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 \pm 0.2 over the length scale 800 { pc } { to } 8 { 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 { pc } .