| Square nautical miles (sq nmi) | Square hectometers (hm²) |
|---|---|
| 1 Square nautical mile | 342.9904 hm² |
| 2 Square nautical miles | 685.9808 hm² |
| 3 Square nautical miles | 1028.9712 hm² |
| 4 Square nautical miles | 1371.9616 hm² |
| 5 Square nautical miles | 1714.952 hm² |
| 10 Square nautical miles | 3429.904 hm² |
| 20 Square nautical miles | 6859.808 hm² |
| 25 Square nautical miles | 8574.76 hm² |
| 50 Square nautical miles | 17149.52 hm² |
| 100 Square nautical miles | 34299.04 hm² |
| Reference | Square nautical miles (sq nmi) | Square hectometers (hm²) |
|---|---|---|
| A sheet of A4 paper | 0.0000000181842 sq nmi | 0.000006237 hm² |
| A tennis court | 0.0000760371 sq nmi | 0.02608 hm² |
| A football pitch | 0.00208169 sq nmi | 0.714 hm² |
| One hectare | 0.00291553 sq nmi | 1 hm² |
| Central Park, New York | 0.994197 sq nmi | 341 hm² |
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.
The square hectometre is a unit of area equal to ten thousand square metres. It is written hm², and it is the area of a square one hundred metres on a side, since a hectometre is one hundred metres. It is exactly one hectare, and the two names describe the same quantity by two different routes.
The routes are worth distinguishing. The hectare is built by putting the prefix hecto in front of the are, so it is a hundred ares. The square hectometre is built by squaring a prefixed length, which is how the International System forms every derived area. The second construction is the strictly coherent one; the first is the one written on every land title in the world.
That difference explains where each name is found. Hectare appears in agriculture, forestry, planning, taxation and international statistics. Square hectometre appears mainly in technical and cartographic work in Spanish-speaking countries, where the hectometre is a familiar prefixed length for other reasons, and in documents where an author has chosen to keep every unit inside strict SI form.
The related cubic form is genuinely common in one field. Spanish and Latin American hydrology reports reservoir capacity and river flow in cubic hectometres, a cubic hectometre being one million cubic metres, so a reservoir holding 500 hm³ is holding half a cubic kilometre of water. Readers used to seeing hm in that context find hm² unremarkable, which is much of why the form survives there and almost nowhere else.
For scale, ten thousand square metres is a square one hundred metres on a side. That is about one and a half association football pitches, a large city block, or a good-sized field. It is the largest area that can still be walked around comfortably in a couple of minutes, and it is where land measurement stops being architectural and starts being agricultural.
When converting, the two names need no arithmetic between them: one square hectometre is one hectare exactly. Going further, multiply by ten thousand for square metres or divide by a hundred for square kilometres, and remember that the imperial comparison, 2.471 acres, is the same for both names.
One square hectometre equals 10,000 square metres, 1 hectare, 0.01 square kilometres, or about 2.471 acres.