code: def test_verticalcs_depth_to_cf():
      vcs = VerticalCS(axis=VerticalCSAxis.DEPTH)
      vcs.to_cf() == [
          {
              "standard_name": "height_above_reference_ellipsoid",
              "long_name": "Depth",
              "units": "metre",
              "positive": "down",
              "axis": "Z",
          }
      ]
  
file_path: pyproj/test/crs/test_crs_coordinate_system.py
imports: import pytest
  from pyproj.crs.coordinate_system import Cartesian2DCS, Ellipsoidal2DCS, Ellipsoidal3DCS, VerticalCS
  from pyproj.crs.enums import Cartesian2DCSAxis, Ellipsoidal2DCSAxis, Ellipsoidal3DCSAxis, VerticalCSAxis
label_desc: Use image node_id 9 for calling a global function with example usage: test_verticalcs_depth_to_cf() without return types