By Geoffrey H. Dutton
Whilst spatial information is digitized to be used in geographic info structures and different software program, information regarding its unique scale, solution and accuracy is usually misplaced. consequently, utilizing such info at diversified scales and mixing it with facts from different resources will be tricky. Mapping vector info at smaller than the unique scale calls for its generalization, that's frequently dealt with through post-processing in ways in which are just weakly supported by way of databases. The types and techniques defined during this ebook triumph over many such difficulties by means of supplying a multi-resolution info illustration that permits retrieval of map info at a hierarchy of scales, in addition to documenting the accuracy of each spatial coordinate.
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Extra resources for A Hierarchical Coordinate System for Geoprocessing and Cartography (Lecture Notes in Earth Sciences)
Example text
Besides octahedra (having 8 facets), we have seen examples of dodecahedra (12), icosahedra (20) and truncated icosahedra (32, itself a polyhedral subdivision). 4 illustrates have been proposed but will not be dealt with here. To our knowledge, no one has suggested using the simplest polyhedron - - a tetrahedron (4 facets) - - as the basis for a global spatial data model. Regardless of its orientation, it would generate an even more non-intuitive coordinate system, a stranger distribution of land masses, and introduce more projection distortions than larger polytopes make necessary.
Relations between GEM's dual triangular hierarchies Although GEM could accept elevation data for any or all points on a planet, it was realized that even a medium-dense elevation model for a whole one would consume huge amounts of storage, even at the rate of less than one bit per triangular facet theoretically possible, Some portions of a GEM pyramid would always be omitted, and some localities would have fewer levels of encoding (resulting in a smaller number of larger facets) than others. GEM's high degree of data compression was based on the implicit ordering of facets in complete sub-hierarchies; the bits in a GEM archive indicated changes in elevation by their values, and identified facets by their ordering, as do elevation grids, only pyramidally.
However, a small amount of more serious interest has been evident in the literature of ecology and geosciences, in which the need for global sampling of environmental data periodically arises. The earliest digital implementation using polyhedra seems to be the one reported in Wickman e t a l (1974). Requiring a sampling regime for global geochemical data that was as uniform as possible, and recognizing that five platonic polyhedra are the only figures that are completely uniform tessellations of a sphere, Wickman compromised, opting for a triangulation of a dodecahedron.
