Canopy Height Model (CHM) Derived From a TanDEM-X InSAR DSM and an Airborne Lidar DTM in Boreal Forest

The first global X-band spaceborne single-pass interferometer mission, TanDEM-X, provides a spatially continuous map of global canopy elevations. In this paper, we assess the use of TanDEM-X data, in combination with an external digital terrain model (DTM), to map boreal canopy heights. A comparison of the TanDEM-X canopy height model (CHM) to a validated reference lidar CHM was performed based on two definitions of canopy height: canopy surface height (CSH) and dominant height (DH) at spatial resolutions ranging from 5 to 25 m, and at stand level. We found the TanDEM-X CHM to have a coarser resolution than the corresponding lidar CHM. This was apparent in the height validation of the TanDEM-X CHM, which had a RMSE of 2.7 m at the 5-m resolution, 1.9 m at the 25-m resolution, and 1.5 m at stand level. The height differences between the InSAR and lidar surfaces varied between 1.3 and 1.5 m, but InSAR heights were below the height of dominant trees by 4.6-7.5 m. Similar discrepancies were observed for the lidar CSH relatively to DH (6.04, 8.98, and 8.05 m, respectively). The results show that the TanDEM-X interferometric heights are very close to the lidar reference height and that penetration below the DH is caused by propagation of the microwave signal between the tree apices and the main foliage surface in boreal forest. Finally, the accuracy of InSAR height estimates was not sensitive to tree density effects, but was moderately affected by local incidence angles (LIAs), gap volume, and canopy height.