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The ArcSDE geometric model is centered around operations on various types of geometric shapes, also called feature classes. Shapes represent geometric objects in two-dimensional planar space that are useful for GIS applications. Shapes can be 0-dimensional points, 1-dimensional lines, and 2-dimensional areas. The available shape types in ArcSDE are NIL, Points, Lines, Simple Lines, and Areas. Each shape has a spatial reference system which describes the coordinate space in which the shape is defined.
The spatial reference system for a shape describes the coordinate system in which it is defined. Spatial data often is defined with different coordinate systems. To integrate spatial data from various sources, the data must be transformed to a common coordinate system. The spatial reference system must be clearly defined not only to verify the integrity of geometric calculations between shapes, but also to allow transformations to take place between shapes in different coordinate systems.