| Square chains (sq ch) | Square nautical miles (sq nmi) |
|---|---|
| 1 Square chain | 0.000117987454529 sq nmi |
| 2 Square chains | 0.000235974909059 sq nmi |
| 3 Square chains | 0.000353962363588 sq nmi |
| 4 Square chains | 0.000471949818117 sq nmi |
| 5 Square chains | 0.000589937272647 sq nmi |
| 10 Square chains | 0.00117987454529 sq nmi |
| 20 Square chains | 0.00235974909059 sq nmi |
| 25 Square chains | 0.00294968636323 sq nmi |
| 50 Square chains | 0.00589937272647 sq nmi |
| 100 Square chains | 0.0117987454529 sq nmi |
| Reference | Square chains (sq ch) | Square nautical miles (sq nmi) |
|---|---|---|
| A sheet of A4 paper | 0.00015412 sq ch | 0.0000000181842 sq nmi |
| A tennis court | 0.644451 sq ch | 0.0000760371 sq nmi |
| A football pitch | 17.6433 sq ch | 0.00208169 sq nmi |
| One hectare | 24.7105 sq ch | 0.00291553 sq nmi |
| Central Park, New York | 8426.29 sq ch | 0.994197 sq nmi |
The square chain is a unit of area equal to 484 square yards, or 404.686 square metres. Its symbol is sq ch. It is the area of a square one chain on a side, a chain being 22 yards, and ten square chains make exactly one acre. That last relationship is the entire reason the unit exists.
The chain was invented by the English mathematician Edmund Gunter around 1620, and it was a physical object before it was a measurement: a steel chain of one hundred links, 22 yards long, that two people carried across a field. Its length was chosen so that ten square chains would be an acre and eighty chains would be a mile, which let a surveyor work in whole numbers and reach acres by shifting a decimal point.
That was a genuine achievement in an age before decimal calculation. Measurements taken in chains and links could be multiplied together and divided by ten to give acres directly, without the awkward factors of 4840 or 43,560 that the yard and the foot force on the same problem. For three centuries almost every land survey in Britain, Ireland, North America and the colonies was recorded this way.
The consequences outlived the tool. American public land records are written in chains, so deeds and plats still describe boundaries as so many chains and links, and anyone tracing an old title has to convert. Wildland firefighting in the United States measures fire line and fire perimeter in chains to this day, because the unit was already embedded in forestry practice when the agencies were formed.
One familiar object preserves the length exactly. A cricket pitch is one chain from wicket to wicket, twenty-two yards, which is why that number appears in the laws of the game and nowhere else in modern sport. Anyone who can picture a cricket pitch can picture a chain, and a square chain is that length squared, roughly a tenth of an acre or a large suburban garden.
For conversion, the useful facts are that ten square chains make an acre and that one square chain is about 405 square metres, so a hectare is nearly twenty-five square chains. Old surveys quoting areas in chains convert cleanly to acres and only awkwardly to metric, which is a good reason to do the acre step first.
One square chain equals 484 square yards, 0.1 acres, about 404.686 square metres, or about 0.0405 hectares.
The square nautical mile is a unit of area equal to 3,429,904 square metres, or about 3.43 square kilometres. Its symbol is sq nmi. It is the area of a square one nautical mile on a side, the nautical mile being defined as exactly 1852 metres. It is the working unit of everything that happens at sea and is measured across a surface rather than along a line.
Its usefulness comes from the definition of the nautical mile itself. One nautical mile is very nearly one minute of latitude, so a square nautical mile is close to a one-minute cell on a chart. That makes it the natural grid square for anything plotted on a nautical chart, because the unit and the graticule agree without any conversion, and a navigator can read distance and area off the same scale.
Maritime law is written in nautical miles and therefore accounted in square ones. Under the international law of the sea, a coastal state has a territorial sea of twelve nautical miles, a contiguous zone of twenty-four and an exclusive economic zone reaching two hundred. The resulting areas are vast: a state with scattered island possessions can control several million square kilometres of ocean, and comparisons between such claims are usually made after converting these figures to metric.
Search and rescue planning depends on the unit directly. A search area is defined in square nautical miles, and the time to cover it follows from the search vessel's speed in knots and the sweep width of its sensors, both of which are already in nautical units. Keeping the whole calculation in one system is what allows a coordinator to estimate coverage without stopping to convert anything.
Fisheries science and naval operations use it the same way. Fish stock density is reported per square nautical mile, survey transects are spaced in nautical miles, exercise and restricted areas are drawn as blocks of them, and aviation control zones over water are described in the same terms because aircraft also navigate in knots.
For conversion, one square nautical mile is 3.43 square kilometres, 1.32 square statute miles, or about 848 acres. The ratio to the statute mile is not intuitive, since the nautical mile is only about 15 per cent longer but the square is a third larger, which is the usual consequence of squaring a length ratio.
One square nautical mile equals 3,429,904 square metres, about 3.43 square kilometres, about 1.32 square miles, or about 848 acres.