Because the interpolation is done in 3D, the grid cells are three dimensional blocks (or voxels) with different thicknesses for each standard depth (e.g. 250 m by 250 m by 5 cm for the first layer; 250 m by 250 m by 10 cm for the second). In all cases, the predictions refer to the centre of the voxel (e.g. "sd1" refers to a depth of 2.5 cm, while “sd2” refers to 10 cm depth).
All maps are projected in the pan-African equal area coordinate system with the following proj4 string:
"+proj=laea +lat_0=5 +lon_0=20 +x_0=0 +y_0=0 +units=m +ellps=WGS84 +datum=WGS84"
Predictions have been made only for areas with vegetation cover. i.e. all deserts and shifting sand areas were excluded using a soil mask. We derived the soil mask for Africa by using the GlobeLand30 data set, by removing all pixels that have >10% of bare land (class 90 in GlobeLand30) and/or >30% of water cover (class 60 in GlobeLand30). The non-vegetated land areas were masked out because soil prediction models could not reliably be obtained from the very few soil point observations non-vegetated land. Extrapolation of models calibrated on vegetated land would lead to unrealistic and erroneous predictions.
Comparison of predicted soil organic carbon content (fine earth) for an area around the town of Arusha (Tanzania): SoilGrids1km (left) and AfSoilGrids250m (right). Vector lines data source: OpenStreetMap. See also a visual comparison in this video.