OSGeodata: Unterschied zwischen den Versionen

Version vom 5. August 2006, 00:58 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 and strictly RESTful harvesting protocols based on Dublin Core (since 1995) whereas WFS allows a RESTful or SOAP client to retrieve geospatial data encoded in GML (since 2000).
• WFS though originated in GIS covers a smaller community than OAI-PMH which today has a larger user base and is supported even by Google and Yahoo!.
• 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 or adapt but much depends on the needs (make sophisticated queries vs. harvesting?) and underlying the architecture (distributed online services vs. local indexing).

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 yet).
• Response:
  • Either an encoded error or unqualified Dublin Core or another format announced with ListMetadataFormats.
• Peculiarities:
  • For identifiers URI must be used (similar requirement to WFS).
  • Has no version negotiation (but see operation Identify)
  • Knows incremental update
  • Another spec. was released called 'OAI Static Repository and an OAI Static Repository Gateway'
  • OAI-PMH may respond results in compressed form which is handled at the HTTP-level (how RESTful!)
• 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