code: def mk_dx_dy_from_geotif_layer(geotif):
      
      ELLIPSOID_MAP = {'WGS84': 'WGS-84'}
      ellipsoid = ELLIPSOID_MAP[geotif.grid_coordinates.wkt]
      d = distance(ellipsoid=ellipsoid)
      dx = geotif.grid_coordinates.x_axis
      dy = geotif.grid_coordinates.y_axis
      dX = np.zeros((dy.shape[0]-1))
      for j in xrange(len(dX)):
          dX[j] = d.measure((dy[j+1], dx[1]), (dy[j+1], dx[0])) * 1000  # km2m
      dY = np.zeros((dy.shape[0]-1))
      for i in xrange(len(dY)):
          dY[i] = d.measure((dy[i], 0), (dy[i+1], 0)) * 1000  # km2m
      return dX, dY
docs: Extracts the change in x and y coordinates from the geotiff file. Presently
      only supports WGS-84 files.