An evaluation of the JPL TOPSAR for extracting tree heights

In this paper, the accuracy of the digital elevation model (DEM) generated by the Jet Propulsion Laboratory (JPL) TOPSAR for extracting canopy height is evaluated. For this purpose, an experiment using C-band TOPSAR at the Mi chigan Forest Test Site (MFTS) in Michigan's Upper Peninsula was conducted, Nearly 25 forest stands were chosen in MFTS, which included a variety of t ree types, tree heights, and densities. For these stands, extensive ground data were also collected. The most important and difficult-to-characterize ground truth parameter was the forest ground level data, which is required for extracting the height of the scattering phase center from the interfero metric SAR(INSAR) DEM. To accomplish this, differential global positioning system (GPS) measurements were done to accurately (+/-5 cm) characterize th e elevation of: 1) a grid of points over the forest floor of each stand and 2) numerous ground control points (GCPs) over unvegetated areas. Significant discrepancies between GPS and TOPSAR DEM and between the two TO PSAR DER ls of the same area were observed. The discrepancies are attribute d to uncompensated aircraft roll and multipath. An algorithm is developed t o remove the residual errors in roll angle using elevation data from: 1) 10 0-m resolution U.S. Geological Survey DEM and 2) the CPS-measured GCPs, Wit h this algorithm, the uncertainties are reduced to within 3 m, Still, compa rison between the corrected TOPSAR DEMs shows an average periodic height di screpancy along the cross-track direction of about +/-5 m, Simulation resul ts show that this might have been caused by multipath from an object near o ne of the INSAR antennas. Careful examination of the coherence image and th e backscatter image also show such periodic patterns. Recommendations are p rovided for the extraction of the best estimate of the scattering phase cen ter height, and a model is provided to estimate actual tree height. It is a ccurate to within 1 m or 10% for the red pine test stands used here.