[postgis-users] PointM, PolyLineM, PolygonM

[Frank Warmerdam]

strk wrote:
> pramsey wrote:
> 
>>Is is possible to have measure *and* elevation in a shapefile? XYZM? Or 
>>is it only XYZ and XYM?
>>Paul
> 
> 
> I think it is *NOT* possible.
> 
>>From the Shapefile Format Specification this is the list of known shapes:
> 	
> 	Value   Shape Type
> 	0	Null Shape
> 	1    	Point
> 	3    	PolyLine
> 	5    	Polygon
> 	8    	MultiPoint
> 	11    	PointZ
> 	13    	PolyLineZ
> 	15    	PolygonZ
> 	18    	MultiPointZ
> 	21    	PointM
> 	23    	PolyLineM
> 	25    	PolygonM
> 	28    	MultiPointM
> 	31    	MultiPatch
> 
> I think using the Z field could be ok, but we need a function that tell
> us wheater a geometry is 2d,3d or measured.  Does OGC say anything about
> measures ? The array option might also work as a temporary solution, as
> would do a just-drop-measure approach.
> ... shp2pgsql could give a few options (--measured-policy=[array|discard])

Strk,

The March 1998 version of the Shapefile specification clearly shows that
the "Z" forms such as PolyLineZ can have Z coordinates *and* M measures
(see circa page 18), so geometries can have Z and M.

OGC's simple features specifications do not address "measures".  It might be
addressed in GML 3.  Measures are supported in some manner by Oracle if you
wanted something to compare to.  I don't know the details though.

Best regards,

-- 
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent