Spatial Reference Identifiers (SRIDs)
Each spatial instance has a spatial reference identifier (SRID). The SRID corresponds to a spatial reference system based on the specific ellipsoid used for either flat-earth mapping or round-earth mapping. A spatial column can contain objects with different SRIDs. However, only spatial instances with the same SRID can be used when performing operations with SQL Server spatial data methods on your data. The result of any spatial method derived from two spatial data instances is valid only if those instances have the same SRID that is based on the same unit of measurement, datum, and projection used to determine the coordinates of the instances. The most common units of measurement of a SRID are meters or square meters.
If two spatial instances do not have the same SRID, the results from a geometry or geography Data Type method used on the instances will return NULL. For example, for the following predicate term to return a non-NULL result, the two geometry instances, geometry1 and geometry2, must have the same SRID:
geometry1.STIntersects(geometry2) = 1
The spatial reference identification system is defined by the European Petroleum Survey Group (EPSG) standard, which is a set of standards developed for cartography, surveying, and geodetic data storage. This standard is owned by the Oil and Gas Producers (OGP) Surveying and Positioning Committee.
geometry Instances Default to Zero SRID
The default SRID for geometry instances in SQL Server is 0. With geometry spatial data, the specific SRID of the spatial instance is not required to perform calculations; thus, instances can reside in undefined planar space. To indicate undefined planar space in the calculations of geometry data type methods, the SQL Server Database Engine uses SRID 0.
geography Instances Must Use Supported SRID
SQL Server supports SRIDs based on the EPSG standards. A SQL Server-supported SRID for geography instances must be used when performing calculations or using methods with geography spatial data. The SRID must match one of the SRIDs displayed in the sys.spatial_reference_systems catalog view. As mentioned previously, when you perform calculations on your spatial data using the geography data type, your results will depend on which ellipsoid was used in the creation of your data, as each ellipsoid is assigned a specific spatial reference identifier (SRID).
SQL Server uses the default SRID of 4326, which maps to the WGS 84 spatial reference system, when using methods on geography instances. If you use data from a spatial reference system other than WGS 84 (or SRID 4326), you will need to determine the specific SRID for your geography spatial data.