投影知识点

投影名称	英文名称	投影类型	投影性质
高斯-克吕格投影（在英美国家称为横轴墨卡托投影）	Gauss-Kruger	等角横切圆柱投影/等角横切椭圆柱投影	投影后中央经线保持长度不变，即比例系数为1
通用横轴墨卡托投影	Universal Transverse Mercator,UTM	等角横轴割圆柱投影	圆柱割地球于南纬80度、北纬84度两条等高圈，投影后两条割线上没有变形，中央经线上长度比0.9996。
横轴墨卡托投影，又称为高斯-克吕格投影	Transverse Mercator,TM	等角横切圆柱投影
墨卡托投影	Mercator	等角正切圆柱投影
Web墨卡托投影 http://hi.baidu.com/liongg/item/2840be728b738b10d0dcb39b	Web Mercator或Spherical Mercator	等角正切圆柱投影	它与常规墨卡托投影的主要区别就是把地球模拟为球体而非椭球体

如下是从ArcGIS帮助文档中摘出来的：

A sphere-based version of Mercator is used by several web mapping sites. The sphere's radius is equal to the WGS 1984 semimajor axis, 6378137.0 meters.

Two methods exist for emulating the Mercator projection used by the web services.

If the Mercator implementation supports spheroids (ellipsoids), the projected coordinate system must be based on a sphere-based geographic coordinate system. This will force the use of sphere equations. The implementation of Mercator auxiliary sphere has sphere equations only.

In addition, it has a projection parameter that identifies what to use for the sphere radius if the geographic coordinate system is ellipsoidal based. The default value of zero (0) uses the semimajor axis.

ArcGIS中实现了Mercator(WGS84)和Mercator(sphere)

看看他们参数，1和2即为上面所述的两种方式，即像Google地图所使用的Web Mercator. 2和4几乎一致，只是2多了个属性：

WGS_1984_Web_Mercator
WKID: 3785 权限: EPSG

Projection: Mercator
False_Easting: 0.0
False_Northing: 0.0
Central_Meridian: 0.0
Standard_Parallel_1: 0.0
Linear Unit: Meter (1.0)

Geographic Coordinate System: GCS_WGS_1984_Major_Auxiliary_Sphere
Angular Unit: Degree (0.0174532925199433)
Prime Meridian: Greenwich (0.0)
Datum: D_WGS_1984_Major_Auxiliary_Sphere
  Spheroid: WGS_1984_Major_Auxiliary_Sphere
    Semimajor Axis: 6378137.0
    Semiminor Axis: 6378137.0
    Inverse Flattening: 0.0

WGS_1984_Web_Mercator_Auxiliary_Sphere
WKID: 3857 权限: EPSG

Projection: Mercator_Auxiliary_Sphere
False_Easting: 0.0
False_Northing: 0.0
Central_Meridian: 0.0
Standard_Parallel_1: 0.0
Auxiliary_Sphere_Type: 0.0
Linear Unit: Meter (1.0)

Geographic Coordinate System: GCS_WGS_1984
Angular Unit: Degree (0.0174532925199433)
Prime Meridian: Greenwich (0.0)
Datum: D_WGS_1984
  Spheroid: WGS_1984
    Semimajor Axis: 6378137.0
    Semiminor Axis: 6356752.314245179
    Inverse Flattening: 298.257223563

Sphere_Mercator
WKID: 53004 权限: ESRI

Projection: Mercator
False_Easting: 0.0
False_Northing: 0.0
Central_Meridian: 0.0
Standard_Parallel_1: 0.0
Linear Unit: Meter (1.0)

Geographic Coordinate System: GCS_Sphere
Angular Unit: Degree (0.0174532925199433)
Prime Meridian: Greenwich (0.0)
Datum: D_Sphere
  Spheroid: Sphere
    Semimajor Axis: 6371000.0
    Semiminor Axis: 6371000.0
    Inverse Flattening: 0.0

WGS_1984_World_Mercator
WKID: 3395 权限: EPSG

Projection: Mercator
False_Easting: 0.0
False_Northing: 0.0
Central_Meridian: 0.0
Standard_Parallel_1: 0.0
Linear Unit: Meter (1.0)

Geographic Coordinate System: GCS_WGS_1984
Angular Unit: Degree (0.0174532925199433)
Prime Meridian: Greenwich (0.0)
Datum: D_WGS_1984
  Spheroid: WGS_1984
    Semimajor Axis: 6378137.0
    Semiminor Axis: 6356752.314245179
    Inverse Flattening: 298.257223563