OGR GeoRSS driver

# GeoRSS : Geographically Encoded Objects for RSS feeds

(Driver available in GDAL 1.7.0 or later)

GeoRSS is a way of encoding location in RSS or Atom feeds.

OGR has support for GeoRSS reading and writing. Read support is only available if GDAL is built with expat library support

The driver supports RSS documents in RSS 2.0 or Atom 1.0 format.

It also supports the 3 ways of encoding location : GeoRSS simple, GeoRSS GML and W3C Geo (the later being deprecated).

The driver can read and write documents without location information as well.

The default datum for GeoRSS document is the WGS84 datum (EPSG:4326). Although that GeoRSS locations are encoded in latitude-longitude order in the XML file, all coordinates reported or expected by the driver are in longitude-latitude order. The longitude/latitude order used by OGR is meant for compatibily with most of the rest of OGR drivers and utilities. For locations encoded in GML, the driver will support the srsName attribute for describing other SRS.

Simple and GML encoding support the notion of a box as a geometry. This will be decoded as a rectangle (Polygon geometry) in OGR Simple Feature model.

A single layer is returned while reading a RSS document. Features are retrieved from the content of <item> (RSS document) or <entry> (Atom document) elements.

## Encoding issues

Expat library supports the following built-in encodings :

- US-ASCII
- UTF-8
- UTF-16
- ISO-8859-1

If your GeoRSS file is not encoded in one of the previous encodings, it will not be parsed by the GeoRSS driver. You may convert it into one of the supported encoding with the iconv utility for example and change accordingly the encoding parameter value in the XML header.
Note : Windows-1252 differs only on a small range of characters with ISO-8859-1. Patching the encoding parameter value to ISO-8859-1 may be enough to make it work...

## Field definitions

While reading a GeoRSS document, the driver will first make a full scan of the document to get the field definitions.

The driver will return elements found in the base schema of RSS channel or Atom feeds. It will also return extension elements, that are allowed in namespaces.

Attributes of first level elements will be exposed as fields.

Complex content (elements inside first level elements) will be returned as an XML blob.

When a same element is repeated, a number will be appended at the end of the attribute name for the repetitions. This is usefull for the <category> element in RSS and Atom documents for example.

The following content :

        <item>
            <title>My tile</title>
            <link>http://www.mylink.org</link>
            <description>Cool descriprion !</description>
            <pubDate>Wed, 11 Jul 2007 15:39:21 GMT</pubDate>
            <guid>http://www.mylink.org/2007/07/11</guid>
            <category>Computer Science</category>
            <category>Open Source Software</category>
            <georss:point>49 2</georss:point>
            <myns:name type="my_type">My Name</myns:name>
            <myns:complexcontent>
                <myns:subelement>Subelement</myns:subelement>
            </myns:complexcontent>
        </item>

will be interpreted in the OGR SF model as :

      title (String) = My title
      link (String) = http://www.mylink.org
      description (String) = Cool descriprion !
      pubDate (DateTime) = 2007/07/11 15:39:21+00
      guid (String) = http://www.mylink.org/2007/07/11
      category (String) = Computer Science
      category2 (String) = Open Source Software
      myns_name (String) = My Name
      myns_name_type (String) = my_type
      myns_complexcontent (String) = <myns:subelement>Subelement</myns:subelement>
      POINT (2 49)

## Creation Issues

On export, all layers are written to a single file. Update of existing files is not supported.

If the output file already exits, the writing will not occur. You have to delete the existing file first.

A layer that is created cannot be immediately read without closing and reopening the file. That is to say that a dataset is read-only or write-only in the same session.

Supported geometries :

- Features of type wkbPoint/wkbPoint25D.
- Features of type wkbLineString/wkbLineString25D.
- Features of type wkbPolygon/wkbPolygon25D.

Other type of geometries are not supported and will be silently ignored.

The GeoRSS writer supports the following dataset creation options:

- FORMAT=RSS|ATOM: whether the document must be in RSS 2.0 or Atom 1.0 format. Default value : RSS

- GEOM_DIALECT=SIMPLE|GML|W3C_GEO (RSS or ATOM document): the encoding of location information. Default value : SIMPLE
  W3C_GEO only supports point geometries.
  SIMPLE or W3C_GEO only support geometries in geographic WGS84 coordinates.

- USE_EXTENSIONS=YES|NO. Default value : NO. If defined to YES, extension fields (that is to say fields not in the base schema of RSS or Atom documents) will be written. If the field name not found in the base schema matches the foo_bar pattern, foo will be considered as the namespace of the element, and a <foo:bar> element will be written. Otherwise, elements will be written in the <ogr:> namespace.

- WRITE_HEADER_AND_FOOTER=YES|NO. Default value : YES. If defined to NO, only <entry> or <item> elements will be written. The user will have to provide the appropriate header and footer of the document. Following options are not relevant in that case.

- HEADER (RSS or Atom document): XML content that will be put between the <channel> element and the first <item> element for a RSS document, or between the xml tag and the first <entry> element for an Atom document. If it is specified, it will overload the following options.

- TITLE (RSS or Atom document): value put inside the <title> element in the header. If not provided, a dummy value will be used as that element is compulsory.

- DESCRIPTION (RSS document): value put inside the <description> element in the header. If not provided, a dummy value will be used as that element is compulsory.

- LINK (RSS document): value put inside the <link> element in the header. If not provided, a dummy value will be used as that element is compulsory.

- UPDATED (Atom document): value put inside the <updated> element in the header. Should be formatted as a XML datetime. If not provided, a dummy value will be used as that element is compulsory.

- AUTHOR_NAME (Atom document): value put inside the <author><name> element in the header. If not provided, a dummy value will be used as that element is compulsory.

- ID (Atom document): value put inside the <id> element in the header. If not provided, a dummy value will be used as that element is compulsory.

When translating from a source dataset, it may be necessary to rename the field names from the source dataset to the expected RSS or ATOM attribute names, such as <title>, <description>, etc... This can be done with a OGR VRT dataset, or by using the "-sql" option of the ogr2ogr utility (see RFC21: OGR SQL type cast and field name alias)

## Example

- The ogrinfo utility can be used to dump the content of a GeoRSS datafile :

      ogrinfo -ro -al input.xml

- The ogr2ogr utility can be used to do GeoRSS to GeoRSS translation. For example, to translate a Atom document into a RSS document

      ogr2ogr -f GeoRSS output.xml input.xml "select link_href as link, title, content as description, author_name as author, id as guid from georss"

  Note : in this example we map equivalent fields, from the source name to the expected name of the destination format.

- The following Python script shows how to read the content of a online GeoRSS feed

          #!/usr/bin/python
          import gdal
          import ogr
          import urllib2

          url = 'http://earthquake.usgs.gov/eqcenter/catalogs/eqs7day-M5.xml'
          content = None
          try:
              handle = urllib2.urlopen(url)
              content = handle.read()
          except urllib2.HTTPError, e:
              print 'HTTP service for %s is down (HTTP Error: %d)' % (url, e.code)
          except:
              print 'HTTP service for %s is down.' %(url)

          # Create in-memory file from the downloaded content
          gdal.FileFromMemBuffer('/vsimem/temp', content)

          ds = ogr.Open('/vsimem/temp')
          lyr = ds.GetLayer(0)
          feat = lyr.GetNextFeature()
          while feat is not None:
              print feat.GetFieldAsString('title') + ' ' + feat.GetGeometryRef().ExportToWkt()
              feat.Destroy()
              feat = lyr.GetNextFeature()

          ds.Destroy()

          # Free memory associated with the in-memory file
          gdal.Unlink('/vsimem/temp')

## See Also

- Home page for GeoRSS format

- Wikipedia page for GeoRSS format

- Wikipedia page for RSS format

- RSS 2.0 specification

- Wikipedia page for Atom format

- Atom 1.0 specification