Heliocentric radial velocity (cz) from radio measurements

vrad is computed from the radio (mostly HI)
measurements in the
compilation of redshifts
by rejecting outliers and measurements marked for their
bad quality and computing a weighted average of the remaining
measurements.
The "optical" measurements are homogenized and combined separately to
produce another parameter, vopt, and
both vopt and vrad are combined to produce
v.

Details of the computation

The radio measurements are selected
and weight-averaged. The weights are the inverse of the square of the errors.
When the precision is not given it is assumed to be
0.3*resc+5.5, and if the resolution (resc) is not known, the
precision is taken as 10 km/s.