What is the difference between universal kriging and ordinary kriging?
Ordinary kriging assumes the absence of a drift and focuses on the spatially correlated component and uses the fitted semivariogram directly for interpolation. Universal kriging, however, assumes that the spatial variation in z values has a drift or a trend in addition to the spatial correlation between the sample points. In the first-order polynomial, M = b_{1}x_{i} + b_{2}y_{i}, M is the drift, x_{i} and y_{i} are the coordinates of the sampled point i, and b_{1} and b_{2} are the drift coefficients.