Klas Folkesson

Model-Based Compensation of Topographic Effects for Improved Stem-Volume Retrieval From CARABAS-II VHF-Band SAR Images

A limiting factor that has been identified for stem-volume retrieval in coniferous forests using VHF synthetic aperture radar is that the backscatter varies depending on ground topography. On sloping ground, the backscatter from a forest is reduced, since the dominant ground-trunk double-bounce scattering mechanism is changed. This leads to underestimation of stem volume, and the variations caused by topography can obscure real variations in stem volume. By using multiple images acquired with different flight headings and combining the image information with ground-topography data in a model-based inversion method, we are able to compensate for the ground-topography influence on the backscatter. The inversion method is based on image segmentation and the optimal estimation method. Using four or more images from the CARABAS-II system and a coarse digital elevation model with 50-m horizontal grid, the stem volume can be retrieved with an average root-mean-square error (rmse) of less than 60 m3 ha-1 for stem volumes in range of 80-700 m3 ha-1 (in terms of above-ground biomass, this is equivalent to an rmse of less than 40 ton ldr ha-1 over the range of 50-400 ton ldr ha-1). The retrieval accuracy is similar to that previously obtained for similar forests standing on flat and horizontal ground.

Effects of Forest Biomass and Stand Consolidation on P-Band Backscatter

In previous studies, P-band synthetic aperture radar (SAR) has shown potential for biomass retrieval in forests. However, while measurements show a general agreement that backscatter increases with increasing biomass, different studies show that the backscatter from stands of similar biomass can significantly vary depending on forest structure, hence making biomass retrieval more challenging. In this letter, we show that, while biomass may be the single most important parameter determining the backscatter from a forest, the number density of trees has also a major impact. This can be explained using simple arguments, leading us to propose the use of the biomass-consolidation index to describe P-band HV-polarized backscatter. This is supported by electromagnetic-modeling studies and by a few measurements from boreal forest made with the AIRSAR system over the BOREAS test site in Canada.

Mapping of wind-thrown forests in Southern Sweden using space- and airborne SAR

Space- and airborne SAR have a potential for providing timely information of forest storm damage. In this paper, we compare Envisat, Radarsat and CARABAS with aerial photography and field observations after a severe storm event. The spaceborne C-band images were not able to detect the storm- damaged areas due to unfavorable frequency band and coarse resolution. The airborne CARABAS VHF SAR, on the other hand, detects most storm-damaged forest areas and even partly damaged which are often not detected in the aerial photography.

ALOS PALSAR Calibration and Validation Results from Sweden

In 2006 calibration activities for ALOS PALSAR were conducted in Sweden. Four five-metre trihedral corner reflectors and three smaller dihedral reflectors were deployed and operated during eight months. 23 PALSAR scenes were acquired over the calibration site allowing an evaluation of the quality and temporal stability of the data. Results show that the co-polarized data have been stable during the whole calibration period with variations in the trihedral responses lower than 0.7 dB. The measured resolution in azimuth was 4.4 m and in slant range 4.7 m for single polarization images and 9.5 m for polarimetric data. For the cross-polarized data large variations in the dihedral responses were found. It is assumed that this is caused by a larger sensitivity to pointing errors. For the polarimetric data, estimation of Faraday rotation gave values ranging from 0.1deg to 3deg.

Mapping of Wind-Thrown Forests Using the VHF-Band CARABAS-II SAR

A severe storm struck the southern part of Sweden in early January 2005 and caused much damage on the forests. Most of the large contiguous patches of wind-thrown timber were cleared during the same year. Numerous of smaller areas with fallen trees located in otherwise untouched forest stands were, however, still present and in many cases not even yet noticed. In an attempt to detect and position such areas, the airborne VHF SAR sensor CARABAS-II mapped 125 km by 125 km of mainly forested land in January 2006. The mission required 15 flights in total and all collected data are now processed and analyzed, with the detection results delivered to the end-user.

Automatic Detection of Wind-Thrown Forest in VHF SAR Images

A method for automatic detection of wind-thrown forest from VHF SAR images has been developed. It has been used to detect wind-thrown trees and estimate the volume of fallen timber within an area of 125 km × 125 km located in southern Sweden.

Mapping of wind-thrown forests using VHF/UHF SAR images

SAR images from the Swedish airborne CARABAS-II and LORA systems have been visually analyzed over simulated wind-thrown forest at both single tree and stand level. In ideal conditions, the results show that LORA is more accurate than CARABAS-II at detecting wind-thrown trees, regardless of tree size and direction of the fallen trees relative to flight heading. Furthermore, the visible single trees in the LORA images appeared more distinct than in the CARABAS-II images, which could be explained by the high resolution in the LORA images. Based on visual interpretation, it is likely that the detection of wind-thrown forests could be improved using VHF/UHF SAR images acquired both prior to and after a storm event.

Automatic segmentation of multiple VHF-band SAR images to improve stem volume retrieval

Ground slope effects have been identified as a limiting factor for stem volume retrieval from VHF-band SAR images since the ground-trunk dihedral scattering, which dominates the measured signal, is highly affected by surface relief and slope. To mitigate this problem coregistered SAR images from different flight headings have been acquired. These images are segmented to find regions with homogenous backscatter amplitude that are used with additional topography data to correct for the ground slope

Stem volume retrieval at stand level using multiple low-frequency SAR images

In this paper the ground slope effect on stem volume retrieval from VHF-band SAR images has been studied using multiple co-registered images acquired from different flight headings. Accurate measurements of the topography were used as additional input data to a model of the backscatter amplitude variation with ground slope and aspect angle. Modeling this topographical effect on the radar backscatter provides a base for improvement of the stem volume retrieval.