  We present a new, inexpensive, bench-top method for measuring groove period
over large areas with high mapping resolution and high measurement accuracy,
dubbed the grating mapper for accurate period (GMAP). The GMAP has the ability
to measure large groove period changes and non-parallel grooves, both of which
cannot be measured via optical interferometry. In this paper, we detail the
calibration and setup of the GMAP, and employ the instrument to measure three
distinct gratings. Two of these measured gratings have customized groove
patterns that prevent them from being measured via other traditional methods,
such as optical interferometry. Our implementation of this tool achieves a
spatial resolution of 0.1 mm$\times$0.1 mm and a period error of 1.7 nm for a 3
$\mu$m size groove period.
