Editing Acre (section)

==US survey acres==
In the [[international yard and pound]] agreement of 1959, the United States and five countries of the [[Commonwealth of Nations]] defined the international yard to be exactly 0.9144 metre.<ref name=NBS1959>{{cite web |publisher= National Bureau of Standards |date= 25 June 1959 |url= http://geodesy.noaa.gov/PUBS_LIB/FedRegister/FRdoc59-5442.pdf |title= Refinement of Values for the Yard and the Pound |website= noaa.gov |archive-url= https://archive.today/20200305184155/https://geodesy.noaa.gov/PUBS_LIB/FedRegister/FRdoc59-5442.pdf |archive-date= 5 March 2020 |url-status= dead |access-date= 3 December 2006 }}</ref> The US authorities decided that, while the refined definition would apply nationally in all other respects, the [[US survey foot]] (and thus the survey acre) would continue 'until such a time as it becomes desirable and expedient to readjust [it]'.<ref name=NBS1959 /> By inference, an "international acre" may be calculated as exactly {{gaps|4,046.856|422|4}} square metres but it does not have a basis in any international agreement.

Both the international acre and the US survey acre contain {{frac|640}} of a square mile or 4,840 square yards, but alternative definitions of a yard are used (see [[Foot (unit)#Survey foot|survey foot]] and [[Yard#Conversions|survey yard]]), so the exact size of an acre depends upon the yard upon which it is based. The US survey acre is about 4,046.872 square metres; its exact value ({{sfrac|4046|13,525,426|15,499,969}}&nbsp;m<sup>2</sup>) is based on an inch defined by 1&nbsp;metre = 39.37&nbsp;inches exactly, as established by the [[Mendenhall Order]] of 1893.<ref>* {{cite journal |last=Mendenhall |first=T.C. |date=6 October 1922 |title=The United States Fundamental Standards of Length and Mass |journal=Science |series=New Series |volume=56 |issue=1449 |pages=337–380 |issn=0036-8075 |doi=10.1126/science.56.1449.377 |jstor=1647062 |pmid=17833047 |bibcode=1922Sci....56..377M |access-date=16 August 2021 |url=https://www.jstor.org/stable/1647062}}</ref> Surveyors in the United States use both international and survey feet, and consequently, both varieties of acre.<ref>[[National Geodetic Survey]], (January 1991), [http://www.ngs.noaa.gov/INFO/Policy/st_plane.html Policy of the National Geodetic Survey Concerning Units of Measure for the State Plane Coordinate System of 1983].</ref>

Since the difference between the US survey acre and international acre (0.016 square metres, 160 square centimetres or 24.8 square inches), is only about a quarter of the size of an [[ISO 216#A series|A4 sheet]] or [[Paper size#North American paper sizes|US letter]], it is usually not important which one is being discussed. Areas are seldom measured with sufficient accuracy for the different definitions to be detectable.<ref>[https://www.nsps.us.com/resource/resmgr/alta_standards/2021_Standards_20201030_grk.pdf Minimum Standard Detail Requirements For ALTA/NSPS Land Title Surveys]. Federick, MD: American Congress on Surveying and Mapping. 2021. [The stated maximum allowable "precision" (page 2) is 2&nbsp;cm and 50 parts per million. An instrument consistently measuring 2&nbsp;cm short would measure the area of a one international acre square, 63.614907&nbsp;m on a side, as 4044.3 square metres, 2.6 square metres less than the true value, a far greater discrepancy than the difference between the international and survey acres.]</ref> In October 2019, the [[US National Geodetic Survey]] and the [[National Institute of Standards and Technology]] announced their joint intent to end the "temporary" continuance of the US survey foot, mile, and acre units (as permitted by their 1959 decision, above), with effect from the end of 2022.<ref>{{cite web |title=NGS and NIST to Retire U.S. Survey Foot after 2022 |url=https://www.ngs.noaa.gov/web/news/us-survey-foot.shtml |publisher=National Geodetic Survey |access-date=4 March 2020 |date=31 October 2019}}</ref><ref>{{cite web |title=U.S. Survey Foot: Revised Unit Conversion Factors |url=https://www.nist.gov/pml/us-surveyfoot/revised-unit-conversion-factors |publisher=NIST |access-date=4 March 2020 |date=16 October 2019}}</ref>

===Spanish acre===
The [[Puerto Rica]]n ''[[cuerda]]'' ({{convert|1|cda|disp=out}}) is sometimes called the "Spanish acre" in the continental United States.<ref name="Rowlett">[http://www.unc.edu/~rowlett/units/dictC.html ''Units: C: cuerda'']. Russ Rowlett. The University of North Carolina at Chapel Hill.</ref>