|
A geodesic grid is a technique used to model the surface of a sphere (such as the Earth) with a subdivided polyhedron, usually an icosahedron. ==Introduction== A geodesic grid is a global Earth reference that uses cells or tiles to statistically represent data encoded to the area covered by the cell location. The focus of the discrete cells in a geodesic grid reference is different from that of a conventional lattice-based Earth reference where the focus is on a continuity of points used for addressing location and navigation. In biodiversity science, geodesic grids are a global extension of local discrete grids that are staked out in field studies to ensure appropriate statistical sampling and larger multi-use grids deployed at regional and national levels to develop an aggregated understanding of biodiversity. These grids translate environmental and ecological monitoring data from multiple spatial and temporal scales into assessments of current ecological condition and forecasts of risks to our natural resources. A geodesic grid allows local to global assimilation of ecologically significant information at its own level of granularity. When modeling the weather, ocean circulation, or the climate, partial differential equations are used to describe the evolution of these systems over time. Because computer programs are used to build and work with these complex models, approximations need to be formulated into easily computable forms. Some of these numerical analysis techniques (such as finite differences) require the area of interest to be subdivided into a grid — in this case, over the shape of the Earth. Geodesic grids have been developed by subdividing a sphere to developing a global tiling (tessellation) based on a geographic coordinates (longitude/latitude) where a rectilinear cell is defined as the intersection of a longitude and latitude line. This approach is easily understood in terms of accepted Earth reference, accessible using the longitude and latitude as an ordered pair, and implemented in a computer coding as a rectangular grid. However, such a pattern does not conform to many of the main criteria for a statistically valid discrete global grid, primarily that the cells' area and shape are not generally similar; this is especially noticeable as the cells are developed towards the poles. Another approach gaining favour uses geodesic sphere grids generated by the subdivision of a platonic solid into cells or by iteratively bisecting the edges of the polyhedron and projecting the new cells onto a sphere. In this ''geodesic grid'', each of the vertices in the resulting geodesic sphere corresponds to a cell. One implementation uses an icosahedron as the base polyhedron, hexagonal cells, and the Snyder equal area projection is known as the Icosahedron Snyder Equal Area (ISEA) grid. Another method, using the intersection of a tetrahedron into triangular quadtrees, is known as the Quaternary Triangular Mesh (QTM). A triangular mesh conforms well to representation in a graphics pipeline, and its dual cells are hexagons, convenient for encoding data. The hexagonal geodesic grid inherits many of the virtues of 2D hexagonal grids, and specifically avoids problems with singularities and oversampling near the poles. Along the same line, different Platonic solids could also be used as a starting point instead of an icosahedron or tetrahedron — e.g. cubes that are common in video games can be used to represent the Earth with a small aperture and an efficient equal area projection. The quadrilateralized spherical cube is a kind of geodesic grid based on subdividing a cube into equal-area cells that are approximately square. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Geodesic grid」の詳細全文を読む スポンサード リンク
|