OSGeodata Discovery: Unterschied zwischen den Versionen

Version vom 27. August 2006, 22:18 Uhr

Need for geographic metadata exchange

Geographic catalog (catalog service) or inventory are rather data provider centric names, so we prefer a user centric approach. See also OSGeodata for further discussion.

Before all we need a geographic metadata exchange model. Then we need a geographic metadata exchange protocol.

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.

Vision

• Users need search services to discover geographic information.
• Geodata owners need a metadata management tool (which implies an internal metadata model).
• Geodata owners and service providers need (i) a metadata exchange model, (ii) an encoding of it, as well as (iii) a protocol for the exchange, dissemination and sharing of metadata.

Remarks: Metadata ...

• Metadata is just data about data, so nothing special regarding modeling and encoding
• Metadata according to metadata exchange model should be strictly free (LGPL?).

Scenario

A preliminary scenario:

• Users search or browse through metadata records. They use a web app. or a search component out of a desktop GIS (remark: Users don't search services per se).
• 'Search service providers' enable the discovery of geodata and 'filter services', like transformation services (Note: WMS is a 'data access service' and belongs to geodata not to filter services)
• 'Search service providers' gather (harvest) their information from 'data/filter service providers' and need a protocol to do this.
• 'Data/filter service providers' offer metadata over this protocol. They typically also implement 'data access services' (WMS, WFS) or they offer 'filter services'.

Building blocks needed

1. A geometadata management model ( e.g. internal to telascience)
2. Tools to manage the geometadata
3. A geometadata exchange model
4. An encoding of the geometadata exchange model (probably XML)
5. Geometadata exchange protocols (see Discovery mechanisms and protocols)

Discovery mechanisms and protocols

We see two types of 'protocols', both already implemented and ready to go (given an encoded information according to a well specified model):

1. Autodiscovery: Publish XML files of geometadata (exchange model) on the web (=HTTP, autodiscovery).
2. Registry: OAI-PMH which uses geometadata exchange model as its specialized metadata model type (data providers needed to register but search services can do better focussed crawling).

There are some ideas around like embedding hints into the geometadata exchange model or THUMP from IETF which lead to something between 1. and 2.

About autodiscovery:

• Pull principle
• Requires at least one relation - an URL or a tag like a class.
• Uses UI to point to XML-encoded feed, therefore crosses from human mode to machine mode, in a way that’s easy for both humans and machines.
• Easy to implement, no further action required from resource owner
• Examples: HTML pages

About registry:

• Push principle
• Awareness and action required from resource owner
• Examples: OAI-PMH, ...

Weblinks

Geographic information search services:

• Catalogs:
  • mapdex.org
  • ESRI's geography network
  • National Portals: US, CA, EU, UK, FR, DE, AT, CH
• Search engines:
  • General: Google Maps, MSN live, Yahoo!
  • Specialized: geometa.info (still german only for linguistic processing reasons)
• GIS with catalog and search components:
  • uDig, ArcGIS, ...?