Overview

The biomass assessment is primarily based on a digital Crown Height Model (CHM or nDSM for normalised Digital Surface Model) and regional forest growth models and allometric formulas.

The modelling of the crown could be done photogrammetrically by stereo-matching of aerial photos or of stereo satellite imagery, such as Worldview-3, or it could be done with Fullwave Laserdata (LiDAR) recorded from an aircraft (Airborne Laser Scanning – ALS). The advantage of ALS is the penetration of the laser beams through the canopy down to the ground. Such ground or bare earth points are important to model a representative Digital Elevation Model (DTM). In stereo photos (no matter if airborne or from satellite) the terrain under a dense forest canopy is hardly visible and the few visible points showing roads or openings, are usually not enough to process a representative DTM. But different from the forest ground, the canopy and the vegetation surface is fully visible in stereo imagery, hence aerial and satellite photos are well suited to process a representative Digital Surface Model (DSM).

The crown height, including the height of automatically detected individual trees is the result of the difference between the surface height, represented by the Digital Elevation Model (DSM) and the height of the terrain, given by the Digital Terrain Model (DTM). That´s why in the beginning a LiDAR survey is essential to get a precise DTM and consequently an accurate Crown Height Model (CHM). But this detailed survey has to be done only once, updates of the CHM can be done later with spectral stereo data.

All three models, DSM, DTM and CHM, are being used for different purposes: The DSM processed from LiDAR, aerial photos or satellite imagery represents the surface of the vegetation and buildings and their heights above sea level at the time when the data has been recorded. This information is needed to create TrueOrtho Photos, which show 3D Objects in an upright position.

Since the vegetation surface changes with every vegetation period or after any calamities or storm damages, the DSM (and also the CHM) is a permanently changing source of information, which should be update regularly or on demand after natural disasters