We have a summary description of the technique listed below, and a 'slide show' presentation can also be viewed. However if you have further questions then please contact us using info@lidar.co.uk or you can write / phone / fax - details at the end of this page.

LIDAR = LIght Detection and Ranging

This is a term used for a method of distance measurement using laser light. (Just as RADAR is 'Radio Detection and Ranging' ). The journey time of the laser beam, from leaving the instrument to it's return after reflection, is measured. And knowing the speed of light - a distance can be computed.

This technology is used in a variety of ways ranging from atmospheric measurements, ground based surveys and aerial 'mapping' type work. This site deals mainly with the latter application - aerial laser scanning. We hope to add the terrestrial laser scanning element to this site as soon as possible.

Photos courtesy of NOAA

An airborne laser scanning system comprises 3 main hardware components:- The laser, the INS, and the GPS.

The Laser

 

By mounting the laser in an aircraft and deflecting the laser beam (using a mirror), from left to right, then a swath of points can be measured, (rather a single profile), beneath the aircraft. Measurement speeds vary depending on the instrument deployed but measurement rates in the order of 5000 times per second are common. This ALS technique is nevertheless often referred to by the generic name for the measurement method - LIDAR.

Inertial Navigation System

The laser instrument is generally fixed into the aircraft such that it will be pointing vertically downwards during normal level flight. However due to the inevitable variations in the aircraft's attitude (roll, pitch and yaw) the laser beam will not always be pointing vertically downward. An inertial navigation system, comprising high accuracy gyroscopes, is used to measure the aircraft's attitude. Data from this device is used to adjust and correct the recorded measurement data.

 

GPS (Global Positioning System)

The aircraft's position, and hence the laser's position, is determined in all 3 dimensions using differential GPS. This requires the installation of a high accuracy GPS instrument into the aircraft, and the establishment of a ground control base station. The inaccuracies of 'navigation GPS' are overcome using 'differential GPS'. For more detailed information on how GPS works, please refer to www.merrettsurvey.co.uk

To obtain the highest accuracy it is essential to use dual frequency GPS and to keep the ground base station as close as possible to the survey aircraft. The highest quality laser scanner systems are capable of achieving height accuracies in the order of +/- 0.15m, but for this it is essential to keep the base station within 20km of the survey aircraft's acquisition area. Careful design of the flight plan relative to the ground station is essential and something that Merrett Survey Partnership can assist with, drawing on considerable experience of the logistics involved.