| Decameters (dam) | Nautical leagues (nlea) |
|---|---|
| 1 Decameter | 0.00179985601152 nlea |
| 2 Decameters | 0.00359971202304 nlea |
| 3 Decameters | 0.00539956803456 nlea |
| 4 Decameters | 0.00719942404608 nlea |
| 5 Decameters | 0.0089992800576 nlea |
| 10 Decameters | 0.0179985601152 nlea |
| 20 Decameters | 0.0359971202304 nlea |
| 25 Decameters | 0.044996400288 nlea |
| 50 Decameters | 0.089992800576 nlea |
| 100 Decameters | 0.179985601152 nlea |
| Reference | Decameters (dam) | Nautical leagues (nlea) |
|---|---|---|
| A sheet of A4 paper (long side) | 0.0297 dam | 0.0000534557 nlea |
| Average adult human height | 0.17 dam | 0.000305976 nlea |
| A football pitch (length) | 10.5 dam | 0.0188985 nlea |
| A marathon | 4219.5 dam | 7.59449 nlea |
| Height of Mount Everest | 884.9 dam | 1.59269 nlea |
The decametre is ten metres, written dam. It is a valid SI unit formed with the prefix deca, but like the decimetre it is rarely used. The prefix deca is the only SI multiplier whose symbol is two letters, which makes it slightly awkward to write and may have contributed to its neglect.
Spelling varies. Decametre is the international form; dekameter appears in some American texts, and the symbol dkm is occasionally seen in older material, though dam is the form recognised by the BIPM. The similarity between dam and the abbreviation for decimetre has caused confusion, and some standards bodies discourage the unit for that reason alone.
Where it does appear, it is usually in surveying, forestry and agronomy. Field dimensions, plot spacing and tree heights sit conveniently in the range of a few decametres. Some European cadastral records use the unit. It also occurs in specifications for cable and hose supplied in ten-metre lengths.
The derived area unit is more familiar than the length unit. A square decametre is 100 square metres, which is exactly one are. The are is itself little used, but its hundredfold multiple, the hectare, is the standard measure of land area across most of the world. A hectare is 100 ares, or 10,000 square metres, and can equally be described as a square hectometre.
In practice most people express ten metres simply as ten metres, or as 0.01 kilometres when working at larger scales. The decametre is best understood as a formally correct unit that everyday usage has passed over.
The unit does appear in one everyday context without being named. Swimming pool lane lengths, running track segments and the spacing of street lighting are frequently set in multiples of ten metres. Planning documents that lay out plot frontages or building setbacks often work to the same grid. The quantity is useful; it is only the word that has fallen out of use.
Its square has fared better than the length itself. The are, a hundred square metres, is the square decametre under another name, and although the are is now rare its hundredfold the hectare governs land measurement across most of the world. The same pattern shows in the decare of a thousand square metres, which is the standard unit of farmland in Turkey, Greece, Norway and parts of the Balkans and is simply ten ares. So the decametre survives chiefly by inheritance: nobody quotes a field boundary in decametres, but a great deal of the world's land is bought and sold in units built from it.
One decametre equals 10 metres, 1000 centimetres, or 0.01 kilometres. It is about 32.8084 feet.
The nautical league is a unit of length equal to three nautical miles, or exactly 5556 metres. It is longer than the land league by about 728 metres, since it is built from the nautical mile of 1852 metres rather than the statute mile.
The unit belonged to an era when it was the practical measure of coastal jurisdiction. The cannon shot rule, articulated in the seventeenth and eighteenth centuries, held that a state's authority extended as far as it could defend from shore, conventionally taken as three nautical miles. That distance became the standard territorial sea limit and was expressed as one marine league. Many older treaties and statutes define maritime boundaries in leagues for this reason.
The three-mile limit held for centuries before eroding through the twentieth. States progressively claimed wider waters, and the United Nations Convention on the Law of the Sea settled the matter in 1982 by establishing a twelve nautical mile territorial sea and a two hundred mile exclusive economic zone. Both figures are stated in nautical miles rather than leagues, and the older unit fell out of legal use.
Charts and pilot books from the age of sail used the league routinely for describing offshore distances and the visibility of landmarks. Reading those sources accurately requires knowing which league is meant, since the same word could refer to a land league, a marine league or a Spanish or Portuguese variant, and the differences are large enough to matter when reconstructing a historical position.
Today the unit has no navigational role. Charts are marked in nautical miles, and distances at sea are quoted the same way. The nautical league appears only in historical and legal texts.
The unit also underlies a familiar figure in visibility calculations. From a height of about three metres above sea level, the horizon lies roughly one nautical league away, which is why the three-mile limit corresponded loosely to what a lookout could see as well as to what a gun could reach.
One nautical league equals 3 nautical miles, 5556 metres, 5.556 kilometres, or about 3.452 statute miles.