The local expansion field ( v _ { 220 } < 1200 \nobreak \mbox { $ $km s$ { } ^ { -1 } $ } ) and the cosmic expansion field out to 30 000 \nobreak \mbox { $ $km s$ { } ^ { -1 } $ } are characterized by H _ { 0 } = 58 [ km s ^ { -1 } Mpc ^ { -1 } ] .
While the random error of this determination is small ( \pm 2 units ) , it may still be affected by systematic errors as large as \pm 10 \% .
The local expansion is outlined by Cepheids and by Cepheid-calibrated TF distances of a complete sample of field galaxies and by nearby groups and clusters ; the cosmic expansion is defined by Cepheid-calibrated SNe Ia .
The main source of systematic errors are therefore the shape and the zero point of the P-L relation of Cepheids and its possible dependence on metallicity .
GAIA will essentially eliminate these systematic error sources .
Another source of systematic error is due to the homogenization of SNe Ia as to decline rate \Delta m _ { 15 } and color ( B - V ) .
GAIA will discover about half of the 2200 SNe Ia which will occur during a four-year lifetime within 10 000 \nobreak \mbox { $ $km s$ { } ^ { -1 } $ } .
For many of them ground-based follow-up will provide useful photometric parameters ( and spectra ) , which will allow to fix the dependence of the SNe Ia luminosity on \Delta m _ { 15 } and ( B - V ) with high accuracy .
At the same time they will yield exquisite distances to a corresponding number of field galaxies .
— GAIA will also revolutionize the very local distance scale by determining fundamental distances of the companion galaxies of the Milky Way and even of some spirals in- and possibly outside the Local Group from their rotation curves seen in radial velocities and proper motions .

Moreover , GAIA will obtain trigonometric parallaxes of RR Lyrae stars , of red giants defining the TRGB , of stars on the ZAMS , of White Dwarf defining their cooling sequence , and of globular clusters , and determine the metallicity dependence of these distance indicators .
It will thus establish a self-controlling network of distance indicators within the Local Group and beyond .