CRS Definition Resolver

Overview Resolver Services Versioning Policy Technology Background CRS Resolver Implementation Resolver Redirection Policy Maintenance and Contact CRSdefinitionResolver Web Utilities

Overview

OGC maintains a resolver for various types of definitions, such as Coordinate Reference Systems (CRSs), coordinate axes, units of measure, etc., identified by a URI (NB: OGC uses URLs). The task of a resolver is, when receiving a URL identifying a concept, to return its definition. Syntax and semantics of CRSs are given by the CRS standard, ISO 19111 (which is identical to OGC Abstract Topic 2).

There is a wide and growing range of CRS definitions maintained, including those composed from horizontal space, vertical, time ("temporal"), and index ("image CRS") axes (see more on this). New definitions arise, such as in planetary sciences. As of today, responses are returned in GML, but other representations (such as WKT) might be added in future (see FAQ and issues).

Resolver Services

The resolver represents a Web service accepting a URL, such as http://www.opengis.net/def/crs/EPSG/0/4326, and responding with a definition of the CRS (or axis, etc.) identified by the input URL.

The following CRS families are provided currently:

• http://www.opengis.net/def/crs/EPSG -- the EPSG CRS database as provided by IOGP
• http://www.opengis.net/def/crs/OGC -- OGC CRS definitions
• http://www.opengis.net/def/crs/IAU -- Planetary CRS definitions, as per International Astronomical Union(IAU); these are experimental currently.

Generally, such definitions sit in the def/ branch of OGC's name type system. CRSs and axes are gathered in def/crs/ and def/axis/, respectively. Among others, the following branches are provided currently (see below for complete list):

• http://www.opengis.net/def/crs -- definitions of CRSs as issued by EPSG, OGC, and others
• http://www.opengis.net/def/crs-compound -- a constructor for combining CRSs into new ones, such as 3-D x/y/t CRSs
• http://www.opengis.net/def/axis--axisdefinitionswhichcan form components of CRS definitions

Versioning Policy

The resolver provides versioning of all definitions. To this end, CRS URLs - following common OGC policy - contain a version indicator. For example, in the following URL the "0" is such a version indicator:

http://www.opengis.net/def/crs/EPSG/0/4326

Version identification adheres to the following rules:

• a concrete version number has a syntax of n.n.n where n is a nonnegative integer number following OGC versioning semantics (first digit change = possible backwards incompatibility; second digit change = backwards-compatible change; third-digit change = bug fix). For EPSG definitions, OGC adopts the IOGP version number unaltered (note that EPSG does not necessarily adhere to the OGC versioning semantics).
  • Example: http://www.opengis.net/def/crs/EPSG/8.9.2/4326
• a special version indicator is written as "0". It indicates "no particular version" in general; currently a "0" in an EPSG definition refers to version 8.5.2 as released by OGP. See herefor more information.
  • Example: http://www.opengis.net/def/crs/EPSG/0/4326

Technology Background

CRS Resolver Implementation

This OGC CRS resolver as described above is implemented using SECORE, the open-source resolver implementation of the rasdaman array engine. The SECORE instance comprising the OGC resolver is operated by OGC member rasdaman GmbHas a complimentary service free of cost for OGC.

Resolver Redirection Policy

Incoming URLs are dispatched to the various domain-specific resolvers by OGC:

• All CRS-related URLs are proxied to the SECORE resolver database:

  http://www.opengis.net/def/(crs|crs-compound|area|axis|axis-name|change-request|coordinateOperation|cs|datum|deprecation|ellipsoid|meridian|method|naming-system|parameter|supersession|version-history)/*

• Further branches of the OGC resolver system - addressing topics outside the CRS domain - are maintained by CSIRO. Consequently, all other namespaces in /def/ are redirected to CSIRO's SEEGRID engine.

Exceptions:

• While /def/uom/ is redirected to CSIRO's SEEGRID, /def/uom/EPSG/ is still redirected to SECORE.
• EPSG KVP (key value pairs) rules are still inspected and rewritten to SECORE.

Maintenance and Contact

• See also the discussion area.
• The resolver database is maintained by Jacobs University through its open-source SECORE technology. Contact: Peter Baumann, baumann attrasdaman.com
• want to see a CRS definition added?Contact the maintainer (see above) providing (i) rationale, (ii) the proposed target URL, (iii) GML definition that is supposed to go behind that URL. See the maintenance history.

CRSdefinitionResolver Web Utilities

• WebTopicList - all topics in alphabetical order
• WebChanges - recent topic changes in this web
• WebNotify - subscribe to an e-mail alert sent when topics change
• WebRss, WebAtom - RSS and ATOM news feeds of topic changes
• WebStatistics - listing popular topics and top contributors
• WebPreferences - preferences of this web