Spatial references in the geodatabase
Spatial references include a coordinate system for x-, y-, and z-values as well as tolerance and resolution values for x-, y-, z-, and m-values.
All spatial reference systems known to the geodatabase are stored in a system table, some of which have editable views. For ArcSDE geodatabases in Oracle using a geometry storage of ST_Geometry, it is the ST_Spatial_References table. In Informix, this table is named Spatial_References. In DB2, it is called ST_Spatial_Reference_Systems. In PostgreSQL, the table is sde_spatial_references, but there is also a view of the sde_spatial_references table, st_spatial_references, that can have spatial references inserted into it.
Internal functions use the parameters of a spatial reference system to translate and scale each floating-point coordinate of the geometry into a 64-bit positive integer prior to storage. Upon retrieval, the coordinates are restored to their external floating-point format.
The floating-point coordinates are converted to integers by subtracting the false x- and false y-values, which translates to the false origin; scaling by multiplying by the x,y units; adding a half unit; then truncating the remainder.
The optional z-coordinates and measures are dealt with similarly, except they are translated with false z- and false m-values and scaled with z-units and m-units, respectively.
For a definition of all the columns in the spatial reference table in each database management system, see their respective system table topics:
The spatial reference system is assigned to a geometry during its construction. The spatial reference system must exist in the spatial reference table. All geometries in a column must have the same spatial reference system.
ESRI recommends assigning or creating spatial references using ArcGIS Desktop tools when creating feature classes. When you use ArcGIS to assign a spatial reference at the time you create a feature class, ArcGIS calculates the x-,y-, z-, and m-units and offsets for you to maintain high-precision data. ESRI understands, though, that some users and application developers want to have options when creating spatial references. The examples in Creating spatial references using SQL show you how to create a spatial reference outside ArcGIS.