The TREND statement enables you to define a spatial trend covariate, where name is a standard SAS variable name that names the trend and the FIELD suboption describes the field as follows:

FIELD (Xvariable, Yvariable, fieldvariable )

specifies a spatial field variable as a trend by using any spatial field covariates that are available in the DATA=
data set, where Xvariable specifies the X coordinate and Yvariable specifies the Y coordinate of the spatial field. The third argument is the fieldvariable, which is a numeric variable in the DATA=
data set. The Xvariable and Yvariable should be the same as the ones in the PROCESS statement. If you specify a different Xvariable and a different Yvariable from the ones specified in the PROCESS statement then, PROC SPP will produce an error.