DCLite4G is short for "Dublin Core Lightweight Profile for Geospatial". It is a minimal information model for metadata about geospatial data. DCLite4G consists of:
- an abstract information model
- an implementation reference exploiting common standard vocabularies - Dublin Core, GeoRSS.
- a namespace used to define extra properties needed to usefully specify the properties of geospatial data.
The URL at which the namespace lives, contains an OWL ontology showing the structure, providing where possible mappings to various metadata schemas.
A Data Set is an abstract object. It corresponds to the ideal of a data set, independent of a physical form or an encoding in which it is being distributed. For example, "TIGER 2005" or "OpenStreetmap from 2007-04-20", or "SRTM" would be considered data sets.
A Data Set can be associated with one or many Data Sources which correspond to concrete resources which can be got at in order to explore the data. All the properties which are shared across multiple Data Sources, are properties of the Data Set. If the properties of a Source are different - e.g. a smaller spatial extent, or a generalised subset of features in the same extent - then it is an instance of a different Data Set.
One goal in this is to make it more possible to trace and annotate the recombination of different sources of data into one package - keeping track of what is known as "lineage" of data packages.
An item of geographic information. Should have at least a title and a URI; ideally the URI is that of a resource where it is possible to access the information directly.
|Title||A name given to a data set. Does not have to be unique or "canonical".|
|Abstract||A text description of the contents of a data set, remarks on anything interesting contained therein.|
|Keywords||A layer may have none or many keywords associated with it.|
|Series||Indicates a Series to which a data set may belong. (see below)|
|Publisher||Contact information for the Agent publishing the data set|
|Originator||Contact information for the original creator of the data set|
|Publication Date||Date on which the data was made available|
|License||URL of a data license whose terms cover the distribution of this data set, OR a text string describing the terms on which the data is made available ("Public Domain")|
|Extents||A bounding box or polygon specified in WGS84, regardless of the original projection and datum of the data.|
|Projection||The original spatial projection of the data|
|Format||If the Source is a service this would be, for example OGC:WMS or OGC:WCS. For files, this is a text identifier which follows as a guideline the output of gdalinfo.|
|URI||A URI giving the location of the data. If this is an OGC web service publishing geographic data, the URI would be that of a HTTP-based GetCapabilities request to it.|
- A Data Set can be part of a Series of associated items
- A Data Set can have optionally many Layers
- A Data Set is made available by an Agent
- A Data Set is originally compiled by an Agent
- A Data Set may be a source of another Data Set
A Series is a collection of Data Sets. It provides a flexible container for a number of Data Sets which have common properties. Thus if all the Data Sets in a Series share a given property (e.g. they all have the same title, or the same date range or the same spatial extents), then that property can be attached to the Series and implicitly inherited by any Data Set belonging to the Series.
Thus a Series can be most usefully used to make an association between:
- A collection of data sets which are adjacent in space, yet represent one temporal snapshot (such as a collection of tiles comprising one larger image or model)
- A collection of data sets which are adjacent in time and may represent one spatial snapshot (such as a sequence of observations collected by an earth imaging satellite)
- A collection of data sets which are "the same" underlying data being made available in different formats, projections, or resolutions.
A Series can carry any of the properties that can be attached to a series.
Vector data formats often provide multiple Layers of data as part of a data set. OGC Web Services also often provide multiple Layers through one service. This is useful metadata which it would be a shame to throw away.
|Name||The layer's name, free text|
|Abstract||A description of the contents of the layer, with any remarks of interest|
|Keywords||A layer may have none or many keywords associated with it.|
- A Data Set can have none or many Layers
(note - though we get information about layers through looking at a Data Source, they are related to the abstract Data Set, because the Layers on all the different Sources will be the same.
An Agent may be a person, an organisation, or a machine agent. This term derives from foaf:Agent, and is used in the same way as 'Principal' is used in security terms.
|Name||The name of the agent|
|A contact email address given as mailto:firstname.lastname@example.org|
|Description||More random details about the Agent|
|Location||A WGS84 lat-lon pair, or a URI indicating a near location|
- An Agent is the original provider of such-and-such a Data Set
- An Agent is the publisher of such-and-such a Data Set
- An Agent is the maintainer of such-and-such a Data Source
Namespace / Ontology
It is possible or advisable to extend many XML-based metadata carrier formats with namespaces which can provide semantics for different properties, taking a "mix-in" approach with the use of small vocabularies for different domains.
Thus DCLite4G attempts not to provide a full model for metadata for geographic information but to reuse properties from other well-known namespaces or ontologies - GeoRSS, Dublin Core, FOAF - and provide a subset of Dublin Core with extra rigour of expression suitable for better machine readability and reuse. This is also what is known as a Dublin Core Application Profile.
http://xmlns.com/2008/dclite4g/ is the namespace reserved for DCLite4G. It should and will come to contain an OWL ontology showing the structure, providing where possible mappings to various metadata schemas.
Example serialisations of a DCLite4G minimal model in different common formats:
DCLite4G Python libraries with genshi templates used to produce them.
- OGC Catalog Services 2 Specification 6.3.3, Core returnable properties
- OGC ebRIM profile of CSW specification, Appendix B.5, Table B.3 - Slots defined in the Basic package
- iGeoResourceInfo class in uDig
- GeodataCommons Metadata Whitepaper
- Open FOSSGIS Community Response to INSPIRE Metadata Draft Implementing Rules
- July 20th 2008
The first version of the DCLite4G model dates to late 2006/early 2007. In mid 2008 the vocabulary is significantly updated and somewhat simplified, after several iterations of a corresponding data registry and search service. The distinction between an abstract "Data Set" which has properties common to potentially many different "Data Sources" has been dropped. A "Series" is introduced, which takes most of the functions of "Data Set". Thus "Data Source" is now renamed "Data Set", resolving the semantic ambiguity in the model. Please see the history of this page