A fully automatic method to extract field boundaries from imagery is described in this paper. The fields are represented together with additional prior knowledge in the form of GIS-data in a semantic model. The approach consists of two main steps: Firstly, a segmentation is carried out in a coarse scale resulting in preliminary field boundaries. In a second step network snakes are used to improve the geometrical correctness of the preliminary boundaries taking into account topological constraints while exploiting the local image information. Focussing on the network snakes and their specialties the results demonstrate the potential of the proposed solution.