Method for determining a surface

Documentation or navigation systems may supply the geographical coordinates of various features, including corners of fields. Each field may be given an identifier code with several numerical or alphanumerical characters. A rectangle surrounding each point may be defined. A portable GPS (Global Positioning System) receiver may be used with a portable computer e.g. a PDA (Personal Digital Assistant) connected to a data bank by a GSM (Global System for Mobile-Communications) or UMTS (Universal Mobile Telecommunication System).