= Maser-disk geometric distance
An orbit model for a resolved <maser> disk combines angular positions, Doppler velocities, accelerations and sometimes <proper motions> to infer physical orbital dimensions and distance. For an ideal matched circular radius, $R=v^2/a_{\rm orb}$ and $d=R/\theta$; velocities and accelerations of unrelated <maser> spots cannot simply be inserted into that expression. Warps, inclination, eccentricity and feature identification enter real disk fits.<A 4% Geometric Distance to the Galaxy NGC4258 from Orbital Motions in a Nuclear Gas Disk by J. R. Herrnstein and others>[1999 abstract; VLBI proper motions and accelerations]{ref}
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