[Gdal-dev] OGR Virtual Layers

[Tyler Mitchell]

Frank, this sounds like a good project.  I can see already how it could
really help, especially for some simple Mapserver applications.  It
overlaps a bit with the GPS coordinate -> PostGIS module I've been working
on.

I'm looking forward to it.

Tyler

gdal-dev-admin at remotesensing.org wrote on 11/03/2003 09:46:31 AM:

> Folks,
>
> For a contract I have, I need to implement "Virtual OGR Datasources".
The
> primary goal in this case is to allow a virtual OGR data source to use
> an existing driver (such as the ODBC driver, or Oracle driver) to read
> non-spatial tables, and select columns to use as X, and Y to return a
> "spatial points" layer to an OGR application.
>
> However, at the same time I am thinking about other things I would like
> to accomplish with such a virtual datasource concept.
>
> So far I have:
>   * Assigning a coordinate system. (required)
>   * Constructing geometry from x/y columns. (required)
>   * Constructing geometry from WKT or WKB columns.
>   * Using geometry directly from the source.
>   * renaming, or subsetting columns list.
>   * Assigning a layer geometry type.
>   * Altering column types (string to int for instance), or precision
> information.
>   * treating an SQL query on the source datasource as a table in the
> virtual layer.
>   * memory or disk based caching.
>   * virtual files should be openable from disk based XML config
> file, or an in memory
>     XML definition.
>   * pass through SetFeature and CreateFeature.
>   * A method to create a virtual datasource cloning a real
> datasource.  This could
>     then be hand edited to make minor changes in configuration.
>   * The ability to control whether attribute and/or spatial queries
> are "passed
>     through".
>
> I am imagining virtual datasources being controlled by an XML
configuration
> file that might look something like this:
>
> <OGR_VirtualDatabase>
>
>      <OGR_VirtualLayer name="RootWormReports">
>
>
<SourceDatasource>ODBC:guest/anonymous at agcan_main</SourceDatasource>
>      <SourceLayer>RootWormReports</SourceLayer>
>    <GeometryType>Point</GeometryType>
>    <GeometryField encoding="xy" x="ReportLong" y="ReportLat"/>
>    <LayerSRS>GEOGCS...</LayerSRS>
>      </OGR_VirtualLayer>
>
> </OGR_VirtualDatabase>
>
> In general, the idea is that the virtual datasource provides an
opportunity to
> "fix up" lots of things that aren't easily done within individual format
> drivers.
>
> 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
>
>
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