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Abstract
Applications of stereo imagery acquired by cameras onboard unmanned aerial vehicles (UAVs) as practical forest inventory tools are hindered by the unavailability of ground surface elevation. It is still a challenging issue to remove the elevation of ground surface in leaf-on stereo imagery to extract forest canopy height without the help of lidar data. This study proposed a method for the extraction of forest canopy height through the synthesis of UAV stereo imagery of leaf-on and leaf-off, and further demonstrated that the extracted forest canopy height could be used for the inventory of deciduous forest aboveground biomass (AGB). The points cloud of the leaf-on and leaf-off stereo imagery was firstly extracted by an algorithm of structure from motion (SFM) using the same ground control points (GCP). The digital surface model (DSM) was produced by rasterizing the point cloud of UAV leaf-on. The point cloud of UAV leaf-off was processed by iterative median filtering to remove vegetation points, and the digital terrain model (DTM) was generated by the rasterization of the filtered point cloud. The mean canopy height model (MCHM) was derived from the DSM subtracted by the DTM (i.e., DSM-DTM). Forest AGB maps were generated using models developed based on the MCHM and sampling plots of forest AGB and were evaluated by those of lidar. Results showed that forest AGB maps from UAV stereo imagery were highly correlated with those from lidar data with R2 higher than 0.94 and RMSE lower than 10.0 Mg/ha (i.e., relative RMSE 18.8%). These results demonstrated that UAV stereo imagery could be used as a practical inventory tool for deciduous forest AGB.
Highlights
Forests cover approximately 30% of the total land area and account for 80% of Earth’s total plant biomass [1,2]
Several satellite missions aiming at the monitoring of forest aboveground biomass (AGB) have been launched or scheduled, such as the L-band synthetic aperture radar (SAR) onboard the advanced land observing satellite (ALOS) launched by the Japanese Space Agency (JAXA) in 2006 [4], the advanced land observing satellite 2 (ALOS-2) equipped with an enhanced L-band SAR (PALSAR-2) [5], the BIOMASS mission of European Space Agency designed for the measurements of forest AGB and its changes [6], and so on
This study reported our new results on the inventory of forest aboveground biomass over deciduous forest through the synthesis of leaf-on and leaf-off unmanned aerial vehicles (UAVs) stereo imagery by taking lidar data as references
Summary
Forests cover approximately 30% of the total land area and account for 80% of Earth’s total plant biomass [1,2]. Several satellite missions aiming at the monitoring of forest aboveground biomass (AGB) have been launched or scheduled, such as the L-band synthetic aperture radar (SAR) onboard the advanced land observing satellite (ALOS) launched by the Japanese Space Agency (JAXA) in 2006 [4], the advanced land observing satellite 2 (ALOS-2) equipped with an enhanced L-band SAR (PALSAR-2) [5], the BIOMASS mission of European Space Agency designed for the measurements of forest AGB and its changes [6], and so on In addition to these satellite missions, several regional maps of forest AGB or related forest structure parameters have been released [7,8,9]. As pointed out by Hall, F.G., et al [13], these existing global or regional datasets were only approximations based on combining land cover types and representative values instead of measurements of actual forest aboveground biomass
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