+According to \cite{valeriano:2002}, variograms calculated with linear trend residues of topographic data have adequate fits to classical variogram models, which present a clear and defined sill. Residues of trend surface analysis are used to guarantee geostationarity of data being modelled.
Since DEMs datasets can be very large, variogram calculation and kriging interpolation can became very time-consuming tasks. The basic idea is to choose a representative subset of the study area, and calculate the variogram over this subset. This variogram model will then be used by kriging to interpolate the hole dataset
Typo: "hole" should be "whole".
Maciek