Editing Geopotential height (section)

{{Short description|Type of altitude above mean sea level}}

'''Geopotential height''', also known as '''geopotential altitude''' or '''geopotential elevation''',<ref>Forrester, W.D. 1983. ''Canadian Tidal Manual''. Chapter 5: Datums and Vertical Control. Department of Fisheries and Oceans, Ottawa, 138pp. [https://psmsl.org/train_and_info/training/reading/canadian_manual/chapter5.pdf]</ref> is a [[vertical coordinate]] (with [[dimension (physics)|dimension]] of length) representing the [[work (physics)|work]] involved in lifting one [[unit of mass]] over one [[unit of length]] through a hypothetical [[space (mathematics)|space]] in which the [[Gravitational acceleration|acceleration of gravity]] is assumed constant.<ref name=NASA>{{cite web|title=NASA Technical Report R-459: Defining Constants, Equations, and Abbreviated Tables of the 1976 Standard Atmosphere|date=May 1976 |url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19760017709.pdf|archive-url=https://web.archive.org/web/20170307211228/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19760017709.pdf |archive-date=2017-03-07 |last1=Minzner |first1=R. A. |last2=Reber |first2=C. A. |last3=Jacchia |first3=L. G. |last4=Huang |first4=F. T. |last5=Cole |first5=A. E. |last6=Kantor |first6=A. J. |last7=Keneshea |first7=T. J. |last8=Zimmerman |first8=S. P. |last9=Forbes |first9=J. M. }}</ref> Geopotential heights are referenced to [[Earth]]'s [[mean sea level]], taking its best-fitting [[equigeopotential]] as a reference surface or [[vertical datum]].
In [[SI units]], a geopotential height difference of one [[metre|meter]] implies the vertical transport of a parcel of one [[kilogram]]; adopting the [[standard gravity]] value (9.80665 [[Metres per second squared|m/s<sup>2</sup>]]), it corresponds to a constant work or [[potential energy]] difference of 9.80665 [[joule]]s. 

Geopotential height differs from geometric height (as given by a [[tape measure]]) because [[Earth's gravity]] is not constant, varying markedly with altitude and latitude; thus, a 1-m geopotential height difference implies a different [[vertical distance]] in [[physical space]]: "the unit-mass must be lifted higher at the equator than at the pole, if the same amount of work is to be performed".<ref name="Bjerknes 1910 p. 13">{{cite book | last=Bjerknes | first=V. | author-link=V. Bjerknes | title=Dynamic Meteorology and Hydrography: Part [1]-2, [and atlas of plates] | publisher=Carnegie Institution of Washington | series=Carnegie Institution of Washington publication | issue=v. 1 | year=1910 | url=https://books.google.com/books?id=ub5XAAAAYAAJ&pg=PA13 | access-date=2023-10-05 | page=13}}</ref>
It is a useful concept in [[meteorology]], [[climatology]], and [[oceanography]]; it also remains a historical convention in aeronautics as the altitude used for calibration of aircraft [[pressure altitude|barometric altimeters]].<ref>{{cite book|last=Anderson|first=John|date=2007|title=Introduction to Flight|publisher=McGraw-Hill Science/Engineering/Math|page=109}} </ref>