest circle are depicted in Figs. 7b, c and d. In the next step, the iterative expansion evolution is carried out

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