All the polygonalisation algorithms we can find in the literature proceed on 4-connected or 8-connected discrete arcs. In this article, we aim to polygonalise “thick arcs”. A first step consists in giving a definition of such arcs based on morphological properties. In a second step, we propose two methods in order to polygonalise such arcs. The first one is based on a squelettisation of the arc and uses the fuzzy arc polygonalisation. The second one is based on a grouping of the cells in order to obtain heterogeneous pixels and then, the polygonalisation of heterogeneous arcs is applied. This work is supported by the french ACI GeomDiGIT (http://liris.cnrs.fr/geodigit/)