The Optech 3033 is a high-resolution airborne laser scanner. Distances from the sensor to objects on the ground are calculated by recording the time taken for a pulse of laser energy emitted by the sensor to be reflected back. Half of this journey time multiplied by the speed of light gives the distance travelled by the pulse. The result is a highly detailed and highly accurate digital elevation model. Typical surveys involve collection of between one and four points per square metre although higher densities are possible. From a flying height of 1200 metres absolute accuracy is better than plus or minus 15 cm. The system is complemented by a video recording system that can provide VHS playback of survey runs The sensor (owned by ULM Cambridge) was deployed by ARSF from 2003 to 2008 on a development basis and is therefore not regarded as a core instrument.