## 4 Discussion
### Crdm2020  
Elevations of CRDTM2020 were set to zero over the Pacific Ocean and the Caribbean Sea; however, some of the cells have an elevation below zero over land, which is expected and should be considered when interpreting the DTM for scientific and engineering applications. First of all, NASADEM, the base DEM in CRDTM2020, is a DSM that does not include any information on bathymetry, as SRTM was not able to resolve the bathymetry of water bodies due to radar reflection from the water surface. NASADEM is provided over the EGM96 geoid. When the reference surface was changed to a more detailed EGM2008, some cells with zero heights or small positive heights resulted in negative elevations. Comparison of negative heights for the study area showed that for the original NASADEM, 30,397 cells had negative heights, while for NASADEM above the EGM2008 geoid, this number of cells increased to 248,469. This is not an issue, first, because both EGM2008 and EGM96 may differ by a few decimetres from the mean sea level. Second, despite this, the advantage of changing the height reference to EGM2008 is significant, as EGM2008 has higher resolution and accuracy as EGM96 and differences between geoids reach 2 m in the Costa Rican mountains, as was shown in Figure 6.  
More importantly, negative heights in NASADEM and CRDTM2020 are attributed to statistical variation of heights around the expected zero mean value. In cases when the points are located at sea level and the expected DTM height is zero or near zero, this means that there would some cells with negative elevations. In the case of NASADEM or any other DSM, as they include positive VB, the heights corresponding to these locations, on  
Figure 17: Vegetation bias models: (**a**) Regional dependence between tree densities and tree heights obtained by DSM and Ice, Cloud,and land Elevation Satellite (ICESat) differences; Figure Sée in [4]. (**a**) VB look-up table for the same region; Figure S6f in [PERSON] et al., 2017 [4]. (**c**) The equivalent of (b) for the VB3 model computed as \(aH_{\text{canopy/canopy}}\). Note that colour bars and vertical axes are different. average, would have positive values depending on vegetation height and density, while DTMs should have values around zero. The mean value for CRDTM2020 negative heights is \(-1.6\) m, and the STD* is \(1.8\) m, as can be seen in Figure 13c, while for the NASADEM above EGM2008, the mean/STD* is \(-0.5\)/\(0.67\) m, and for NASADEM above EGM96, it is \(-1.6\)/\(1.3\) m. Depending on the application for CRDTM2020, users may choose to set all negative elevations to zero. The uncertainty of CRDTM2020 in the cells that were not vegetated should be equal to the uncertainty of NASADEM. In the validation with the levelling dataset, bare-earth points were separated using the canopy density model. The results provided in Table 5 show mean/MED/STD*/MAD of \(-2.5\)/\(-2.0\)/\(7.3\)/\(2.7\) m for both models in bare-earth points. For the points in vegetated areas, CRDTM2020 has much better performance in terms of mean and median error: the mean/MED/STD*/MAD is \(-0.1\)/\(-0.7\)/\(11.8\)/\(5.1\) m, while for NASADEM, the same metrics are \(-4.5\)/\(-4.1\)/\(12.4\)/\(5.1\) m. It is expected that the levelling dataset is more representative of bare-earth statistics than the LVIS datasets, as levelling is more accurate and typically performed in non-vegetated locations. Comparison between CRDTM2020 and NASADEM using LVIS datasets used LVIS ZT elevations above \(3\) m to separate vegetated and bare-earth points. Table 5 shows that the statistical values for NASADEM (highlighted in grey) for bare-earth points are superior to CRDTM2020, e.g., using LVIS2019 for CRDTM2020, the mean/MED/STD*/MAD is \(2.3\)/\(1.7\)/\(6.0\)/\(2.1\) m, while for NASADEM, these are \(-0.4\)/\(0.6\)/\(5.4\)/\(2.1\) m. The reason may lie in the accuracy of CHM2000 used in the computation of CRDTM2020 or in errors during the separation between vegetated and non-vegetated areas in this validation.