  A new method for measuring the large-scale structure of the Galactic magnetic
field is presented. The Galactic magnetic field has been probed through the
Galactic disk with near-infrared starlight polarimetry, however the distance to
each background star is unknown. Using red clump stars as near-infrared
standard candles, this work presents the first attempt to decompose the line of
sight structure of the sky-projected Galactic magnetic field. Two example
lines-of-sight are decomposed: toward a field with many red clump stars and
toward a field with few red clump stars. A continuous estimate of magnetic
field orientation over several kiloparsecs of distance is possible in the field
with many red clump stars, while only discrete estimates are possible in the
sparse example. toward the Outer Galaxy, there is a continuous field
orientation with distance that shows evidence of perturbation by the Galactic
warp. toward the Inner Galaxy, evidence for a large-scale change in the
magnetic field geometry is consistent with models of magnetic field reversals,
independently derived from Faraday rotation studies. A photo-polarimetric
method for identifying candidate intrinsically polarized stars is also
presented. The future application of this method to large regions of the sky
will begin the process of mapping the Galactic magnetic field in a way never
before possible.
