OSGeodata: Unterschied zwischen den Versionen

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* OGC's CSW 2.0
 
* OGC's CSW 2.0
 
* OGC's WFS
 
* OGC's WFS
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* [[OAI-PMH]] includine home, wikipedia, documents, tutorials, tools and demos
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* [[OAI-PMH]] - including home, wikipedia, documents, tutorials, tools and demos

Version vom 6. August 2006, 22:15 Uhr

These pages are about our search of "a protocol for the incremental exchange of metadata about geographic resources between systems" which is open, lean and mean. Proposals like OGC's CSW 2.0 do not fulfill these requirements. Profiled specifications like WFS or OAI-PMH (see below) are currently on our short list.

These are experimental pages originated from OSGeodata Mailing List on OSGeo.org.

Towards a geographic metadata harvesting protocol

For protocol requirements read the OSGeodata Mailing List mentioned above, for requirements of the metadata information model (e.g. for response data) see this OSGeo Wiki page as well as the OSGeodataMetadataModel. Note that it's not a search protocol we seek (OpenSearch for example would be one)!

Appropriate name sought:

  • Geographic Metadata Harvesting Protocol (GMH)
  • Geographic Data Discovery Protocol (GDDP)
  • Geographic Metadata Dissemination Protocol (GMDP)
  • ...?

Conceptual Issue

There is a dilemma about what constitutes a 'resource' the metadata points to: Is a geographic service, like WMS, a resource on its own? In this case a harvester must subsequently try to request it for more information based on the indicated protocol hopefully indicated by a type value. If the geographic data remains the single source of resource, every file format (like GeoRSS, GML, DXF, Shapefile) as well a service type, like a 'WMS layer' or a 'WFS feature set' needs to be mentioned in the metadata record.

Personal remark: The dilemma get's even stronger if we think of style sheets that render the same geographic data differently. Still, I tend to the latter choice, because all are data streams originating from the single most precious source: geographic data as interpreted, digitized and stored geoinformation) -- Stefan 20:07, 6. Aug 2006 (CEST)

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 (c.f. Filter) includes spatial constraints (but no temporal?)
  • Response:
    • Must be GML 2 and may be other XML encodings.
  • Peculiarities:
    • Knows version negotiation (which isn't implemented yet...)
    • Uses URN as unique identifier
  • Availability and outreach of services, tools and (open source) software:
  • 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-PMH is short for 'Open Archives Initiative Protocol for Metadata Harvesting'. See OAI-PMH for a short description and architectural diagrams.
  • Spec. size: Version 2.0 still is < 50 pages(!)
  • 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
    • 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)
    • ListSets - retrieve set structure of a repository (optional)
  • Query:
    • Standard query includes temporal constraints (but no spatial yet).
  • Response:
    • Either an encoded error or unqualified Dublin Core XML 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?
    • ...?

Background

Let's finally unleash geographic information through open access and dissemination of geographic data and (metadata) search appliances!

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.... (Citation from Jo Walsh)

Keywords: Open access to and dissemination of geographic data (geodata) and information; Metadata; Finding, harvesting or discovery of geodata and web map services; Interoperability; Integration; Service binding; Spatial data infrastructure; Standards.

Weblinks

Information:

Geographic information search services:

Specs.:

  • OGC's CSW 2.0
  • OGC's WFS
  • OAI-PMH - including home, wikipedia, documents, tutorials, tools and demos
Von «https://giswiki.hsr.ch/index.php?title=OSGeodata&oldid=2580»