/SUBGRID keyword in
forces linear interpolation in
x at the specified coordinates.
Without this keyword, floating-point subscripts are converted to integer
type and the elements corresponding to those integer subscripts are then
extracted from the source variable.
This keyword can only be used with numerical arrays
x, and only
numerical arrays and scalars are allowed as subscripts. The result is
/SUBGRID is specified, then
implied. For example,
x([1.2,2.3], [3.3,4.1], /SUBGRID) is
[x(1.2, 3.3, /SUBGRID), x(2.3, 4.1, /SUBGRID)].