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The BeSSeL Survey is mapping the spiral structure of the Milky Way bymeasuring trigonometric parallaxes of hundreds of maser sources associated withhigh-mass star formation. While parallax techniques for water masers at highfrequency (22 GHz) have been well documented, recent observations of methanolmasers at lower frequency (6.7 GHz) have revealed astrometric issues associatedwith signal propagation through the ionosphere that could significantly limitparallax accuracy. These problems displayed as a "parallax gradient" on the skywhen measured against different background quasars. We present an analysismethod in which we generate position data relative to an "artificial quasar" atthe target maser position at each epoch. Fitting parallax to these data cansignificantly mitigate the problems and improve parallax accuracy.

Techniques for Accurate Parallax Measurements for 6.7-GHz Methanol Masers