Retrieve the set of all possible starting positions

In two-dimensional systematic samples, all possible samples can be obtained merely by moving the starting position within a "starting region" contained by the top-left-most sample group. For a sampling interveal a the hexagonal and rectangular configurations will contain a^2 possible samples. This function retrieves those starting positions and is used internally as part of compare_estimators().

subsample_starts(sys_frame, a)

Arguments

sys_frame	A SysFrame object
a	The sampling interval

Value

A dataframe of starting positions for all possible systematic samples for the interval a.