View source for Triangulation (surveying)

{{Short description|Using measures of converging rays to improve fixed points for mapping}}
{{Other uses|Triangulation (disambiguation)}}
[[File:Theb1604 - Flickr - NOAA Photo Library.jpg|thumb|Triangulation of [[Kodiak Island]] in [[Alaska]] in 1929.]]

In [[surveying]], '''triangulation''' is the process of determining the location of a point by measuring only [[angle]]s to it from known points at either end of a fixed baseline by using [[trigonometry]], rather than measuring distances to the point directly as in [[trilateration]]. The point can then be fixed as the third point of a triangle with one known side and two known angles.

Triangulation can also refer to the accurate [[surveying]] of systems of very large triangles, called '''triangulation networks'''. This followed from the work of [[Willebrord Snellius|Willebrord Snell]] in 1615–17, who showed how a point could be located from the angles subtended from ''three'' known points, but measured at the new unknown point rather than the previously fixed points, a problem called [[Position resection and intersection|resectioning]]. Surveying error is minimized if a mesh of triangles at the largest appropriate scale is established first. Points inside the triangles can all then be accurately located with reference to it. Such triangulation methods were used for accurate large-scale land surveying until the rise of [[global navigation satellite system]]s in the 1980s.

== Principle ==
[[Image:Triangulation-boat.png|thumb|310px]]Triangulation may be used to find the position of the ship when the positions of A and B are known. An observer at ''A''  measures the [[angle]] ''α'', while the observer at ''B'' measures ''β''.

The position of any vertex of a triangle can be calculated if the position of one side, and two angles, are known.  The following [[formulae]] are strictly correct only for a flat surface. If the curvature of the Earth must be allowed for, then   [[spherical trigonometry]] must be used.

=== Calculation ===
{{See also|Geometric mean theorem|inverse Pythagorean theorem}}

With <math>\ell</math> being the distance between ''A'' and ''B'' gives:
:<math> \ell = \frac{d}{\tan \alpha} + \frac{d}{\tan \beta}</math>

Using the [[trigonometric identities]] tan&nbsp;α =&nbsp;sin&nbsp;α&nbsp;/&nbsp;cos&nbsp;α and sin(α&nbsp;+&nbsp;β) =&nbsp;sin&nbsp;α&nbsp;cos&nbsp;β&nbsp;+&nbsp;cos&nbsp;α&nbsp;sin&nbsp;β, this is equivalent to:
:<math>\ell= d \left(\frac{\cos \alpha}{\sin \alpha} + \frac{\cos \beta}{\sin \beta}\right)</math>

:<math>\ell = d\ \frac{ \sin(\alpha + \beta)}{\sin\alpha \sin\beta }</math>
therefore:
:<math>d = \ell\ \frac{ \sin\alpha \sin\beta }{ \sin(\alpha + \beta)}</math>

From this, it is easy to determine the distance of the unknown point from either observation point, its north/south and east/west offsets from the observation point, and finally its full coordinates.

==History==
[[Image:Sea island survey.jpg|thumb|right|[[Liu Hui]] ({{circa|263}}), How to measure the height of a sea island.  Illustration from an edition of 1726]]
[[Image:G-F triangulation.jpg|thumb|right|[[Gemma Frisius]]'s 1533 proposal to use triangulation for mapmaking]]
[[Image:L-Triangulierung.png|thumb|right|Nineteenth-century triangulation network for the triangulation of Rhineland-Hesse]] 
Triangulation today is used for many purposes, including [[surveying]], [[navigation]], [[metrology]], [[astrometry]], [[binocular vision]], [[model rocketry]] and gun direction of [[weapon]]s.

In the field, triangulation methods were apparently not used by the Roman specialist land surveyors, the ''[[agrimensores]]''; but were introduced into medieval Spain through [[Geography in medieval Islam|Arabic treatises]] on the [[astrolabe]], such as that by [[Ibn al-Saffar]] (d. 1035).<ref name="Hill">[[Donald Routledge Hill]] (1984), ''A History of Engineering in Classical and Medieval Times'', London: Croom Helm & La Salle, Illinois: Open Court. {{ISBN|0-87548-422-0}}. pp. 119–122</ref> [[Abu Rayhan Biruni]] (d. 1048) also introduced triangulation techniques to [[Abu Rayhan Biruni#Geodesy and geography|measure the size of the Earth]] and the distances between various places.<ref name=MacTutor>{{MacTutor|id=Al-Biruni|title=Abu Arrayhan Muhammad ibn Ahmad al-Biruni}}</ref> Simplified Roman techniques then seem to have co-existed with more sophisticated techniques used by professional surveyors.  But it was rare for such methods to be [[Latin translations of the 12th century|translated into Latin]] (a manual on geometry, the eleventh century ''Geomatria incerti auctoris'' is a rare exception), and such techniques appear to have percolated only slowly into the rest of Europe.<ref name="Hill" />  Increased awareness and use of such techniques in Spain may be attested by the medieval [[Jacob's staff]], used specifically for measuring angles, which dates from about 1300; and the appearance of accurately surveyed coastlines in the [[Portolan charts]], the earliest of which that survives is dated 1296.

=== Gemma Frisius ===
On land, the cartographer [[Gemma Frisius]] proposed using triangulation to accurately position far-away places for map-making in his 1533 pamphlet ''Libellus de Locorum describendorum ratione'' (''Booklet concerning a way of describing places''), which he bound in as an appendix in a new edition of [[Peter Apian]]'s best-selling 1524 ''Cosmographica''.  This became very influential, and the technique spread across Germany, Austria and the Netherlands.  The astronomer [[Tycho Brahe]] applied the method in Scandinavia, completing a detailed triangulation in 1579 of the island of [[Ven, Sweden|Hven]], where his observatory was based, with reference to key landmarks on both sides of the [[Øresund]], producing an estate plan of the island in 1584.<ref>Michael Jones (2004), "[https://books.google.com/books?id=_tsvf51Z4ocC&dq=portolan+triangulation&pg=PA210 Tycho Brahe, Cartography and Landscape in 16th Century Scandinavia]", in Hannes Palang (ed), European Rural Landscapes: Persistence and Change in a Globalising Environment, p.210</ref>  In England Frisius's method was included in the growing number of books on surveying which appeared from the middle of the century onwards, including [[William Cuningham]]'s ''Cosmographical Glasse'' (1559), Valentine Leigh's ''Treatise of Measuring All Kinds of Lands'' (1562), [[William Bourne (mathematician)|William Bourne]]'s ''Rules of Navigation'' (1571), [[Thomas Digges]]'s ''Geometrical Practise named Pantometria'' (1571), and [[John Norden]]'s ''Surveyor's Dialogue'' (1607). It has been suggested that [[Christopher Saxton]] may have used rough-and-ready triangulation to place features in his county maps of the 1570s; but others suppose that, having obtained rough bearings to features from key vantage points, he may have estimated the distances to them simply by guesswork.<ref>Martin and Jean Norgate (2003), [http://www.geog.port.ac.uk/webmap/hantsmap/hantsmap/saxton1/sax1svy1.htm Saxton's Hampshire: Surveying], University of Portsmouth</ref>

=== Willebrord Snell ===
{{main|Snellius' triangulation}}

The modern systematic use of triangulation networks stems from the work of the Dutch mathematician [[Willebrord Snellius|Willebrord Snell]], who in 1615 surveyed the distance from [[Alkmaar]] to [[Breda]], approximately 72 miles (116 kilometres), using a chain of quadrangles containing 33 triangles in all. Snell underestimated the distance by 3.5%. The two towns were separated by one degree on the [[Meridian (geography)|meridian]], so from his measurement he was able to calculate a value for the circumference of the earth – a feat celebrated in the title of his book ''Eratosthenes Batavus'' (''The Dutch [[Eratosthenes]]''), published in 1617. Snell calculated how the planar formulae could be corrected to allow for the curvature of the earth. He also showed how to [[Snellius–Pothenot problem|resection]], or calculate, the position of a point inside a triangle using the angles cast between the vertices at the unknown point. These could be measured much more accurately than bearings of the vertices, which depended on a compass. This established the key idea of surveying a large-scale primary network of control points first, and then locating secondary subsidiary points later, within that primary network.

===Further developments===
Snell's methods were taken up by [[Jean Picard]] who in 1669–70 surveyed one degree of latitude along the [[Paris Meridian]] using a chain of thirteen triangles stretching north from [[Paris]] to the clocktower of [[Sourdon]], near [[Amiens]]. Thanks to improvements in instruments and accuracy, Picard's is rated as the first reasonably accurate measurement of the radius of the earth. Over the next century this work was extended most notably by the Cassini family: between 1683 and 1718 [[Giovanni Domenico Cassini|Jean-Dominique Cassini]] and his son [[Jacques Cassini]] surveyed the whole of the Paris meridian from [[Dunkirk]] to [[Perpignan]]; and between 1733 and 1740 Jacques and his son [[César-François Cassini de Thury|César Cassini]] undertook the first triangulation of the whole country, including a re-surveying of the [[meridian arc]], leading to the publication in 1745 of the first map of France constructed on rigorous principles.

Triangulation methods were by now well established for local mapmaking, but it was only towards the end of the 18th century that other countries began to establish detailed triangulation network surveys to map whole countries. The [[Principal Triangulation of Great Britain]] was begun by the [[Ordnance Survey]] in 1783, though not completed until 1853; and the [[Great Trigonometric Survey]] of India, which ultimately named and mapped [[Mount Everest]] and the other Himalayan peaks, was begun in 1801. For the Napoleonic French state, the French triangulation was extended by [[Jean-Joseph Tranchot]] into the German [[Rhineland]] from 1801, subsequently completed after 1815 by the Prussian general [[Karl von Müffling]]. Meanwhile, the mathematician [[Carl Friedrich Gauss]] was entrusted from 1821 to 1825 with the triangulation of the [[kingdom of Hanover]] ({{ill|Gaussian land survey|de|Gaußsche Landesaufnahme}}), on which he applied the [[least squares|method of least squares]] to find the best fit solution for problems of large systems of [[simultaneous equation]]s given more real-world measurements than unknowns.

Today, large-scale triangulation networks for positioning have largely been superseded by the [[global navigation satellite system]]s established since the 1980s, but many of the control points for the earlier surveys still survive as valued historical features in the landscape, such as the concrete [[trig point|triangulation pillars]] set up for [[retriangulation of Great Britain]] (1936–1962), or the triangulation points set up for the [[Struve Geodetic Arc]] (1816–1855), now scheduled as a UNESCO [[World Heritage Site]].

== See also ==
*[[Anglo-French Survey (1784–1790)]]
* [[Bilby tower]]
*[[Great Trigonometrical Survey]]
* [[Multilateration]], where a point is calculated using the time-difference-of-arrival between other known points
* [[Parallax]]
* [[Resection (orientation)]]
* [[SOCET SET]]
* [[Spherical trigonometry]]
* [[Stellar triangulation]]
* [[Stereopsis]]
* [[Trig point]]

==References==
{{Reflist|2}}

==Further reading==
<!-- not consulted, but probably useful-->
* Bagrow, L. (1964) ''History of Cartography''; revised and enlarged by R.A. Skelton.  Harvard University Press.
* Crone, G.R. (1978 [1953]) ''Maps and their Makers: An Introduction to the History of Cartography'' (5th ed).
* Tooley, R.V. & Bricker, C. (1969) ''A History of Cartography: 2500 Years of Maps and Mapmakers''
* Keay, J. (2000) ''The Great Arc: The Dramatic Tale of How India Was Mapped and Everest Was Named''. London: Harper Collins. {{ISBN|0-00-257062-9}}.
* Murdin, P. (2009) ''Full Meridian of Glory: Perilous Adventures in the Competition to Measure the Earth''. Springer. {{ISBN|978-0-387-75533-5}}.

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[[Category:Angle]]
[[Category:Elementary geometry]]
[[Category:Euclidean geometry]]
[[Category:Surveying]]
[[Category:Geodetic surveys]]