Another approach for incorporating secondary information is cokriging, a multivariate extension of
kriging (Goovaerts, 1997, pp. 203–248). When the secondary variable is known everywhere and varies
smoothly across the study area (e.g. elevation) there is little loss in retaining in the cokriging system only the
secondary datum co-located with the location u being estimated (Xu et al., 1992; Goovaerts, 1998).