M (geography Data Type)
The M (measure) value of the geography instance. The semantics of the measure value are user-defined but generally describe the distance along a linestring. For example, the measure value could be used to keep track of mileposts along a road.
The following example creates a Point instance with Z (elevation) and M (measure) values and uses M to fetch the M value of the instance.
DECLARE @g geography; SET @g = geography::STGeomFromText('POINT(-122.34900 47.65100 10.3 12)', 4326); SELECT @g.M;