Morphometric parameters of the Coral Belt Mounds (CBM), Namibian coral mound province

Quantitative morphometric analyses were carried out for each mound following the workflows presented by Purkis et al. (2007) The coral mound base was defined following the methodological approach of Correa et al. (2012) using the dip angle map, generated from the digital elevation model (DEM), to extract closed polygons that follow the 3°-contour line. This 3°-cutoff has been qualitatively validated with a comparison between the DEM and the dip angle. Small-scaled polygons within mound perimeters and resulting from bathymetry artefacts were filtered out. Manual editing was applied to split simple merged mound structures (e.g. twin-peak mounds) based on higher cut-off slope values (4-5°). Furthermore, polygons describing the mound footprint have been corrected to remove unrealistic shapes especially common for the CBM. The DEM was subsequently re-gridded to generate hypothetical bathymetric maps without mounds, for which the vertical relief beneath each removed mound was interpolated from the mound perimeters. The newly interpolated surfaces were then subtracted from the original DEMs to evaluate the volume and heights of the coral mounds. Only features with a footprint area greater than 900 suared meters (corresponding to a two-dimensional array of 3 × 3 DEM grid cells) and with a height of >2 m above the surrounding seafloor (4 × 0.5 m of vertical precision) were considered as coral mounds and quantitatively analyzed.

本研究遵循Purkis等人（2007）提出的工作流程，对每座珊瑚丘开展定量形态计量分析。珊瑚丘的基底参照Correa等人（2012）的方法进行界定：基于由数字高程模型（DEM）生成的倾角图，提取沿3°等高线分布的闭合多边形。该3°阈值已通过对比DEM与倾角图完成定性验证。珊瑚丘周边范围内由水深地形伪影产生的小型多边形将被滤除。基于更高的坡度阈值（4°-5°），通过手动编辑拆分合并的单一丘体结构（例如双峰珊瑚丘）。此外，表征珊瑚丘基底覆盖范围的多边形亦经过修正，以移除在珊瑚丘群（CBM）中尤为常见的非合理形状。随后对DEM进行重网格化处理，生成不含珊瑚丘的假想水深地形图：基于各移除丘体的周边边界，插值得到其下方的地形起伏。将新插值得到的地形表面从原始DEM中减去，以此计算珊瑚丘的体积与高度。仅将基底覆盖面积大于900平方米（对应3×3的DEM网格单元二维阵列）、且高于周围海底高度超过2米（垂直精度为4×0.5米）的地貌特征认定为珊瑚丘，并开展定量分析。