Validating the ICESat-2 Land Water Vegetation Elevation Product (ATL08) with Airborne Lidar Data

The ICESat-2 Land Water Vegetation Elevation product (ATL08) is offering estimates of terrain and canopy heights at unprecedented fine spatial scales, which will provide improved prospects for the assessment of forest structure worldwide. Given the novelty of these data to the vegetation community and their susceptibility to solar background noise, there is great need to validate them to determine usability. The goal of this study is to assess the usability of the ATL08 product by 1) validating it against reference datasets of airborne lidar, 2) comparing ATL08 photon classification to our own multi-level photon filtering and classification algorithm, and 3) determining the level of usable ATL08 data compared to collected photon data (ATL03). We compared relative and absolute height metrics from 100-m ATL08 segments with corresponding estimates derived from airborne lidar data at selected sites in California, New Mexico and Oregon. The level of usable data was determined by comparing the number of data segments in ATL08 with real estimates to the total number of segments in a granule. Results show that ATL08 absolute canopy heights are highly correlated (R² > 0.9) with airborne data and height estimates from our own algorithm. However, high inconsistencies (R² < 0.1) were observed between ATL08 relative canopy heights and aboveground lidar heights likely attributable to errors in the applied ground reference elevation and the photon classification. The level of usable data from the ATL08 based on 15 tracks ranged from 50% for weak beams to 76% for strong beams. These moderate levels of usable data are also indicative of the high levels of solar background noise and photon misclassifications. Overall, the ATL08 product shows great promise for assessment of forest resources but will require continued improvement in the photon classification and relative canopy height estimation.