Mapping accuracy AfSoilGrids250m
The accuracy of the maps (random forests part of the model) was assessed using 5-fold cross-validation, with model re-fitting, and reported using RMSE and % variance explained. The latter is defined as 1-MSE/sigma-square, where MSE is the mean square error at cross-validation points and sigma-square is the variance of the target variable. Some soil properties were log-transformed prior to prediction using regression kriging. In such cases the maps are the back-transformed regression kriging maps, but the % variance explained was derived in the transformed (trans-Gaussian) space which differ from the % variance explained in the original space. The figure and table below shows scatter plots and summary statistics of cross-validation results.
Example of a mapping accuracy assessment plot for soil organic carbon, pH and bulk density (obtained with 5-fold cross-validation).
Table: Mapping accuracy for AfSoilGrids250m maps assessed using 5-fold cross-validation (using only random forests models).
Code |
Soil property name |
Units |
Sample size |
Range |
RMSE |
% variance |
ORCDRC |
Soil organic carbon concentration |
g kg^{-1} |
64,010 |
0.9–42‰ |
10.6 |
61.3 |
PHIHOX |
Soil pH in H2O |
- |
68,458 |
4.4–8.7 |
0.67 |
66.9 |
CRFVOL |
Coarse fragments volumetric |
cm^{3} cm^{-3} |
39,206 |
7–90% |
18.4 |
20.3 |
SNDPPT |
Soil texture fraction sand |
kg kg^{-1} |
55,578 |
7–94% |
15.9 |
61.1 |
SLTPPT |
Soil texture fraction silt |
kg kg^{-1} |
54,164 |
1–47% |
8.3 |
56.1 |
CLYPPT |
Soil texture fraction clay |
kg kg^{-1} |
54,167 |
3–73% |
13.7 |
52.4 |
BLD |
Bulk density (fine earth) |
t m^{-3} |
8732 |
0.9–1.9 |
0.14 |
70.4 |
CEC |
Cation Exchange Capacity (fine earth) |
cmol+/kg |
47,875 |
1.2–57 |
7.9 |
66.3 |
NTO |
Total nitrogen |
g kg^{-1} |
50,997 |
0.1–3.1‰ |
0.69 |
61.0 |
ALUM3S |
Aluminium concentration |
ppm |
18,055 |
150–1800 |
160 |
86.3 |
EACKCL |
Exchangeable acidity |
cmol+/kg |
24,242 |
0–6.4 |
1.30 |
77.3 |
ECAX |
Exchangeable Calcium |
cmol+/kg |
65,158 |
0.1–46 |
12.7 |
67.2 |
EXKX |
Exchangeable Potassium |
cmol+/kg |
64,518 |
0.01–2.4 |
0.60 |
58.6 |
EMGX |
Exchangeable Magnesium |
cmol+/kg |
63,261 |
0.04–15 |
2.5 |
60.0 |
ENAX |
Exchangeable Sodium |
cmol+/kg |
42,986 |
0–8.3 |
3.6 |
46.7 |
EXBX |
Sum of exchangeable bases |
cmol+/kg |
64,270 |
0.31–66 |
11.0 |
68.8 |