[postgis-devel] Oracle SDO_GEOMETRY vs PostGIS WKT

[Paul Ramsey]

First, be careful to not mistake internal representation for external
serialization. People ask "do you store geometries in OGC format", to
which I answer "why do you care?" (the answer is, of course "sort of"
(until 2.0, when the answer will be "less so"), but is still
irrelevant to end users).

Now, regarding the text serialization of SDO_GEOMETRY (a) it predates
the OGC SFSQL specification and (b) it includes some extra information
like curve interpolation types that are not representable in WKT. So
it's not a completely arbitrary ignoring of OGC standards.

That said, it took Oracle a very very long time to provide utility
methods for creating geometries from WKT and writing them out as WKT,
so I wouldn't count Oracle as a particularly fervent follower of the
standards base.

The standards are a good starting point, in my opinion. Certainly they
give you an open door into PostGIS, DB2, SQLServer and ESRI's new SQL
support. The mappings from the standards SQL into Oracle spatial SQL
are murky but doable, which is testament to the fact that Oracle must
at least give a sidelong glance at the standards from time to time.
You find things like the DE9IM model in Oracle, and the semantics of
valid polygons match the OGC scheme, so that's all good.

Paul

2010/8/24 Jorge Arévalo <jorge.arevalo at deimos-space.com>:
> Hello,
>
> I'm studying Oracle SDO_GEOMETRY type [1], and I find it really
> tangled. But, AFAIK, the SFS standard [2] ST_Geometry is implemented
> too, provided by ESRI [3]. And even a third ST_Geometry object,
> provided by Oracle and basically equal to SDO_GEOMETRY type, is
> available [4].
>
> On the other hand, and please correct me if I'm wrong, PostGIS simply
> implements the standard ST_Geometry type. I'm reading the SFS
> documents [2], and the PostGIS documentation [5], and I find it
> simpler to specify if the geometry is compound or not, if it has
> holes... this is: the geometry's properties.
>
> So, my questions, as a beginner, are:
> - Why does Oracle use a tangled format like SDO_GEOMETRY, if they
> could simply implement the easier standard? Any obvious reason I'm
> missing? Apart from "we are a private company and define our own
> formats".
> - If I want to really understand spatial formats used in Oracle
> Spatial, PostGIS... are the [2] standards the best source? I think
> they are THE source... Am I right?
>
> Thanks in advance, and best regards
>
> --
> Jorge Arévalo
> http://gis4free.wordpress.com
>
> [1] http://download.oracle.com/docs/cd/B28359_01/appdev.111/b28400/sdo_objrelschema.htm#i1004087
> [2] http://www.opengeospatial.org/standards/sfs
> [3] http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?id=2709&pid=2704&topicname=The_ST_Geometry_storage_type
> [4] http://download.oracle.com/docs/cd/B28359_01/appdev.111/b28400/sdo_sql_mm.htm#CHDJHAFA
> [5] http://postgis.refractions.net/docs/ch04.html
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