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Remote sensing has evolved into a key resource for monitoring ecosystems and the global carbon cycle. Remote sensing data has been used to quantify the amount of carbon stored and forest loss due to fire, deforestation, and reforestation. Optical remote sensing has long been used to assist in monitoring global forests. However, optical remote sensing is only suitable for investigations from the top of the canopy. Airborne light detection and ranging (lidar) has proven to be valuable for determining forest attributes across larger spatial scales by providing 3- dimensional structural information. Using models created by combining field inventory samples and lidar data, variables such as canopy height, cover, and aboveground biomass can be estimated over larger regions of forest, allowing for wall-to-wall mapping. Spaceborne lidar (GEDI and ICESat-2) can provide forest attributes metrics at the global scale.
This study aims (1) to validate ICESat-2 and GEDI’s canopy height using ALS canopy height, (2) to develop wall-to-wall canopy height using GEDI, field data with Sentinel-1 and Sentinel-2 data, (3) to validate GEDI canopy cover using ALS cover, and (4) to validate and develop locally calibrated GEDI above-ground biomass (AGB) using field-measured AGB. In this study, all experiments are undertaken in three locations with distinct geography and land cover conditions, such as the US, Pakistan, and Nepal. We anticipate that this analysis will provide users with an extensive understanding of ICESat-2 and GEDI's product accuracy under varied topography, land cover, and climate conditions.
The findings indicate that spaceborne LiDAR offers reliable canopy height estimates. GEDI outperforms ICESat-2 in most of the scenarios. ICESat-2 provides better canopy height estimation in drylands and shrublands. The optimum scale to capture the heterogeneity of canopy height for both missions' sample density is 500 m or above. The CART model performs better in generating a wall-to-wall tree species-based canopy height model in the mountainous region (Nepal). Post-registration of the GEDI canopy cover with ALS cover provides higher estimation accuracy. The trends indicate that the GEDI canopy cover product is associated with GEDI estimated canopy height. GEDI provides reliable AGB estimation globally. However, locally calibrated AGB improves biases in low and high-density AGB regions of South Asia (focusing on Pakistan).
This research recommends to GEDI and ICESat-2 users on what factors they should consider when combining spaceborne height estimates, GEDI cover, and AGB mapping applications. ICESat-2 height and GEDI products can be used with confidence, but they should be supplemented with additional data sources such as ALS and field measurements, or any ground truth measures, to ensure quality at the local scale. These findings are intended to assist global consumers in understanding that the reliability of GEDI data is reliant on topography relief and land cover.