OSGeodata: Unterschied zwischen den Versionen

Version vom 5. August 2006, 00:13 Uhr

Experimental pages for OSGeodata Mailing List on OSGeo.org.

Goal for a lean and mean geographic metadata protocol

Citation from Jo Walsh: "Managing metadata well is the key to making a repository of open geographic data really useful and re-usable. Creating metadata can be a dull chore. Part of the problem is over-focus on production, rather than consumption; on standards, rather than on usage. The Geodata Committee at OSGeo has been working on a "simplest useful thing" approach to geographic metadata...".

Appropriate name sought:

• geographic metadata protocol
• geographic metadata harvesting protocol
•  ?

See also: OSGeodataMetadataModel.

Comparison between WFS and OAI-PMH

Relative (biased?) comparison:

• OAI-PMH is a set of simple harvesting protocols based on Dublin Core (since 1995) whereas WFS allows a client to retrieve geospatial data encoded in GML (since 2000).
• WFS though originated in GIS could cover a smaller community than OAI-PMH which today probably has a larger user base and is supported even by Google.
• Both have in common that they can be made (WFS) or are (OAI-PMH) RESTful and both can be profiled to respond an 'output content model' - which has yet to be defined (see OSGeodataMetadataModel).
• WFS needs to be profiled (spec. size is ~200 p.) whereas OAI-PMH needs probably to be extended (spec. size is ~50 p.) - so WFS seems more complex and more costly to implement.

WFS

• WFS is short for 'Web Feature Service'.
• Spec. size: About 100 pages plus mandatory reference to Filter spec. of about 40 pages, plus reference to GML 2 (60 pages).
• Background: A de-facto standard in GIS world owned by OGC
• Request operations (Basic WFS, read only WFS):
  • GetCapabilities - describe service capabilities;
  • DescribeFeatureType - describe structure of a feature type
  • GetFeature - retrieve feature instances
• Query:
  • Standard query includes spatial constraints (but no temporal?)
• Peculiarities:
  • Knows version negotiation (which isn't implemented yet...)
  • Uses URN as unique identifier
• Availability and outreach of services, tools and (open source) software:
  • Open source software see freegis.org
• Expected changes to spec. (profile or extension):
  • Profile to KVP binding
  • Profile Filter to Boolean and Wildcard matches as well as 'within'
  • ...

OAI-PMH

• OAI is short for 'Open Archives Initiative', PMH is short for 'Protocol for Metadata Harvesting'.
• Spec. size: < 50 pages(!) plus??
• Background: Was initiated by libraries, universities, museums and galleries to 'open access' (OA) free online availability of digital content.
• Request operations:
  • Identify - describe an archive (similar to WFS' GetCapabilities)
  • ListMetadataFormats - retrieve available metadata formats from archive
  • ListSets - retrieve set structure of a repository
  • ListIdentifiers - abbreviated form of ListRecords, retrieving only headers
  • ListRecords - harvest records from a repository (similar to WFS' GetFeature)
  • GetRecord - retrieve individual metadata record from a repository (also similar to WFS' GetFeature)
• Query:
  • Standard query includes temporal constraints (but no spatial).
• Peculiarities:
  • Uses identifier mechanism (numbers) from digital archives.
  • Has no version negotiation
  • Knows incremental update
  • Recently, another spec. was released called 'OAI Static Repository and an OAI Static Repository Gateway'
• Availability and outreach of services, tools and (open source) software:
  • Open source software see OAI-PMH tools
  • Gateways available from publishing files to others
  • Google reads it as a Sitemaps file (how to submit) and Internet Archive uses it among others.
• Expected changes to spec. (profile or extension):
  • special treatment of ID's?
  • Need to be extended if spatial constraints are needed here
  • Add version negotiation?
  • ...?

Weblinks

• OGC's CSW 2.0
• OGC's WFS
• OAI-PMH - Open Archives Initiative Protocol for Metadata Harvesting 2.0