This is an example of a "space-driven" or data independent method, as opposed to "data-driven" or data dependent method, as discussed further in Rigaux et al. In practice, construction of grid-based spatial indices entails allocation of relevant objects to their position or positions in the grid, then creating an index of object identifiers vs. Coordinates are given as the radius and angle from the center of the grid. Polar grids utilize the polar coordinate system, using circles of a prescribed radius that are divided into sectors of a certain angle. Quadtrees are a specialised form of grid in which the resolution of the grid is varied according to the nature and complexity of the data to be fitted, across the 2-d space. Criteria for optimal discrete global gridding have been proposed by both Goodchild and Kimerling in which equal area cells are deemed of prime importance. 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. This grid was also employed as the basis of the rotatable globe that forms part of the Microsoft Encarta product. It eventually resulted in a thesis entitled "A Hierarchical Coordinate System for Geoprocessing and Cartography" that was published in 1999. 100 km, 10 km) but not in degrees of longitude, in particular.Ī commonly used triangular grid is the "Quaternary Triangular Mesh" (QTM), which was developed by Geoffrey Dutton in the early 1980s. Grids that are " equal area" ( statistical grids), that have cell sizes that are constant in distance on the ground (e.g. Grids that are " equal angle" have cell sizes that are constant in degrees of latitude and longitude but are unequal in area (particularly with varying latitude). In general, these grids fall into two classes, " equal angle" or " equal area". Such grids may or may not be aligned with the grid lines of latitude and longitude for example, Marsden Squares, World Meteorological Organization squares, c-squares and others are aligned, while Universal Transverse Mercator coordinate system and various local grid based systems such as the British national grid reference system are not. 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.
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