WKT简介

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WKT - 概念

  WKT(Well-known text)是一种文本标记语言，用于表示矢量几何对象、空间参照系统及空间参照系统之间的转换。它的二进制表示方式，亦即WKB(well-known binary)则胜于在传输和在数据库中存储相同的信息。该格式由开放地理空间联盟(OGC)制定。

WKT - 几何对象

WKT可以表示的几何对象包括：点，线，多边形，TIN（不规则三角网）及多面体。可以通过几何集合的方式来表示不同维度的几何对象。几何物体的坐标可以是2D(x,y),3D(x,y,z),4D(x,y,z,m),加上一个属于线性参照系统的m值。
以下为几何WKT字串样例：

Geometry primitives (2D)

Type	Examples
Point	POINT (30 10)
LineString	LINESTRING (30 10, 10 30, 40 40)
Polygon	POLYGON ((30 10, 10 20, 20 40, 40 40, 30 10))
	POLYGON ((35 10, 10 20, 15 40, 45 45, 35 10), (20 30, 35 35, 30 20, 20 30))

Multipart geometries (2D)

Type	Examples
MultiPoint	MULTIPOINT ((10 40), (40 30), (20 20), (30 10))
	MULTIPOINT (10 40, 40 30, 20 20, 30 10)
MultiLineString	MULTILINESTRING ((10 10, 20 20, 10 40), (40 40, 30 30, 40 20, 30 10))
MultiPolygon	MULTIPOLYGON (((30 20, 10 40, 45 40, 30 20)), ((15 5, 40 10, 10 20, 5 10, 15 5)))
	MULTIPOLYGON (((40 40, 20 45, 45 30, 40 40)), ((20 35, 45 20, 30 5, 10 10, 10 30, 20 35), (30 20, 20 25, 20 15, 30 20)))

WKT - 空间参照系统

一个表示空间参照系统的WKT字串描述了空间物体的测地基准、大地水准面、坐标系统及地图投影。 WKT在许多GIS程序中被广泛采用。ESRI亦在其shape文件格式(*.prj)中使用WKT。以下是空间参照系统的WKT表示样例：

COMPD_CS["OSGB36 / British National Grid + ODN",
    PROJCS["OSGB 1936 / British National Grid",
        GEOGCS["OSGB 1936",
            DATUM["OSGB_1936",
                spheroid["Airy 1830",6377563.396,299.3249646,AUTHORITY["EPSG","7001"]],
                TOWGS84[375,-111,431,0,0,0,0],
                AUTHORITY["EPSG","6277"]],
            PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],
            UNIT["DMSH",0.0174532925199433,AUTHORITY["EPSG","9108"]],
            AXIS["Lat",NORTH],
            AXIS["Long",EAST],
            AUTHORITY["EPSG","4277"]],
        PROJECTION["Transverse_Mercator"],
        PARAMETER["latitude_of_origin",49],
        PARAMETER["central_meridian",-2],
        PARAMETER["scale_factor",0.999601272],
        PARAMETER["false_easting",400000],
        PARAMETER["false_northing",-100000],
        UNIT["metre",1,AUTHORITY["EPSG","9001"]],
        AXIS["E",EAST],
        AXIS["N",NORTH],
        AUTHORITY["EPSG","27700"]],
    VERT_CS["Newlyn",
        VERT_DATUM["Ordnance Datum Newlyn",2005,AUTHORITY["EPSG","5101"]],
        UNIT["metre",1,AUTHORITY["EPSG","9001"]],
        AXIS["Up",UP],
        AUTHORITY["EPSG","5701"]],
    AUTHORITY["EPSG","7405"]]