  The generation mechanism of compressible fluid turbulence at kiloparsec
scales in the Interstellar Medium (ISM) is a long-lasting puzzle. In this work,
we explore the nature of large-scale turbulence in the external spiral galaxy
NGC~6946. We use the Visibility Moment Estimator (VME) to measure the \HI
column density and line of sight turbulent velocity power spectra combining the
new observations of A array configuration of Karl G. Jansky Very Large Array
(VLA) with the VLA B, C, D array observations from The \HI Nearby Galaxy Survey
(THINGS). The estimated power spectra are obeying a power law with a slope of
$-0.96\pm0.05$ in column density and $-1.81\pm0.07$ in line of sight velocity
in length scales ranging from $6$ kpc to $170$ pc. This points towards a
forward energy cascade in the plane of the disc with a driving scale at least
as large as $6$ kpc. The values of the power law indices indicate a combination
of solenoidal and compressive force responsible in driving the measured
turbulence. The presence of strong regular magnetic fields from the magnetic
spiral arms in the galaxy is possibly contributing to the solenoidal part,
while self-gravity or gravitational instability can mostly be the input for the
compressive part of the forcing in the driving mechanism.
