In geodesy, a meridian arc measurement is a highly accurate determination of the distance between two points with the same longitude. Two or more such determinations at different locations then specify the shape of the reference ellipsoid which best approximates the shape of the geoid. This process is called the determination of the Figure of the Earth. The earliest determinations of the size of a spherical Earth required a single arc. The latest determinations use astro-geodetic measurements and the methods of satellite geodesy to determine the reference ellipsoids.
The Earth as a sphereEdit
Early estimations of Earth's radius are recorded from Egypt in 240 BC, and from Baghdad caliphs in the 9th century, but it was the Alexandrian scientist Eratosthenes who first calculated the circumference a reasonably good approximate value for the radius. He knew that on the summer solstice at local noon the sun goes through the zenith in the ancient Egyptian city of Syene (Assuan). He also knew from his own measurements that, at the same moment in his hometown of Alexandria, the zenith distance was 1/50 of a full circle (7.2°).
Assuming that Alexandria was due north of Syene, Eratosthenes concluded that the distance between Alexandria and Syene must be 1/50 of Earth's circumference. Using data from caravan travels, he estimated the distance to be 5000 stadia (about 500 nautical miles)—which implies a circumference of 252,000 stadia. Assuming the Attic stadion (185 m) this corresponds to 46,620 km, or 16% too great. However, if Eratosthenes used the Egyptian stadion (157.5 m) his measurement turns out to be 39,690 km, an error of only 1%. Syene is not precisely on the Tropic of Cancer and not directly south of Alexandria. The sun appears as a disk of 0.5°, and an estimate of the overland distance traveling along the Nile or through the desert couldn't be more accurate than about 10%.
Eratosthenes' estimation of Earth’s size was accepted for nearly two thousand years. A similar method was used by Posidonius about 150 years later, and slightly better results were calculated in AD 827 by the Gradmessung of the Caliph al-Ma'mun.
The Earth as an ellipsoidEdit
Comment: early literature uses the term oblate spheroid to describe a sphere "squashed at the poles". Modern literature uses the term "ellipsoid of revolution" although the qualifying words "of revolution" are usually dropped. An ellipsoid which is not an ellipsoid of revolution is called a tri-axial ellipsoid. Spheroid and ellipsoid are used interchangeably in this article.
The method of determining the ellipsoidEdit
High precision land surveys can be used determine the distance between two places at "almost" the same longitude by measuring a base line and a chain of triangles. (Suitable stations for the end points are rarely at the same longitude). The distance Δ along the meridian from one end point to a point at the same latitude as the second end point is then calculated by trigonometry. The surface distance Δ is reduced to Δ', the corresponding distance at mean sea level. The intermediate distances to points on the meridian at the same latitudes as other stations of the survey may also be calculated.
The geographic latitudes of both end points, φs (standpoint) and φf (forepoint) and possibly at other points are determined by astrogeodesy, observing the zenith distances of sufficient numbers of stars. If latitudes are measured at end points only, the radius of curvature at the midpoint of the meridian arc can be calculated from R = Δ'/(|φs-φf|). A second meridian arc will allow the derivation of two parameters required to specify a reference ellipsoid. Longer arcs with intermediate latitude determinations can completely determine the ellipsoid. In practice multiple arc measurements are used to determine the ellipsoid parameters by the method of least squares. The parameters determined are usually the semi-major axis, , and either the semi-minor axis, , or the inverse flattening , (where the flattening is ).
The eighteenth centuryEdit
In 1687 Newton had published in the Principia a proof that the earth was an oblate spheroid (of inverse flattening equal to 230). This was disputed by some, but not all, French scientists. A meridian arc of Picard was extended to a longer arc by Cassini (J.D.) over the period 1684–1718. The arc was measured with at least three latitude determinations, so they were able to deduce mean curvatures for the northern and southern halves of the arc, allowing a determination of the overall shape. The results indicated that the Earth was a prolate spheroid (with an equatorial radius less than the polar radius). (The history of the meridian arc from 1600 to 1880 is fully covered in the first chapter of Geodesy by Alexander Ross Clarke.).
To resolve the issue, the French Academy of Sciences (1735) proposed expeditions to Peru (Bouguer, Louis Godin, de La Condamine, Antonio de Ulloa, Jorge Juan ) and Lappland (Maupertuis, Clairaut, Camus, Le Monnier, Abbe Outhier, Celsius). (The expedition to Peru is described on the page French Geodesic Mission and that to Lappland is described on the page Torne Valley.) The resulting measurements at equatorial and polar latitudes confirmed that the earth was best modelled by an oblate spheroid, supporting Newton.
By the end of the century the French arc had been remeasured and extended from Dunkirk to the Mediterranean. (By Delambre). It was divided into five parts by four intermediate determinations of latitude. By combining the measurements together with those for the arc of Peru, ellipsoid shape parameters were determined and the distance between the equator and pole along the Paris Meridian was calculated as 5130762 toise (as specified by the standard toise bar in Paris). Defining this distance as exactly 10,000,000 m led to the construction of a new standard metre bar as 0.5130762 toise. (See Clarke, pp18–22).
The nineteenth and twentieth centuriesEdit
In the 19th century, many astronomers and geodesists were engaged in detailed studies of the Earth's curvature along different meridian arcs. The analyses resulted in a great many model ellipsoids such as Plessis 1817, Airy 1830, Bessel 1830, Everest 1830 and Clarke 1866. A comprehensive list of ellipsoids is given under Figure of the Earth.
Meridian distance on the ellipsoidEdit
The determination of the meridian distance, that is the distance from the equator to a point at a latitude on the ellipsoid is an important problem in the theory of map projections, particularly the Transverse Mercator projection. Ellipsoids are normally specified in terms of the parameters defined above, , , , but in theoretical work it is useful to define extra parameters, particularly the eccentricity, , and the third flattening . Only two of these parameters are independent and there are many relations between them:
The radius of curvature is defined as
so that the arc length of an infinitesimal element of the meridian is (with in radians). Therefore the meridian distance from the equator to latitude is
The distance formula is simpler when written in terms of the parametric latitude.
where and . The distance from the equator to the pole, the polar distance, is
Relation to elliptic integralsEdit
The length of a meridian from the equator to the pole can be expressed in terms of the complete elliptic integral of the second kind,
The calculation (to arbitrary precision) of the elliptic integrals and approximations are also discussed in the NIST handbook. These functions are also implemented in computer algebra programs such as Mathematica and Maxima.
Series in eccentricityEdit
The above integral may be approximated by a truncated series in the square of the eccentricity (approximately 1/150) by expanding the integrand in a binomial series. Setting ,
Using simple trigonometric identities the powers of may be reduced to combinations of factors of . Collecting terms with the same cosine factors and integrating gives the following series, first given by Delambre in 1799.
The numerical values for the semi-major axis and eccentricity of the WGS84 ellipsoid give, in metres,
The first four terms have been rounded to the nearest millimetre whilst the eighth order term gives rise to sub-millimetre corrections. Tenth order series are employed in modern "wide zone" implementations of the transverse Mercator projection. (See Stuifbergen).
For the WGS84 ellipsoid the distance from equator to pole is given (in metres) by
The perimeter of a meridian ellipse is . Therefore is the radius of the circle whose circumference is the same as the perimeter of a meridian ellipse. This defines the mean Earth radius as 6,367,449.146 m. Dividing by 90 gives the mean value of the meridian distance per degree of latitude as 111,132 m.
On the ellipsoid the exact distance between parallels at and is . For WGS84 an approximate expression for the distance between the two parallels at one half of a degree from the circle at latitude is given (in metres) by
Series in third flattening (n)Edit
The third flattening is related to the eccentricity by
With this substition the integral for the meridian distance becomes
This integral has been expanded in several ways, all of which can be related to the Delambre series.
Since n is approximately one quarter of the value of the squared eccentricity, the above series for the coefficients converge 16 times as fast as the Delambre series.
and expanding the numerator terms.
Despite the simplicity and fast convergence of Helmert's expansion the U.S. Defense Mapping Agency adopted the fully expanded form of the Bessel series reported by Hinks in 1927. This expansion is important, despite the poorer convergence of series in , because it is used in the definition of UTM.
where the coefficients are given to order n5 by
The above series, to eighth order in eccentricity or fourth order in third flattening, are adequate for most practical applications. Each can be written quite generally. For example, Kazushige Kawase (2009) derived following general formula:
Truncating the summation at j = 2 gives Helmert's approximation.
By analogy of the above result, it is not so hard to write down a general formula for meridian arc length in terms of the parametric latitude as
Compared with the Helmert result, the series in terms of the parametric latitude converge faster due to the coefficient of reduced by .
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- Clarke, Alexander Ross, 1880: Geodesy. Clarendon Press. Recently republished at Forgotten Books
- F. W. J. Olver, D. W. Lozier, R. F. Boisvert, and C. W. Clark, editors, 2010, NIST Handbook of Mathematical Functions (Cambridge University Press).
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- N. Stuifbergen, 2009, Wide zone transverse Mercator projection, Technical Report 262, Canadian Hydrographic Service.
- Bessel, F. W. (1837): Bestimmung der Axen des elliptischen Rotationssphäroids, welches den vorhandenen Messungen von Meridianbögen der Erde am meisten entspricht, Astronomische Nachrichten, 14, 333–346
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- Krüger, L. (1912): Konforme Abbildung des Erdellipsoids in der Ebene. Royal Prussian Geodetic Institute, New Series 52, page 12
- Hinks, A. R. (1927): New geodetic tables for Clarke's figure of 1880, with transformation to Madrid 1924 and other figures, Royal Geographical Society, London, page x (in introduction)
- J. W. Hager, J.F. Behensky, and B.W. Drew, 1989. Defense Mapping Agency Technical Report TM 8358.2. The universal grids: Universal Transverse Mercator (UTM) and Universal Polar Stereographic (UPS).
- A guide to coordinate systems in Great Britain, Ordnance Survey of Great Britain.
- Kawase, K. (2009): A General Formula for Meridional Distance from the Equator to Given Latitude, Journal of the Geographical Survey Institute, 119, 45–55 (ISSN 0430-9081, in Japanese)
- Kawase, K. (2011): A General Formula for Calculating Meridian Arc Length and its Application to Coordinate Conversion in the Gauss-Krüger Projection, Bulletin of the Geospatial Information Authority of Japan, 59, 1–13