PhotonLabeler: Collection of ground truth data through visual Interpretation and manual labeling of ICESat-2 ATL03 photons

The ICESat-2 geolocated photon data (ATL03) is enabling the generation of various standard data products to characterize land ice, sea ice, inland water and vegetation elevation. While these derived products have opened avenues for new Earth and vegetation science research, there is also a need for software tools to facilitate photon-level analyses for product validation and development of new analysis methods. The objectives of this study are two-fold: first, showcase PhotonLabeler, a free graphic user interface (GUI) for manual labeling and visualization of ATL03 data. PhotonLabeler facilitates the reading and display of ATL03 Hierarchical Data Format (HDF) files, the manual labeling of individual photons into target classes of choice using a number of point selections tools and enables eventual saving of labeled data in ASCII format. We expect labeled data generated using PhotonLabeler to serve both training and validation purposes, especially for study sites without existing airborne lidar. Second, provide guidelines and recommendations for effective visual interpretation of ATL03 data into various point class for vegetation and land applications. In line with these objectives, we demonstrate the use of PhotonLabeler for collecting ground truth data to validate canopy and terrain height estimates from the Land and Vegetation product (ATL08) over sites in northwestern Zambia, using night-time granules and eastern Texas using daytime granules. For both scenarios, ATL08 absolute (above reference ellipsoid), relative (above-ground) and terrain height estimates showed high agreement (R² > 0.8) with matching estimates generated from manually labeled data.