An optional coordinate to define a measure value for the point.
M-values are used in linear referencing scenarios and may represent things like mile markers along a highway. Like z-values, every geometry can optionally store m-values with the point coordinates that comprise it. The default m-value is null. If an m-value is specified when a geometry is created, the new geometry has m-values (Geometry.hasM() is true). Note that when you get m-values back from a geometry, the default value of null is returned for vertices that do not have m-values. A geometry with m-values is sometimes known as an m-aware geometry.