[gdal-dev] Call for discussion for "RFC 41 : Support for multiple geometry fields in OGR"

[Even Rouault]

Hi,

This is a call for discussion for "RFC 41 : Support for multiple geometry 
fields in OGR" :

http://trac.osgeo.org/gdal/wiki/rfc41_multiple_geometry_fields

As an introduction, you'll find below the first paragraphs of the RFC. Much more 
to read at the above link !

== Summary ==

Add read/write support in the OGR data model for features with multiple 
geometry fields.

== Motivation ==

The OGR data model is currently tied to a single geometry field per feature,
feature definition and layer. But a number of data formats support multiple
geometry fields. The OGC Simple Feature Specifications also do not limit to
one geometry field per layer (e.g. §7.1.4 of 
[http://portal.opengeospatial.org/files/?artifact_id=25354 OGC 06-104r4 
"OpenGIS® Implementation Standard for Geographic information - Simple feature 
access -Part 2: SQL option]).

There are workarounds : using geometries of type GEOMETRYCOLLECTION, or
advertizing as many layers as there are geometry columns in the layer (like
currently done in the PostGIS or SQLite drivers). All those approach are at 
best workarounds that suffer from limitations :
  * GEOMETRYCOLLECTION approach : no way to know the name/semantics of each
    sub-geometry. All sub-geometries must be expressed in the same SRS. No way
    of guaranteeing that the GEOMETRYCOLLECTION has always the same number of
    sub-geometries or that there are of a consistent geometry type.
  * one layer per geometry column approach : only appropriate for read-only
    scenarios. Cannot work in write scenarios.

The purpose of this RFC is to make support for multiple geometry fields per
feature to be properly taken into account in the OGR data model.

[...]

Best regards,

Even

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