On the Role of Structure Metrics in the Combination of the GEDI Waveforms and TanDEM-X Coherences for Biomass Estimation

Abstract

The full waveform GEDI lidar mission will map forest height and biomass at 1 km resolution globally in temperate and tropical forests. In order to obtain biomass maps with higher resolution and overcome the limitations of the footprint sampling on ground, the possibility to combine GEDI waveforms and TanDEM-X coherences is currently considered using forest height as a key parameter. First of all, continuous and high resolution height estimates are obtained from TanDEM-X coherences by means of an inversion initialized by the GEDI waveforms. Then, a relationship between the footprint-level GEDI biomass estimates and TanDEM-X forest height estimates at the same location is defined. This relationship is used to obtain a biomass map from the TanDEM-X forest height at footprint scale. Finally, the obtained biomass estimates are aggregated at a coarser resolution (e.g. 1 ha). The objective of this work is to investigate the role of structure metrics derived from TanDEM-X coherences in the combined GEDI-TanDEM-X biomass mapping. Helped by the limited X-band penetration, recently considered structure metrics quantify the local variability of high resolution TanDEM-X digital elevation model as a proxy to forest height variations, linked to horizontal forest heterogeneity and density. Such structural information might support the definition of height-biomass relationships, whose coefficients can indeed vary with varying forest structure. On the other hand, structure information might also support the initialization of scattering models with GEDI waveforms for improving height estimates. These aspects will be addressed in the presentation by means of real data results obtained by processing TanDEM-X data and lidar waveforms acquired in relevant campaigns in different forest sites. The final accuracy obtained in the estimation of biomass will be evaluated as well.