Grid (spatial index): Difference between revisions

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{{Short description|Partition of a surface into contiguous cells}}

{{Refimprove|date=September 2018}}

In the context of a [[spatial index]], a '''grid''' or '''mesh''' is a regular{{fact|reason=What about irregular grids?|date=October 2023}} [[tessellation]] of a [[manifold]] or [[Surface (topology)|2-D surface]] that [[Space partitioning|divides it]] into a series of contiguous cells, which can then be assigned unique identifiers and used for spatial indexing purposes. A wide variety of such grids have been proposed or are currently in use, including grids based on "[[Square tiling|square]]" or "rectangular" cells, [[Triangular tiling|triangular grids]] or meshes, [[Hexagonal tiling|hexagonal grids]], and grids based on diamond-shaped cells.also A "[[Discrete_Global_Griddiscrete global grid|global grid]]" ifis ita kind of grid that covers the entire surface of the [[globe]]).

==Types of grids==

[[File:Geodesic Grid (ISEA3H) illustrated.png|right | 500px]]

Square or rectangular grids are frequently used for purposes such as translating spatial information expressed in Cartesian coordinates ([[latitude]] and [[longitude]]) into and out of the grid system. Such grids may or may not be aligned with the grid lines of latitude and longitude; for example, [[Marsden Square]]s, [[World Meteorological Organization squares]], [[c-squares]] and others are aligned, while [[Universal Transverse Mercator coordinate system]] and various nationallocal grid based systems such as the [[British national grid reference system]] are not. In general, these grids fall into two classes, those"''equal angle''" or "''[[equal-area map|equal area]]''". Grids that are "''equal angle''", that have cell sizes that are constant in degrees of latitude and longitude but are unequal in area (particularly with varying latitude),. or thoseGrids that are "''equal area''" ([[statistical grid]]s), that have cell sizes that are constant in distance on the ground (e.g. 100 km, 10 km) but not in degrees of longitude, in particular.

A commonly used triangular grid is the "Quaternary Triangular Mesh" (QTM), which was developed by Geoffrey Dutton in the early 1980s. It eventually resulted in a thesis entitled "A Hierarchical Coordinate System for Geoprocessing and Cartography" that was published in 1999.<ref>Geoffrey This grid was also employed as the basis of the rotatable globe that forms part of the Microsoft [[Encarta]] productDutton.

[http://www.spatial-effects.com/SE-papers1.html "Spatial Effects: Research Papers and Data"] {{Webarchive|url=https://web.archive.org/web/20070219054353/http://www.spatial-effects.com/SE-papers1.html |date=2007-02-19 }}.</ref>

This grid was also employed as the basis of the rotatable globe that forms part of the Microsoft [[Encarta]] product.

Hexagonal grids may also be used. In general, triangular and hexagonal grids are constructed so as to better approach the goals of equal-area (or nearly so) plus more seamless coverage across the poles, which tends to be a problem area for square or rectangular grids since in these cases, the cell width diminishes to nothing at the pole and those cells adjacent to the pole then become 3- rather than 4-sided. Criteria for optimal discrete global gridding have been proposed by both Goodchild and Kimerling<ref>[http://www.ncgia.ucsb.edu/globalgrids-book/comparison Criteria and Measures for the Comparison of Global Geocoding Systems, Keith C. Clarke, University of California] {{webarchive|url=https://web.archive.org/web/20100623002915/http://www.ncgia.ucsb.edu/globalgrids-book/comparison/ |date=2010-06-23 }}</ref> in which equal area cells are deemed of prime importance.

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==See also==

*[[Discrete global grid]]

*[[Euclidean tilings by convex regular polygons]]

*[[Geodesic grid]]

*[[Spatial index]]

* [[Grid plan]]

*[[Grid reference]]

*[[Geocode]]

* [[hex map]]

*[[quadrilateralized spherical cube]]

*[[Quadtree]]

*[[R-tree]]