  We use Spitzer 24 $\mu$m, 70 $\mu$m and ground based H$\alpha$ data for a
sample of 40 SINGS galaxies to establish a star formation rate (SFR) indicator
using 70 $\mu$m emission for sub--galactic ($\sim0.05-2\ \rm{kpc}$)
line-emitting regions and to investigate limits in application. A linear
correlation between 70 $\mu$m and SFR is found and a star formation indicator
SFR(70) is proposed for line-emitting sub-galactic regions as $\rm \Sigma(SFR)\
({M_{\odot}\cdot yr^{-1}\cdot kpc^{-2}})=9.4\times10^{-44}\ \Sigma(70)\
\rm{(ergs\cdot s^{-1}\cdot kpc^{-2})}$, for regions with
$12+\rm{log(O/H)}\gtrsim8.4$ and $\rm \Sigma(SFR)\gtrsim10^{-3}\
(M_{\odot}\cdot yr^{-1}\cdot kpc^{-2})$, with a 1-$\sigma$ dispersion around
the calibration of $\sim0.16$ dex. We also discuss the influence of metallicity
on the scatter of the data. Comparing with the SFR indicator at 70 $\mu$m for
integrated light from galaxies, we find that there is $\sim40%$ excess 70
$\mu$m emission in galaxies, which can be attributed to stellar populations not
involved in the current star formation activity.
