Observations of forest stand top height and mean height from interferometric SAR and LIDAR over a conifer plantation at Thetford Forest, UK

Estimates of forest stand mean height using airborne LiDAR (Light Detection And Ranging) instruments have been previously reported with accuracies comparable to traditional ground-based measurements. However, the small area covered by a LiDAR sensor in a single aircraft overpass is a significant hindrance for large scale forest inventories. In comparison, airborne interferometric Synthetic Aperture Radar (InSAR) systems are also able to make estimates of surface height, but the swath coverage is often far greater, typically five or ten times that of the LiDAR coverage. A set of interferometric data takes was acquired by the ESAR airborne sensor over a managed pine plantation at Thetford Forest, UK. Scattering phase centre height estimates were made from two single-pass X-band acquisitions and polarimetric repeat-pass L-band acquisitions and compared with height estimates made from a separate LiDAR acquisition. The relationship between the scattering phase centre heights and stand top height is described, and the accuracy of stand top height estimates estimated from InSAR and LiDAR is quantified by the root mean square error (rmse). General yield class models by the Forestry Commission (UK) were used to estimate stand top height from a GIS database used for forest management. The longer wavelength L-band radiation penetrates deeper into the canopy than X-band and the scattering phase centre height is affected by both forest structural parameters (canopy density, understorey and gaps) and sensor parameters (look-angle and reduced coherence through temporal and volume decorrelation). Consequently, a simple translation of scattering phase centre height into stand top height gives noisy results for L-band, with observed rmse values between ±3.1m in the near range and ±6.4m in the far range. The X-band based top height estimates are more accurate with rmse between ±2.9m in the near and ±4.1m in the far range, which can be further reduced by an empirical incidence angle correction. Stand top height estimates from LiDAR achieved an rmse of only ±2.0m. The X-band scattering phase centre heights have also been related to mean stand height and are comparable with heights observed from the LiDAR sensor and field measurements. An rmse of ±2.5 m for the mean stand height estimates based the X-band data set was found. Finally, we briefly discuss error propagation from the use of a terrain model, here provided by the Ordnance Survey.