A method automatically determines a boundary of a partial area of a map, the partial area indicating the remaining range of a motor vehicle. The total area is divided into a matrix of rectangular area segments, with which a connected subset of the partial area is associated. Initially starting from a region of the total area not belonging to the partial area, a boundary area segment of the partial area is located and a first point describing the position of the boundary area segment is entered into a list of points, following which the process proceeds along the edge of the partial area and a point describing the position of the boundary area segment is entered into the list for each further boundary area segment located, until the boundary area segment corresponding to the first point is reached again. The boundary is determined from the list of points.