  We have measured the HI power spectrum of the nearly face-on spiral galaxy
NGC628 (M74) using a visibility based estimator. The power spectrum is well
fitted by a power law $P(U)=AU^{\alpha}$, with $\alpha =- 1.6\pm0.2$ over the
length scale $800 {\rm pc} {\rm to} 8 {\rm kpc}$. The slope is found to be
independent of the width of the velocity channel. This value of the slope is a
little more than one in excess of what has been seen at considerably smaller
length scales in the Milky-Way, Small Magellanic Cloud (LMC), Large Magellanic
Cloud (SMC) and the dwarf galaxy DDO210. We interpret this difference as
indicating a transition from three dimensional turbulence at small scales to
two dimensional turbulence in the plane of the galaxy's disk at length scales
larger than galaxy's HI scale height.
  The slope measured here is similar to that found at large scales in the LMC.
Our analysis also places an upper limit to the galaxy's scale height at $800\
{\rm pc}$ .
