Assessment of digital elevation models obtained in Brazilian Amazon based on P and X band airborne interferometric data

An airborne campaign in September of 2000 over Tapajo´s National Forest, which is a region of Brazilian Amazon, Para´ State, has acquired P and X band interferometric data over a region which comprises primary forest, secondary forest in several stages of regrowth, pasture, crop plantations, bare soil, water and other classes. The AeS-1 polarimetric system, from AeroSensing Radarsysteme GmbH, Germany, provided P band polarimetric data for two-pass interferometry and X band single polarization, single-pass interferometric data. During the radar mission, a ground survey was carried out for target identification and collection of tri-dimensional differential GPS data for P and X band corner reflectors and other characteristic points. Georeferenced Digital Elevation Models (DEMs), with spatial resolution of 2.5 m, were generated considering X and P band interferograms. The X band DEM generally shows higher altitude than the P band DEM, especially over forested areas, because the considerably higher penetration of P band towards the forest floor, while X band DEM reflects the canopy altitude. X band DEM is called a Digital Surface Model (DSM), because it is related mainly with the top of the land cover. The difference between the DSM and the DEM (P band) potentially gives the forest height. The actual internal height of the forest was estimated, in the field, in transects opened in the primary and secondary forest and compared with the DSM-DEM difference. The results showed that the DSM-DEM difference tends to underestimate the forest height under secondary forest, probably due to higher volume scattering of P band emission. The DSM-DEM difference over primary forest is closely related to the average height, in the transects, standing between the global average tree height and the average height of the upper storey trees of the forest transects.