Using multiple external drifts to estimate a soil variable

Kriging with topographic variable (beta) and electrical resistivity (rho) measurements used as external drifts was compared with universal kriging and kriging either with a topographic variable or with electrical resistivity measurements taken as external drift, to predict the depth of a limestone bedrock upper boundary (LUB) in the "Petite Beauce" region, southwest of the Paris Basin. Two sets of limestone depth apparition (LUB) were recorded. A first set of 67 measurements was used for the prediction and another set of 50 measurements was kept for the validation. The slope gradient (beta) and the resistivity (rho), which are linearly related to the limestone upper boundary depth (LUB) and exhaustively sampled over the whole study area, were used as external drift variables for kriging. Two indices, (i) the mean error (ME) and (ii) the root mean square error (RMSE), as well as residuals analysis, were computed from the validation sample (observed data) and predicted values. On the 50 test data, the results showed that kriging using two external drifts proved to be less unbiased and more precise compared to kriging using only one external drift variable in predicting the target variable. (C) 2002 Elsevier Science B.V. All rights reserved.