The centimetre and millimetre are submultiples of the base unit metre. The kilometre is a multiple of the base unit.
The name of the millibar should be changed to the hectopascal, which is a multiple of the SI derived unit the pascal.
The hectare is a non-SI unit currently accepted for use with the SI.
The square metre and cubic metre are SI derived units, whereas the square kilometre is a unit derived from the multiple of an SI base unit. Note that
1 km2 = 1 000 000 m2.
The hectare is useful to fill this gap.
1 km2 = 100 ha, 1 ha = 10 000 m2.
I might add that it is good engineering practice to not use the prefixes hecto-. deca-, deci- and centi-. This avoids the need to state the unit symbol after every dimension number on a drawing..
-- Joseph B. Reid 17 Glebe Road West Toronto M5P 1C8 Telephone 416-486-6071
