OGIS规范定义的几何对象定义
Curve：A Curve is a 1-dimensional geometric object usually stored as a sequence of Points, with the subtype of Curve specifying the form of the interpolation between Points. This standard defines only one subclass of Curve, LineString, which uses linear interpolation between Points.
LineString: 表示曲线，内部包含多个点，相邻点对构成直线段。
A LineString is a Curve with linear interpolation between Points. Each consecutive pair of Points defines a Line segment.
A Line is a LineString with exactly 2 Points. Line表示只有2个点的曲线段。
A LinearRing is a LineString that is both closed and simple. 闭合简单，线环
MultiLineString 类似于AE中的Polyline。
A Polygon is a planar Surface, defined by 1 exterior boundary and 0 or more interior boundaries.
MultiPolygon 类似于AE中的Polygon，不过AE中A polygon is a collection of one or many exterior and interiorrings.
SharpMap的实现
