def ocean_sigma_z_coordinate(sigma, eta, depth, depth_c, nsigma, zlev):
    """
    Creates a dimensioned version of ocean sigma over z.

    Definition:
        for k <= nsigma
            z(n,k,j,i) = eta(n,j,i) + sigma(k)*(min(depth_c,depth(j,i))+eta(n,j,i))
        for k > nsigma
            z(n,k,j,i) = zlev(k)

        where z(n,k,j,i) is height, positive upwards, relative to ocean datum
        (e.g. mean sea level) at gridpoint (n,k,j,i) , eta(n,j,i) is the height
        of the ocean surface, positive upwards, relative to ocean datum at
        gridpoint (n,j,i) , sigma(k) is the dimensionless coordinate at
        vertical gridpoint (k) for k <= nsigma , and depth(j,i) is the distance
        from ocean datum to sea floor (positive value) at horizontal
        gridpoint (j,i) . Above depth depth_c there are nsigma layers.

    """
    raise NotImplemented
