Multibaseline TanDEM-X Mangrove Height Estimation: The Selection of the Vertical Wavenumber

Abstract

We generated a large-scale mangrove forest height map using multiple TanDEM-X (TDX) interferometric synthetic aperture radar (InSAR) acquisitions with various spatial baselines in order to improve the height estimation accuracy across a wide range of forest heights. The forest height inversion using InSAR data is strongly dependent upon the vertical wavenumber (i.e., perpendicular baseline). First, we investigated the role of the vertical wavenumber in forest height inversion from InSAR data using the sensitivity of the interferometric (volume) coherence to forest height. We used corrected but lower resolution and accuracy Shuttle Radar Topography Mission (SRTM) mangrove height maps as a priori information over Akanda and Pongara National Parks in Gabon to estimate lower and upper boundaries of the vertical wavenumber over test sites from the measured coherence-to-height sensitivity. Only TDX acquisitions within the boundaries of the vertical wavenumber were selected and combined for multibaseline mangrove height inversion. Mangrove forest height was obtained with multibaseline TDX acquisitions and was validated against the reference height derived from field measurement data providing improvements in multibaseline inversion over existing height estimates (i.e., SRTM height) and single-baseline inversions (multibaseline inversion: $r^{2}= {\text{0.98}}$ , root mean square error (RMSE) of 2.73 m; SRTM height: $r^{2}= {\text{0.86}}$ , ${\text{RMSE}}= {\text{7.21}}$ m; single-baseline inversions: $r^{2}= {\text{0.08}}{\hbox{-}} {\text{0.97}}$ , ${\text{RMSE}}= {\text{3.86}}$ –11.10 m). As a result, to accurately estimate forest heights over a wide range (3–60 m), multibaseline InSAR acquisitions (at least three different baselines) are needed to exclude biases associated with the vertical wavenumber in forest height inversion.