[geos-devel] WkbReader and Z-geometries

[Oyvind Idland]

Oh.. sweet chaos!

The lack of a WKB-spec with Z (and M) ordinates seems to have triggered a
bunch of WKB-flavours, such as
- OGR's "Two-and-a-half-D extensions for Simple Features"
- FDO's FGF-format
- Postgis EWKB

I found the EWKB proposal here, but probably not the latest:
http://postgis.refractions.net/pipermail/postgis-devel/2004-December/000710.html

As far as I can see, it is fairly similar in structure as the SQL/MM, except
for the optional SRID. The object-type ID's are of course not the same.

In addition, SQL/MM specifies CircularString/CurvePolygon/CompundCurve etc
aswell, would be nice to have these implemented in GEOS aswell (PostGIS has
done some work here, not sure what)

I got two versions of the SQL/MM spec in PDF, the earliest (2nd edtion)
dates from 2002. I'll send you the newest right away.

For now, I'll try to make a simple hack on the existing WkbReader.

Would be nice with a more united WKB-world...


-- Oyvind





On Thu, Aug 4, 2011 at 2:24 AM, Martin Davis <mtnclimb at telus.net> wrote:

> **
> The EWKB that JTS and GEOS implement is based on the PostGIS EWKB
> specification (which predates the SQL/MM spec, I think.)
>
> I don't have a good reference for the PostGIS spec - it would be nice if
> there was one online somewhere (hint, hint).
>
> It might be nice if JTS/GEOS supported the SQL/MM spec as well.  Do you
> have a reference available for the spec?
>
> Martin
>
>
> On 8/3/2011 4:19 AM, Oyvind Idland wrote:
>
> Hello,
>
> i am trying to read WKB chunks based on the iso SQL/MM part3 spec. However,
> this doesn't seem to work.
>
> From the WkbReader source code doc:
> "This implementation supports the extended WKB standard for representing
> 3-dimensional coordinates. The presence of 3D coordinates is signified by
> setting the high bit of the wkbType word."
>
> Which extended WKB standard is being refered to here ?
>
>
>
> For example, In the SQL/MM spec, various point types are defined as the
> following unsigned values (not hex):
>
> <wkbpoint>     1
> <wkbpointz>   3000001
> <wkbpointzm> 2000001
> <wkbpointm>   4000001
>
> However, in the WkbReader.cpp the following logic is used:
>
>     int typeInt = dis.readInt();
>     int geometryType = typeInt & 0xff;
>     ......
>     bool hasZ = ((typeInt & 0x80000000) != 0);
>     if (hasZ) inputDimension = 3;
>     else inputDimension = 2; // doesn't handle M currently
>
>
>
>
> Is there any plans to implement support for SQL/MM ?  (I can volunteer for
> a patch, if desired)
>
>
> -- Oyvind
>
>
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