Geodata Metadata Requirements

One goal of the Public Geospatial Data Project is to offer, in the future, a repository of reusable public geographic data that can support open source geospatial software projects, both inside and outside the foundation.

One big requirement for a potential Geodata Repository is that there be a well-defined baseline for metadata. This can be seen as a quality assurance effort - data won't be accepted without a certain amount of metadata.

The |US Federal Geographic Data Committee metadata standard emphasises conformance, but doesn't emphasise exchangeability / reusability. FDGC is standard for "Spatial Data Infrastucture" efforts, but doesn't have much of a "geospatial web" orientation.

There are some properties in addition to FGDC which it would be really useful to have - different distribution channels like WFS, bittorrent which have come into existence since FGDC was originally defined. For many elements, FGDC asks for full-text descriptions. More structure in descriptions would help with automating discovery or re-use.

This is perpetual work in progress. See also:


 * Geodata Metadata Translata
 * Simple Catalog Interface
 * Geometa Engine

= Draft Metadata model =

Graph illustrating a basic metadata model generated from an RDF model of what OSGeo Geodata Committee participants have identified as their core needs for metadata.

This picks an arbitrary namespace for an OWL schema that maps to most, if not all, of the FGDC mandatory properties and provides some extra ones.



title
Title of the data set. Corresponds to Dublin Core title

description
Text description of the data set. Corresponds to Dublin Core description element.

Person
A person responsible for publication of the data set - name and contact email address. These properties are well-defined in the FOAF vocabulary.

Organization
A organization responsible for publication of the data set - name and contact email address. These properties are well-defined in the FOAF vocabulary.

Spatial Data Organization
Vector, Raster or Point data, as described in FGDC. (cf http://biology.usgs.gov/fgdc.metadata/version2/sdorg.htm )

datasource
URL from which the data can be downloaded via different protocols

WFS
For Vector data in GML

WMS
For Raster data described in GML

BitTorrent
URL of bittorrent .torrent tracker file.

Other Web API
For example, OpenStreetmap API ( http://wiki.openstreetmap.org/index.php/REST )

License information
Emphasis on public geographic data licenses: PGL, possible LPGL, Public Domain, Creative Commons-type licenses. These can be represented by URLs.

Publication date
Corresponds to Dublin Core date: ISO compliant date of publication.

Spatial Domain
A lot of this can be inferred either using GDAL/OGR or collected from a WMS/WFS GetCapabilities. It would be nice to bypass human error on collecting this kind of metadata.

bounding coordinates
FGDC specifies north, east, west, south bounding co-ordinates. It doesn't specify a projection in which these should be described. For reasons of simplicity it could make sense to require these be in WGS84 (EPSG:4236) - for the same reasons GeoRSS decided to mandate WGS84, rather than complicate matters by dictating that people also specify an SRS.

Projection (Raster, Vector, Coverage)
Original projection of the data (reference to an ?)

Horizontal and vertical datum;

Horizontal and vertical units.

Resolution (Raster,Coverage)
(property of DataSet)

e.g. map units per pixel where map units are defined by SRS

can be different in horizontal / vertical axes e.g. non square pixels

Colour Depth (Raster)
8/16/24 bit etc - this is useful rather than required

Scale (Vector)
Map scale at which vectors are considered accurate

Quantified as a fractional/dimensionless number - 'inches per inch' - on a scale between 1 and 0 - or inverse scale such as 1:50000 - and we would want to store this in a consistent way.

Layers
DataSet has multiple Layers

Extent
can be non-rectangular

Scale Hinting
minscale / maxscale - cf resolution and scale - are these actually properties of layers and not really of data sets? (eg data set contains multiple layers - will they be in any way likely to contain different scale properties?)

Taxonomy/Ontology
Currently undecided; would be good to refer this to current well-known thesauri for data themes.

= Discovery =

Requirements
A discovery resource is essential to expose resultant metadata as per this document. Below are requirements:

Publish:
 * ability to publish/register a web service
 * ability to publish/register a static resource
 * ability to harvest and classify public and private resources
 * ability to establish and maintain user/group/role based authentication
 * ability to provide a RESTful authentication mechansim

Find:
 * ability to discover the existence of a web service
 * ability to discover the existence of a resource which is available via web services (i.e WMS layer, WFS feature type)
 * ability to discover the existence of a static resource (dataset, document, etc.)

Bind:
 * ability to perform discovery operations with spatial, aspatial and temporal predicates
 * ability to provide a RESTful request API
 * ability to provide responses in XML
 * ability to expose resource/service metadata in a manner which facilitates dynamic connection to a resource/service

Information model for metadata exchange
This is the http://tinyurl.com/kfkyv model (:->).

Some design considerations:
 * This is a minimal metadata information model regarding to a metadata exchange protocol for harvesting (e.g. no filter nor GML implementation needed) and according to the ideas about a Simple Catalog Interface/protocol.
 * Based on Dublin Core (DC) and Catalogue Services Specification 2.0.1, OGC 04-021r3, p.22.
 * Dublin Core need refined semantics of some properties/attributes.
 * Have had hard times with the abundance use of namespaces. This is because DC specs and other XML 'practices' specialize properties/attribute types instead of specializing whole classes.
 * All properties/attributes have cardinality [0..1] except for identifiers (which are mandatory) and for those attributes which are really needed (as unbounded) for automation!
 * Take all information one can in an automated manner, e.g. from data set resource.

Details:
 * Services are included in attribute 'format' in the sense that WMS, etc. are just protocol bindings to geodata. Real well known services on it's own like filter or label placement services have a place there too. They could be still detected by challenging them with GetCatabilites (taken from OWS/WxS).
 * Indicating of quality of service could be a nice task for search service provider; no need to add it as attribute
 * Relationships between features is part of schema metadata: How to handle this...?

Minimal Information Model
'Basic' (not so highly disputed attributes/properties):

'Extended' (highly disputed attributes/properties):

Legend: 'Poss. to autom.' means Possible to automate

Remarks:
 * General:
 * DC attributes/properties left as they are...: Audience; Contributor; Creator.
 * All attributes/properties have cardinality 1 except dc:relation and dct:format.
 * No additinal DC attributes/properties required; few them needed to be specialized (see dct:...); ** See for some general explanations about dc/dct here.
 * still some attributes/properties need some specialized recommended meaning (see tbd.).
 * Assume metadata (as opposite to geodata) is always free and open information, like Creative Commons Share Alike
 * An encoding still has to be discussed (see following example). need schemaLocation in OSGeo!?
 * Details:
 * dct:modified and dct:spatial can be sync'ed from dataset.
 * Attribute 'relation': This was'nt discussed yet. Simply helps harvesters to discover more (meta) data providers.
 * Attribute 'publisher': Carl mentioned such a structure here which includes StreetAddress, addressee, primaryAddressNumber, streetName, city, state, zipCode, countryCode (like in KML and behind Google geocoding service!?)
 * Keywords is included in attribute 'dc:subject'; I think people have a hard time to agree on an enumerated list (see the success of folksonomy).
 * GetCapabilities adds following attributes (not yet modelled here): Fees, ScaleHint and Style.
 * Note that OAM-PMH...
 * puts a XML envelope around this metadata and adds a header containing two attributes: 'identifier' to identify an metadata record and 'datestamp' as date of last (published) change of metadata record.
 * requires to define a name for metadata sets. Let's don't care about this yet.

Example
Notes:
 * Example values are only for explanation purposes and purely fictive.
 * XML Schema (= geometadc.xsd) still tbd.
 * This record is not yet validated!
 * Took 'geometadc' as envelope name.

 f264-77d2-09ce-aa39-f0f0 National Elevation Mapping Service for Texas Elevation data collected for the National Elevation Dataset (NED). Elevation, Hypsography, and Contours g264-77d2-09ce-aa39-g0g0 grid geodata uri:http://www.osgeo.org/services/wms/ uri:http://www.osgeo.org/geodata/ned_grid_georss.xml</dc:format> uri:http://www.osgeo.org/geodata/ned_grid.shp</dc:format> <dct:modified>2004-03-01</dct:modified> <dct:spatial> <Box projection="EPSG:4326" name="Geographic"> 34.353        -96.223         28.229         -108.44       </Box> </dct:spatial> <dc:language>en</dc:language> <dc:source>Lineage: Based on 30m horizontal and 15m vertical accuracy.</dc:source> <dc:rights>uri:http://www.usgs.gov/pubprod/</dc:rights> <dc:publisher>U.S. Geological Survey</dc:publisher> </dct:description>

Other Relevant Info

 * Simple_Catalog_Interface
 * OSGeodata on GISpunkt Wiki - These pages are about the search of an open, lean and mean "protocol for the incremental exchange of metadata about geographic resources between systems". Profiled specifications like WFS or OAI-PMH are currently on our short list. Delving into 'Open Archives Initiative Protocol for Metadata Harvesting' (OAI-PMH) is strongly encouraged. It's a low barrier interoperability specification based around metadata harvesting model, it's stable (subsequent revisions are backwards compatible) and uses unqualified Dublin Core as default metadata information model; there exist open source tools (like OAICat) and it has been adopted among others by Google and Yahoo! but it's not a search protocol.
 * See here a comparison between CSW, WFS and OAI-PMH.

= References =

Geospatial

 * FGDC geospatial metadata model


 * GEON geospatial metadata model


 * DIF geospatial metadata model


 * GeoRSS


 * WFS


 * GeoAPI contains an implementation of ISO 19115

RDF

 * Resource Description Framework


 * RDF Primer


 * OWL Web Ontology Language Guide


 * Semantic Web for Earth and Environmental Terminology OWL Ontologies at NASA


 * Dublin Core metadata model for documents


 * FOAF metadata model for people and organisations


 * DOAP metadata model for open source software projects and code repositories

From Geodata Packaging Working Group:

 * Specifications of a data set
 * Creator
 * Date
 * License
 * Data Type
 * Topic
 * Spatial Extent
 * Coordinate System/Projection
 * Target Scale/Precision
 * Attribute Data