Create the data set. REFLIB.CLAY contains the percent of clay at various locations of a test site. 
data reflib.clay;
   input x y pct_clay;
   datalines;
-10    -10       2.316
-10     -9       1.816
-10     -8       2.427
...more data lines...
 10      8        .
 10      9        .
 10     10        .
;