$ gemmi map -h
Usage:
 gemmi map [options] CCP4_MAP[...]

  -h, --help         Print usage and exit.
  -V, --version      Print version and exit.
  -v, --verbose      Verbose output.
  --deltas           Statistics of dx, dy and dz.
  --check-symmetry   Compare the values of symmetric points.
  --write-xyz=FILE   Write transposed map with fast X axis and slow Z.
  --write-full=FILE  Write map extended to cover whole unit cell.
  --write-mask=FILE  Make a mask by thresholding the map.

Options for making a mask:
  --threshold        Explicit threshold value for 0/1 mask.
  --fraction         Threshold is selected to have this fraction of 1's.