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 @regi         


        

Answer 1

There are a lot of systems to map the earth. For example you want to map some state in USA. You can set the most south-east point as 0,0 and map all other spatial coordinates according to this point. On the other hand you may want to map some spatial data that span all over the map. In any case you must choose some point as 0,0. In addition you must select some sort of measurement unit: miles/kilometers/degrees/some other magical unit that suits you better. Over the years a lot of such systems where developed. Each has its own zero point, its own coordinates, its own rules about if the earth is flat or not. SRID or SRS is the id of such system. Using this id you can map point expressed in one system to another system, although sometimes it involves some pretty complex math.

And about 4326 SRID. It also called "WGS 84" (http://en.wikipedia.org/wiki/World_Geodetic_System) system. It's the most common system to represent point on spherical(not flat) earth. It uses degree,minute,second notation and its x and y coordinates are usually called latitude and longitude.

Most used non-spherical earth projection is called UTM. You can read about it here: http://en.wikipedia.org/wiki/Universal_Transverse_Mercator_coordinate_system

Anyway, as long you are not doing any spatial conversions from one system to other, you don't really care about the system that you data uses.