SQL Server supports spatial data defined in a wide variety of spatial reference systems used across the world. When you create an individual item of
geography data, you can use any one of the 392 spatial reference systems listed in the
sys.spatial_reference_systems table. And if you create
geometry data, you can use any spatial reference system you like; you can even define your own coordinate system that uses coordinates measured from an origin somewhere in your living room.
Coordinate values are only valid for the spatial reference system in which they were defined, and you can only compare two coordinates if they were obtained from the same coordinate system. In SQL Server, this means that ...