sql server - Geometry column: STGeomFromText and SRID (what is an SRID?)

Question

I'm playing with the new geography column in SQL Server 2008 and the STGeomFromText function. Here is my code (works with AdventureWorks2008)

DECLARE @region geography;
set @region = geography::STGeomFromText('POLYGON((
        -80.0 50.0, -90.0 50.0,
        -90.0 25.0, -80.0 25.0,
        -80.0 50.0))', 4326);

SELECT @region;

My question is about the 4326 in the code. It is supposed to be a spacial Reference ID. When I go to MSDN there isn't a lot on it. If I change the value to 56 I get an error telling me the value must be in the sys.spatial_reference_systems table.

You can look at that table by executing:

select * from sys.spatial_reference_systems 

There is a well_known_text column in that table, but it doesn't tell me much. The value for 4326 is:

GEOGCS["WGS 84", DATUM["World Geodetic System 1984", ELLIPSOID["WGS 84", 6378137, 298.257223563]], PRIMEM["Greenwich", 0], UNIT["Degree", 0.0174532925199433]]

Can anyone explain this mystery to me? What is the SRID?

Answer 1

So I ended up talking with an ex-military guy yesterday who was a radar/mapping specialist. Basically, he knew exactly what that number (4326) was, where it came from, and why it is there.

It is an industry standard for computing geography. The problem is that the earth is not a perfect sphere (it bulges in the middle), and SRID 4326 accounts for that.

As I stated, the table sys.spatial_reference_systems lists all of the code and what they are. But the short version is that you are really only going to use 4326 unless you have a very specific reason to use something different.