Stem volume retrieval in boreal forests from ERS-1/2 interferometry

C-band repeat-pass interferometry, in particular, the coherence, has been shown to be of great potential for stem volume retrieval. For boreal forests, we have investigated a stem volume retrieval method based on inversion of ERS-1/2 coherence measurements by means of a semiempirical model. A multitemporal combination of several stem volume estimates has been used in order to reduce errors in the estimation. The retrieval procedure was first applied in a forest estate located in Kنttbِle, Sweden, where accurate in situ measurements were taken. Stem volume was determined both at the stand level (between 2 and 14 ha) and at the pixel level (25×25 m). A multitemporal combination of coherence data acquired in stable winter-type conditions gave the most accurate results. Based on the results obtained in Kنttbِle, the retrieval procedure was extended to a large area of 4235 km2 around Kنttbِle. Retrieval was performed in all forested areas on a pixel basis (25×25 m), generating stem volume maps. In Kنttbِle, at the stand level, stem volume up to 350 m3/ha was estimated with an error comparable to the ground truth, i.e. 10 m3/ha. At the pixel level, the error reached the value of 55 and 71 m3/ha in the forest estate and in the large area, respectively. Compared to the results from the stand analysis, the higher error is believed to be mainly due to the higher uncertainty of coherence estimation at high stem volume and to geometric mismatch between field data and coherence data. Moreover, over large areas, spatial variation of the parameters in the model should be considered.