  Any calibration of the present value of the Hubble constant requires
recession velocities and distances of galaxies. While the conversion of
observed velocities into true recession velocities has only a small effect on
the result, the derivation of unbiased distances which rest on a solid zero
point and cover a useful range of about 4-30 Mpc is crucial. A list of 279 such
galaxy distances within v<2000 km/s is given which are derived from the tip of
the red-giant branch (TRGB), from Cepheids, and from supernovae of type Ia (SNe
Ia). Their random errors are not more than 0.15 mag as shown by
intercomparison. They trace a linear expansion field within narrow margins from
v=250 to at least 2000 km/s. Additional 62 distant SNe Ia confirm the linearity
to at least 20,000 km/s. The dispersion about the Hubble line is dominated by
random peculiar velocities, amounting locally to <100 km/s but increasing
outwards. Due to the linearity of the expansion field the Hubble constant H_0
can be found at any distance >4.5 Mpc. RR Lyr star-calibrated TRGB distances of
78 galaxies above this limit give H_0=63.0+/-1.6 at an effective distance of 6
Mpc. They compensate the effect of peculiar motions by their large number.
Support for this result comes from 28 independently calibrated Cepheids that
give H_0=63.4+/-1.7 at 15 Mpc. This agrees also with the large-scale value of
H_0=61.2+/-0.5 from the distant, Cepheid-calibrated SNe Ia. A mean value of
H_0=62.3+/-1.3 is adopted. Because the value depends on two independent zero
points of the distance scale its systematic error is estimated to be 6%.
Typical errors of H_0 come from the use of a universal, yet unjustified P-L
relation of Cepheids, the neglect of selection bias in magnitude-limited
samples, or they are inherent to the adopted models.
