Hectare
Non-SI unit accepted for use with the SI · quantity: area
The surveyor who arrived in the village in the spring of 1796 brought a chain, an astrolabe and a new word. He meant to measure in ares — squares ten metres by ten, the principal land measure in the intention of the metric system's authors. The peasants listened politely and went on counting in arpents. The are took root nowhere but in dacha talk, while its hundred survived: the hectare, a square one hundred metres on a side, a unit whose prefix has grown into its name and become inseparable.
01 · Definition
A hectare is the area of a square one hundred metres on a side, exactly ten thousand square metres. Formally it is a hectoare — a prefix placed on a prefix, a form the SI rules forbid.
The way out of the contradiction was clerical: the hectare was declared a name of the unit in its own right rather than a derived form, and so no further prefix may be attached to it. There is no “kilohectare”; beyond a few hundred hectares one moves to square kilometres. The are meanwhile stayed in the tables as a — one hundred square metres. The Russian word sotka is its literal translation: a hundredth part of a hectare.
The scale of the unit explains its durability. A field, a forest compartment, a city park, a factory site — all of these are single or double digits of hectares, whereas in square metres the figures run to five digits and in square kilometres to fractions. The hectare took the empty place between two awkward scales and holds it still: in cadastres, in forest management, in agricultural statistics, in fire reports.
One relation is worth holding firmly: a square kilometre contains a hundred hectares, not a thousand. Area grows as the square of linear size, and the tenfold error is the commonest one in newspaper reports of clear-cuts and burnt-over land.
Nobody measures a parcel directly: the boundary is walked, angles and lengths are recorded, and the area is computed. Add up the measured angles and the sum will not match the geometric one — the traverse fails to close, and the gap shows what the resulting hectare is worth.
The red cell is a hectare, the whole grid a square kilometre. A kilometre holds ten hundreds of metres, but a square kilometre holds only a hundred hectares. In fire reports these two numbers are substituted for one another more often than any others.
02 · Conversion
Enter an area — the sheet will convert it
Land measures proved the most durable of all: the hectare displaced neither the acre, nor the mu, nor the rai, nor the dunam. The reason is plain — land is not shipped anywhere, so a local measure faces no competitor from outside, and changing it means redrawing every deed at once.
Half the world still counts its fields in something other than hectares: fifteen mu to the Chinese hectare, six and a quarter rai to the Thai one, ten dunams to the Turkish one.
The small red square is the are, one hundredth of a hectare, one hundred square metres. The standard six ares of a Soviet garden plot are 600 square metres, 0.06 hectare; the figure grew out of the 1949 decree on collective gardens, where the size was chosen so that a plot would feed a family vegetables without becoming a commercial holding.
| Unit | Name | Value | Where it is met |
|---|---|---|---|
| ha | hectare | 1 | land management, agriculture |
| m² | square metre | 10 000 | SI unit, definition of the hectare |
| a | are | 100 | garden plots in Russia |
| km² | square kilometre | 0.01 | districts, forestries, maps |
| dam² | square decametre | 100 | the same as the are, but through an SI prefix |
| ac | acre | 2.4710538 | USA, Britain, India |
03 · Orders of magnitude
from a flat to all dry land04 · Measuring instruments
What hectares are measured with
Ten sazhens — 21.3 metres — in links of one arshin each. The chain stretched under tension and was checked against a standard before every outing; the relative error of a traverse came to about 1 : 300.
A circle with sights on a tripod: angles read to a quarter of a degree. Boundaries in eighteenth- and nineteenth-century Russia were laid out with it, and the area was then obtained by breaking the parcel into triangles.
Angles to five seconds, distance by laser to millimetres. The coordinates of the turning points are written to memory and the area is computed by the polygon formula on the spot.
It does not measure angles at all: each point receives a coordinate accurate to about two centimetres, and traverse misclosure simply does not arise. The area of ploughland and clear-cuts is now more often taken from satellite imagery — a ten-metre pixel gives a cell of one hundredth of a hectare.
05 · Writing rules
Takes no prefixes
Hecto is already inside the name, so a second prefix may not be attached: for large areas one uses square kilometres, for small ones ares and square metres. The symbol is written with two lower-case letters. The are is denoted by a single letter a, and it takes no prefixes either.
Yield is recorded in decitonnes or tonnes per hectare: thirty decitonnes per hectare is an ordinary result for wheat in the temperate belt, three tonnes of grain from a square one hundred metres on a side. English-language reports give the same quantity in bushels per acre, and the conversion factor depends on the crop, because a bushel is a measure of volume, not of weight.
06 · Neighbouring units
A hectare is a square hectometre, and it stands exactly midway between the are and the square kilometre: a hundredfold each way. The whole chain is set by the metre, and at the opposite end of the scale of areas lives the barn — the cross-section of an atomic nucleus, thirty-two orders of magnitude below the hectare.
100 m²
10⁴ m²
100 ha
The land area of the Earth is 148.9 million square kilometres, that is 14.9 billion hectares: about 1.8 hectares per person, of which ploughland yields roughly 0.17. Agricultural land takes up about a third of the dry surface, and since the 1990s its area has barely grown — only the yields have.
07 · Historical section
How land was measured before the hectare
The strip a pair of oxen would plough in a day: a furlong long, four rods wide — 43 560 square feet. The shape of the measure is written into its definition, because turning a team around was expensive and a field was made long.
2400 square sazhens made the state desyatina — nine per cent larger than a hectare. Alongside it ran the oblique, the round and the household desyatina, all of different sizes, and in boundary disputes the first thing settled was which one was meant.
The Homestead Act gave a settler 160 acres — a quarter of a square mile, 64.7 hectares. The rectangular grid of that division is still visible in imagery of the Midwest: the field boundaries run strictly along the meridians.
A measure the metric reform did not abolish but adjusted: the mu was set at 666⅔ square metres so that exactly fifteen would make a hectare. Chinese rural statistics are kept in mu to this day.
A unit with two prefixes had to be declared indivisible
The SI forbids compound prefixes: kilokilogram and micromillimetre may not be written. The hectare breaks the ban in its very name — it is a hectoare, a prefix upon a prefix. The resolution proved to be lexical rather than physical: the word was declared a name of the unit in its own right, after which taking it apart became illegitimate, and further prefixes are simply not applied to it.
The are met the opposite fate. It was meant to be the principal land measure of the new system, but its scale suited neither a garden plot nor a field: one hundred square metres is too little for the one and laughably little for the other. The unit died in the documents and survived in speech — as the Russian sotka, still used to measure dachas by people who have no idea they are pronouncing the name of a French measure of 1795.
Catalogue · units of measurement
A passport for every quantity
Seven SI base units, twenty-two derived ones with names of their own, and the non-SI units neither engineering nor daily life can do without. Each has a sheet of its own: definition, conversion, instruments, writing rules, history. In 36 languages.
Non-SI units
in vermilion — the sheet now openSI base units
the metre is highlighted — the hectare is defined through itSI derived units with names of their own
twenty-two sheetsSheet language
36 languages. The symbol ha and the formulas are not translated — they are the same in every locale; what is translated is the prose, the local land measures and the writing rules.