Airborne laser scanning can provide a dense 3D point cloud of the earth surface. The sensor, carried by an aeroplane or helicopter, emitters laser pulses which are reflected by a surface. By measuring the runtime of the signal the distance can be determined. To derive a 3D point, the additional information of the sensor position needs to be known as well as the direction in which the laser pulse has been emitted. This required information can be obtained by a GPS / INS (inertial navigation system) unit (Lohr,Weil,1999).