WKT (Well Known Text)

http://hi.baidu.com/liongg/blog/item/a60e35fa1900078c9e5146b8.html
Well-Known Text (WKT) Representation of Spatial Reference Systems：顾名思义用众所周知的文字形式来描述看见坐标参照系统，它与EPSG（2005年已经换名）的坐标文字表述模型一致。

基础知识：坐标参照系有三种最常见的子类：地心坐标系(geocentric cs、GEOCCS)，地理坐标系(geographic cs、GEOGCS)，和投影坐标系(projected cs、PROJCS)以及相互之间的关系，可以参考《坐标系、坐标参照系、坐标变换、投影变换》。投影参数内容：Ellipsoid 、 Datum ；Projection，可以参考《地图投影为什么》。

坐标系的文字描述的扩展BN范式(EBNF)定义如下：
<coordinate system> = <projected cs> | <geographic cs> | <geocentric cs>
 
<projection> = PROJECTION["<name>"]
<parameter> = PARAMETER["<name>", <value>]
<value> = <number>
  
<datum> = DATUM["<name>", <spheroid>]
<spheroid> = SPHEROID["<name>", <semi-major axis>, <inverse flattening>]
<semi-major axis> = <number> NOTE: semi-major axis is measured in meters and must be > 0.
<inverse flattening> = <number>
<prime meridian> = PRIMEM["<name>", <longitude>]
<longitude> = <number>
  
<angular unit> = <unit>
<linear unit> = <unit>
<unit> = UNIT["<name>", <conversion factor>]
<conversion factor> = <number>
以下示例说明，参照上述参数，然后比瓢画葫芦即可自行用WKT创建坐标系。

地理坐标系的格式：<geographic cs> = GEOGCS["<name>", <datum>, <prime meridian>, <angular unit>]

WGS1984的地理坐标系WKT形式：
GEOGCS["WGS 84",
     DATUM["WGS_1984",
          SPHEROID["WGS 84", 6378137, 298.257223563, AUTHORITY["EPSG", "7030"]],    
          AUTHORITY["EPSG", "6326"]],
     PRIMEM["Greenwich", 0, AUTHORITY["EPSG", "8901"]],
     UNIT["degree", 0.0174532925199433, AUTHORITY["EPSG", "9122"]],
     AUTHORITY["EPSG", "4326"]]

投影坐标系的格式：<projected cs> = PROJCS["<name>", <geographic cs>, <projection>, {<parameter>,}* <linear unit>]

WGS1984地理坐标，统一横轴墨卡托(UTM)投影，中央经线117E的投影坐标系WKT形式：

PROJCS["WGS 84 / UTM zone 50N",
     GEOGCS["WGS 84", DATUM["WGS_1984", SPHEROID["WGS 84", 6378137, 298.257223563, AUTHORITY["EPSG", "7030"]], AUTHORITY["EPSG", "6326"]], PRIMEM["Greenwich", 0, AUTHORITY["EPSG", "8901"]], UNIT["degree", 0.0174532925199433, AUTHORITY["EPSG", "9122"]], AUTHORITY["EPSG", "4326"]],
     PROJECTION["Transverse_Mercator"],
     PARAMETER["latitude_of_origin", 0],
     PARAMETER["central_meridian", 117],
     PARAMETER["scale_factor", 0.9996],
     PARAMETER["false_easting", 500000],
     PARAMETER["false_northing", 0],
     UNIT["metre", 1, AUTHORITY["EPSG", "9001"]],
     AUTHORITY["EPSG", "32650"]]

地心坐标系格式相似于地理坐标系：<geocentric cs> = GEOCCS["<name>", <datum>, <prime meridian>, <linear unit>]

参数中出现AUTHORITY是EPSG的玩意，在自定义坐标系时可以忽略，我会在后面详细介绍EPSG相关内容。WKT更具体的可参考OGC相关文档：如SFA、《SF for OLE/COM》等。

练习：给出下属投影坐标系参数，请用WKT方式表述。
投影参数：椭球体，Krasovsky_1940；基准面，北京1954；投影：兰勃特双标准纬线，25N，47N；中央经线，117E。

参考答案：
PROJCS["liongg",
    GEOGCS["GCS_Beijing_1954",
         DATUM["D_Beijing_1954",SPHEROID["Krasovsky_1940",6378245.0,298.3]],
         PRIMEM["Greenwich",0.0],
         UNIT["Degree",0.0174532925199433]],
    PROJECTION["Lambert_Conformal_Conic"],
    PARAMETER["False_Easting",20500000.0],
    PARAMETER["False_Northing",0.0],
    PARAMETER["Central_Meridian",0.0],
    PARAMETER["Standard_Parallel_1",25.0],
    PARAMETER["Standard_Parallel_2",47.0],
    PARAMETER["Scale_Factor",1.0],
    PARAMETER["Latitude_Of_Origin",0.0],
    UNIT["Meter",1.0]]
有个偷懒的方法就是用商业软件譬如ArcGIS按照参数新建投影，然后在.prj文件中提取投影内容并修改。