Abstract
This paper report experiences from the processing and mosaicking of 518 TanDEM-X image pairs covering the entirety of Sweden, with two single map products of above-ground biomass (AGB) and forest stem volume (VOL), both with 10 m resolution. The main objective was to explore the possibilities and overcome the challenges related to forest mapping extending a large number of adjacent satellite scenes. Hence, numerous examples are presented to illustrate challenges and possible solutions. To derive the forest maps, the observables backscatter, interferometric phase height and interferometric coherence, obtained from TanDEM-X, were evaluated using empirical robust linear regression models with reference data extracted from 2288 national forest inventory plots with a 10 m radius. The interferometric phase height was the single most important observable, to predict AGB and VOL. The mosaics were evaluated on different datasets with field-inventoried stands across Sweden. The root mean square error (RMSE) was about 21%–25% (27–30 tons/ha and 52–65 m3/ha) at the stand level. It was noted that the most influencing factors on the observables in this study were local temperature and geolocation errors that were challenging to robustly compensate against. Because of this variability at the scene-level, determinations of AGB and VOL for single stands are recommended to be used with care, as an equivalent accuracy is difficult to achieve for all different scenes, with varying acquisition conditions. Still, for the evaluated stands, the mosaics were of sufficient accuracy to be used for forest management at the stand level.
Highlights
The amount of remote sensing data of the Earth is increasing, and the quality of the collected data increases
For forest-related purposes, field inventories paired with airborne laser scanning (ALS) data have made forest maps of several vital variables, e.g., tree height, basal area, above-ground biomass (AGB) and stem volume (VOL), of yet unprecedented accuracy available [1,2,3]
National satellite-based forest maps with about 25 m resolution have been available in Sweden for almost two decades, and ongoing work to derive world-wide ground height mosaics or large-scale forest height mosaics have been presented for other regions [5,6]
Summary
The amount of remote sensing data of the Earth is increasing, and the quality of the collected data increases. SLU and the Swedish Forest Agency have used these ALS data to generate nation-wide maps of forest variables, similar to the earlier ones based on kNN and optical data, but with higher accuracy and with 12.5 m pixels [3]. This freely available ALS-based product has changed how forest companies cope with inventories and remote sensing data, and it has enabled small individual forest owners to obtain as good estimates of their forest stocks, as only timber buyers earlier would possess. People interested in Swedish forest due to various reasons (e.g., local ecology, stored biomass, timber values, etc.) are demanding frequent updates of the published ALS-based forest maps
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