因为模型是系统定义的一部分，比起文档，它们从不会过时，而且是可靠的。
Because models are part of the system definition, rather than documentation, they are never out of date and are reliable.
椭圆体模型是空间引用系统定义的第一部分。
The ellipsoid model is the first part of a spatial reference system definition.
按照这个定义，模型是关于系统的描述。
According to this definition, a model is a description of a system.
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