To limit cumulative errors due to the odometer, we propose an absolute location method based on the extraction and the use of geometrical landmarks. This method also avoids the need of prior knowledge of the robot environment (no training steps) and its specified fitting (no beacons). If different measurements of the environment around the robot taken by an ultrasonic telemeter are merged, geometrical primitives appear. They are then discriminated and rebuilt. Those primitives are the landmarks used by the locating system. Hence a set of definitions and laws have been established to conjointly use few landmarks, in order to obtain the mobile robot absolute location by triangulation.