Monday September 28 2009 at 07:47
The OGC standard has an STDimension() method defined on geometry/geography objects and this is implemented in SQL Server 2009. This function returns the inherent (or topological) dimensionality of an object: for a point this is 0; for a linestring this is 1; and for a polygon this is 2.
SELECT [name], [geom].STDimension() as [dim]
FROM ( select 'Point' as name, geometry::STGeomFromText('POINT(3 3)', 0) as geom
union all select 'LineString' as name, geometry::STGeomFromText('LINESTRING(0 0, 3 3)', 0) as geom
union all select 'Polygon' as name, geometry::STGeomFromText('POLYGON((0 0, 3 0, 0 3, 0 0))', 0) as geom) as a;
name 
dim 
Point 
0 
LineString 
1 
Polygon 
2 
But what if you wanted to know the coordinate dimensionality of an object? That is whether an object is:
 2D ie has only X,Y ordinates; or
 3D ie has X,Y and Z ordinates defined; or
 4D ie has X,Y,Z and M ordinates defined?
The OGC standard cannot help here and no such function has been implemented in SQL Server.
Now, in Oracle, a method called GET_DIMS() is defined on the sdo_geometry object that returns the coordinate dimensionality of an object and, in PostGIS, a function called ST_NDims() is defined. What can we do for SQL Server 2008 Spatial?
Here is a function called NDims that implements what we are after.
USE [GISDB]  Change to your database
GO
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
Create Function [dbo].[NDims]( @p_geom geometry )
Returns Int
As
BEGIN
Declare
@v_ndims Int;
Begin
SELECT @v_ndims = ( 2 +
case when c.point.Z IS NULL then 0 else 1 end +
case when c.point.M IS NULL then 0 else 1 end )
FROM (SELECT geometry::STGeomFromText(b.PointText,0) as point
FROM (select Top 1
case when geom.STGeometryType() = 'Point' then geom.AsTextZM()
when geom.STGeometryType() = 'MultiPoint' then geom.STGeometryN(1).AsTextZM()
when geom.STGeometryType() = 'LineString' then geom.STPointN(1).AsTextZM()
when geom.STGeometryType() = 'MultiLineString' then geom.STGeometryN(1).STPointN(1).AsTextZM()
when geom.STGeometryType() = 'Polygon' then geom.STExteriorRing().STPointN(1).AsTextZM()
when geom.STGeometryType() = 'MultiPolygon' then geom.STGeometryN(1).STExteriorRing().STPointN(1).AsTextZM()
end as pointText
from (select case when @p_geom.STGeometryType() = 'GeometryCollection'
then @p_geom.STGeometryN(1)
else @p_geom
end as geom ) as a
) as b
) as c;
RETURN @v_ndims;
END;
END
GO
Note that the function is defined only on the geometry object type. The function can be called for geography object types via the use of my toGeometry/toGeography functions.
Testing
Testing this function with a variety of data returns the following:
select dbo.ndims(geometry::STGeomFromText('POINT(0 1)',0)) as pDim
union all select dbo.ndims(geometry::STGeomFromText('POINT(0 1 2)',0)) as pDim
union all select dbo.ndims(geometry::STGeomFromText('POINT(0 1 2 3)',0)) as pDim;
select dbo.ndims(geometry::STGeomFromText('MULTIPOINT((0 0 0), (1 1 1), (2 2 2), (3 3 3))',0)) as mpDim;
select dbo.ndims(geometry::STGeomFromText('LINESTRING(0 0, 1 1)',0)) as lDim
union all select dbo.ndims(geometry::STGeomFromText('LINESTRING(0 0 0, 1 1 1, 2 2 2, 3 3 3)',0)) as lDim
union all select dbo.ndims(geometry::STGeomFromText('LINESTRING(0 0 0 0, 1 1 1 1, 2 2 2 2, 3 3 3 3)',0)) as lDim;
select dbo.ndims(geometry::STGeomFromText('MULTILINESTRING((0 0,1 1,1 2),(2 3,3 2,5 4))',0)) as mlDim
union all select dbo.ndims(geometry::STGeomFromText('MULTILINESTRING((0 0 0,1 1 1,2 2 2),(3 3 3,4 4 4,5 5 5))',0)) as mlDim
union all select dbo.ndims(geometry::STGeomFromText('MULTILINESTRING((0 0 0 0,1 1 1 1,2 2 2 2),(3 3 3 3,4 4 4 4,5 5 5 5))',0)) as mlDim;
select dbo.ndims(geometry::STGeomFromText('POLYGON ((0 0, 1 0, 1 1, 0 1, 0 0))',0)) as dPoly
union all select dbo.ndims(geometry::STGeomFromText('POLYGON ((0 0 0, 1 0 0, 1 1 0, 0 1 0, 0 0 0))',0)) as dPoly;
select dbo.ndims(geometry::STGeomFromText('MULTIPOLYGON (((20 20, 40 20, 40 40, 20 40, 20 20)), ((0 0, 10 0, 10 10, 0 10, 0 0), (4 4, 4 6, 6 6, 6 4, 4 4)))',0)) as dMPoly
union all select dbo.ndims(geometry::STGeomFromText('MULTIPOLYGON (((20 20 0, 40 20 0, 40 40 0, 20 40 0, 20 20 0)), ((0 0 0, 10 0 0, 10 10 0, 0 10 0, 0 0 0), (4 4 0, 4 6 0, 6 6 0, 6 4 0, 4 4 0)))',0)) as dMPoly;
select dbo.ndims(geometry::STGeomFromText('GEOMETRYCOLLECTION(POINT(2 3 4),LINESTRING(2 3 4,3 4 5),POLYGON ((0 0 0, 1 0 0, 1 1 0, 0 1 0, 0 0 0)))',0)) as dGC;
Geography
If we have a geography object we can still find its coordinate dimensionality via use of the toGeometry/toGeography functions.
select dbo.NDims(
dbo.toGeometry(geography::STGeomFromText('POLYGON ((148.0 44.0, 148.0 43.0, 147.0 43.0, 147.0 44.0, 148.0 44.0), (147.4 43.6, 147.2 43.6, 147.2 43.2, 147.4 43.2, 147.4 43.6))',4326),
0)) as dGeogPoly;
I hope this function is useful to somebody.

Comment [1]
Thank you, this function was very useful to me, since I wanted to find all geometries that are not 3dimensional. It’s strange to me that function STDimension() is used for getting the type of geometry, when by the name of it you would think it get you dimension (2D, 3D…).
— Krešimir Kovačić · 13 July 2016, 20:33 · #