A method and apparatus generates data concerning a partition of land such as farmland based on an observation image of the surface of the earth photographed by a high-altitude flying vehicle such as a satellite or airplane at high speed while reducing the burden on the user. The method comprises the steps of extracting edges in the observation image; extracting a rectangle region on the image by determining and connecting points of intersection of a plurality of straight lines extending radially from a point on the image and the edges; and adjusting the extracted rectangle region by refining and correcting it.