Precise Measurements of Peatland Topography and Tree/Canopy Height with a High-Resolution Airborne Laser-Scanner to calculate Carbon- and Bio-Mass; 1-2006
Workshop on Vulnerability of Carbon Pools of Tropical Peatlands in Asia,
Pekanbaru, Riau, Sumatra, Indonesia, 24-26.Jan 2006
by Dr. H.-D.V. Boehm
High-resolution Airborne Laser-Scanners (ALS) installed in helicopters or fixed wings have increased the Z-range-resolution (height) measurement to a value of better than +/-0.15m over the last ten years.
With these precise, dense and geo-referenced 3D-measurements (x, y and z) demanding applications are now possible, such as: - forest inventory and bio-mass monitoring of PSF; finding illegal logging activities - flood plain mapping and costal monitoring - hydrological simulations and hydrological models - inventory of precise topographic maps and land use mapping - environmental protection, disaster management, erosion measurements - urban, pipelines and wireless network planning and city models - power line and power pole mapping and forest growth near power lines - monitoring of disposals and mines - archaeology and change detection - calibration of airborne or satellite SAR products After processing laser row data with intensity information a high-quality Digital Surface Model (DSM) and a high-quality Digital Terrain Model (DTM) can be derived as standard product. The digital surface model uses the first echo (FE) of the laser beam, the digital terrain model its last echo (LE).
Other products derived from this are difference models, TINs (Triangular Information), break lines and contours. The x-, y-resolution grid can be of the order 0.5m or 1m or 3m or 5m with 30 to 3 points/m². This laser-scanner can measure the height of peat swamp forest (PSF) at the canopy area with the first echo and peat surface and peat dome with the last echo. Tree height and Bio-Mass of PSF are obtained by subtracting the two echoes and multication of the area.
With additional peat depth drillings at selected way-points, this method can evaluate the amount of stored Carbon in peatland better than used any other. Monitoring of illegal logging is easily possible. High quality hydrological models can be analysed and thus offer a better understanding of peatlands. In combination with airborne or satellite SAR-systems (synthetic aperture radar), that are able to monitor large land areas, the airborne laser-scanner can calibrate SAR-data. Laser-scanner technology has come out of the research phase and is now fully mature. In combination with line scanner, true ortho images (RGB, CIR) can be produced. Most laser-scanners have an inertial navigation system (INS) including a roll stabilisation and a differential-global positioning system (D-GPS), to achieve a geo-referenced product, which can be processed and stored in a Geographical Information System (GIS). Helicopters or small aircrafts equipped with laser-scanner are handled easily by one pilot and one operator, travelling at a speed of approx. 60m/s at an altitude of approx. 800m. Analysing the data can be done by skilled personnel or the operator with the help of available software.
The equipment requires little maintenance. For reliable data, the laser beam should hit the canopy with an angle of incidence as close to normal as possible to achieve a good last echo from the ground. When clouds or haze are between the laser-scanner and the ground, the equipment cannot be used. The wavelengths of laser-scanners are in the near infrared band between 1µm and 1.5µm. Some laser-products are eye-safe, others only in a higher flight-path at 800m. The laser-beam divergence has mostly a value of 1mrad. Three companies, who offer flight services with Laser Scanners and providing the DSM- and DTM-products have been contacted and compared: - Swissphoto (Switzerland), - Terra Digital (Germany) and - TopoSys (Germany). Three Laser Scanner devices and their manufacturers are: - ALTM 3100 from Optech, Canada, with a rotating mirror scanner; - ALS 50 from Leica, Switzerland, with an oscillating mirror scanner; - Falcon II/III from TopoSys, Germany, with a 128/300 fiber scanner; The measurement rate is 83kHz/125KHz; calibration of the scanner is required only once. - Harrier240 + 560 from Riegl/TopoSys, with full wave analysis All laser-scanners have advantages and disadvantages concerning technical parameters and operational and commercial aspects, but all have the advantage of cost efficient acquisition of precise laser and image data at the same time with 3 to 6 €/ha depending on laser-pixels/m².