While sometimes used interchangeably with the term "drift", in geostatistics the two are considered separate. The term is usually reserved to denote the deterministic representation obtained by the use of Trend Surface Analysis, i.e, a functional representation for spatially located data (usually taken as a polynomial in the position coordinates). The "trend" is obtained by a least squares fit to the data. As an estimator to the mean of a random function it is sub-optimal. If the residuals from trend surface analysis are used to model the variogram, a biased variogram estimator results.