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The hectare ( or ; symbol ha) is an SI accepted metric system unit of area equal to and primarily used in the measurement of land. An acre is about 0.4047 hectare and one hectare contains about 2.47 acres. In 1795, when the metric system was introduced, the "are" was defined as 100 square metres and the hectare ("hecto-" + "are") was thus 100 "ares" or km2. When the metric system was further rationalised in 1960, resulting in the International System of Units (), the ''are'' was not included as a recognised unit. The hectare, however, remains as a non-SI unit accepted for use with the SI units, mentioned in Section 4.1 of the SI Brochure as a unit whose use is "expected to continue indefinitely".〔 == History == The metric system of measurement was first given a legal basis in 1795 by the French Revolutionary government. The law of 18 Germinal, Year III (7 April 1795) defined five units of measure: *The metre for length *The are (100 m2) for area (land ) *The stère (1 m3) for volume of stacked firewood *The litre (1 dm3) for volumes of liquid *The gram for mass Although the law defined the length of the metre, there was no practical way of accurately measuring the metre (and hence the ''are'') until 1799 when the first standard metre was manufactured and adopted. The standard metre remained in the custody of successive French governments until 1875 when, under the Convention of the Metre, its supervision passed into international control under the auspices of the General Conference on Weights and Measures (). At the first meeting of the CGPM in 1889 when a new standard metre, manufactured by Johnson Matthey & Co of London was adopted, the ''are'' and ''hectare'' were automatically redefined. In 1960, when the metric system was updated as the International System of Units (SI), the ''are'' did not receive international recognition. The International Committee for Weights and Measures () makes no mention of the ''are'' in the current (2006) definition of the SI, but classifies the hectare as a "Non-SI unit accepted for use with the International System of Units" In 1972, the European Economic Community (EEC) passed directive 71/354/EEC,〔(【引用サイトリンク】 title=Council Directive of 18 October 1971 on the approximation of laws of the member states relating to units of measurement, (71/354/EEC) )〕 which catalogued the units of measure that might be used within the Community. The units that were catalogued replicated the recommendations of the CGPM, supplemented by a few other units including the ''are'' (and implicitly the ''hectare'') whose use was limited to the measurement of land. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Hectare」の詳細全文を読む スポンサード リンク
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