# Spatial variance structures

## Kriging models

Kriging models apply to points in an irregular spatial arrangement. They require coordinates of the points so that the distances can be calculated. For example
• the distance between time points in a one-dimensional longitudinal analysis,
• the spatial distance between plot coordinates in a two-dimensional field survey analysis.

Information for determining distances is supplied by the Sortkey argument on the structure line.
• For one dimensional cases, Sortkey may be
 the name of a data field containing the coordinate values when it relates to an R structure 0 in which case a vector of coordinates of length order must be supplied after all R and G structure lines. fac(x) when it relates to model term fac(x).
• In two directions ( IEXP, IGAU, IEUC, AEXP, AGAU, MATn), the Sortkey argument also depends on whether it relates to an R or G structure.
 For an R structure, use the form rrcc where rr is the number of a data field containing the coordinates for the first dimension and cc is the number of a data field containing the coordinates for the second direction. For example, in the analysis of spatial data, if the x coordinate was in field 3 and the y coordinate was in field 4, the second argument would be 304. fac(x,y) when it relates to model term fac(x,y). For example ``` ... y ~ mu ... !r fac(x,y) ... fac(x,y) 1 fac(x,y) fac(x,y) IEUCV .7 1.3 ```