Factors affecting relative height and ground elevation estimations of GEDI among forest types across the conterminous USA

Assessing canopy height measurements from ICESat-2 and GEDI orbiting LiDAR across six different biomes with G-LiHT LiDAR

“…This study found height estimate accuracy based on RMSE and Bias (table 2) comparable to those of other studies for both ICESat-2 and GEDI (ICESat-2: Liu et al 2021, Malambo and Popescu 2021, GEDI: Adam et al 2020, Liu et al 2021, Fayad et al 2021a, 2021b, Kutchartt et al 2022, Wang et al 2022, though most of these found less of a height bias than found here perhaps because the overall height range was smaller for earlier studies. Other assessments of ICESat-2 also showed the importance of restricting samples to night estimates for height (Liu et al 2021, Malambo andPopescu 2021).…”

“…Other GEDI and ICESat-2 assessments found similar biases, but not all. In a study across a wide range of biomes in NEON sites, Wang et al (2022) found bias ranged from −0.24 to −4.69 for RH100 for NEON forested sites with GEDI. Adam et al (2020) showed that GEDI overestimated RH100 by ∼5 m for <20 m tall canopies, with bias decreasing to ∼1 m for canopies >30 m. In a different comparison over NEON sites, Liu et al (2021) found that overall bias for RH100 was 0.77 m for ICESat-2 and −0.79 m for GEDI (night + day acquisitions) and −0.87 m for ICESat-2 and −0.18 m for GEDI (night acquisitions, with strong beam for ICESat-2 and power beam for GEDI).…”

“…These results likely arise because ground detection may be easier for ICESat-2 because of its elongated (∼100 m) data sampling and aggregation strategy (Neuenschwander et al 2020, figure S3). Several studies found that ICESat-2 or GEDI height estimates were biased in samples with steep slopes (ICESat-2: Liu et al 2021, GEDI: Adam et al 2020, Liu et al 2021, Fayad et al 2021a, Kutchartt et al 2022, Wang et al 2022. For GEDI, the ground estimation for GEDI waveforms could be broadened in steep terrain where canopy returns are gradually mixed with the ground returns (Wang et al 2019, Yang et al 2011a, leading to potential overestimation of canopy height (figure 7 in Wang et al 2019).…”

First validation of GEDI canopy heights in African savannas

“…While these studies provided essential early insights into the global and continental-scale reliability of GEDI metrics with very promising results, such comparisons to ALS CHM-derived height metrics alone are not the most appropriate validation approaches, as they essentially compare different structural metrics which may confound interpretation. The present study, therefore, followed the novel approach of first simulating, simulated GEDI waveforms from the ALS point cloud data before , followed by the extractingon of ground elevation and RH metrics (Hancock et al, 2019;Silva et al, 2018) to generate an ALS-based reference dataset before performing an error assessment (Wang et al, 2022) (Roy et al, 2021).…”

“…We would argue that a study on the validation of GEDI canopy height measurements in short, sparse savannas is fully justified at this point in time and should precede regional ecological studies based on GEDI, especially since scientists do not yet know how reliable the GEDI measurements are in savannas. Our study's novelty is based on (i) its overall timing given the fairly recent launch of GEDI (ii) the first validation study on date of submission (Feb 2022) to simulate GEDI waveforms from ALS for direct validation of GEDI's on-orbit relative height measurements (only one other paper, Wang et al 2022, has been published recently using this approach -May 2022), (iii) the study provides detailed insights into the causes of the lower limit of detectability of short stature shrubs that are essential to ecological studies in savannas, and (iv) the validation of GEDI canopy height measurements in savannas, despite the fact that GEDI sensor and algorithms were designed for dense, tall forests, as stated in the mission objectives.…”