   a decametre or dekametre (american spelling: dekameter, earlier decameter; symbol dam, sometimes dm or dkm) is a very rarely used unit of length in the metric system, equal to ten metres, the si base unit of length. it can be written in scientific notation as  (exponential notation), meaning . this measure is included in the si mostly for completeness: in principle, any combination of prefix and unit is permissible, but many are rarely used in practice. one practical use of the decameter is for altitude of geopotential heights in meteorology. meteorologists also use another seldom encountered si prefix: hecto- in hectopascal (hpa). the volumetric form (see below) cubic decametre is convenient for describing large volumes of water such as in rivers and lakes. *for surface the square decametre (dam2) is a common unit, albeit by the name are (a). the are is a measurement of area the size of 1 decametre by 1 decametre â the square decametre â 10 metres by 10 metres, equalling 100 square metres (100 m2). the standard metric unit of land registry, the hectare, is thus equivalent to 100&nbsp;dam2. *for volumes the cubic decametre (dam3) is also used, 10&nbsp;m by 10&nbsp;m by 10&nbsp;m, equalling 1,000&nbsp;cubic metres (1,000&nbsp;m3). one cubic decametre is equal to 0.811 acre feet. ==see also== *si prefix *deca- *for length: 1 e+1 m and orders of magnitude (length) *for surface: 1 e+2 mÂ² and orders of magnitude (area) *for volume: 1 e+3 mÂ³ and orders of magnitude (volume) *conversion of units, for comparison with other units 
Orders_of_magnitude_(length) Units_of_length