OSGeodata Discovery
Inhaltsverzeichnis
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
"We don’t know what people will want to do, or how they will want to do it, but we may know how to allow people to do what they want to do when they want to do it". (ockham.org)
- 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
- A geometadata management model ( e.g. internal to telascience)
- Tools to manage the geometadata
- A geometadata exchange model
- An encoding of the geometadata exchange model (probably XML)
- Geometadata exchange protocols (see Discovery mechanisms and protocols)
Discovery mechanisms and protocols
We are aware of two types of 'protocols' both already implemented and ready to go (given an encoded information according to a well specified model):
- Autodiscovery: Publish XML files of geometadata (exchange model) on the web (=HTTP).
- 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 for augmented autodiscovery. See also THUMP from IETF which leads 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 content-rich XML-encoded feed, therefore crosses from human mode to machine mode, in a way that’s easy for both humans and machines.
- Autonomous/independent resource owner; no further action required from him.
- Examples: HTML pages, Extended Dublin Core.
About registry:
- Push principle.
- More load on resource owner.
- Awareness and action required from resource owner.
- Examples: OAI-PMH, RSS pinging search services...
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, ...?
