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

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