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Subject: CRS Registry article
Title: CRS Registry article
Hi Folks,
This is a draft of the CRS (Coordinate Reference System) which
may be a feature proposal for the next version of CAP which Carl Reed
authored, and I added a small edit. I wanted to give you a chance to
eyeball it as another article for the Java Location Service Newsletter
on CAP. This is a more GIS-specific article, which is very appropriate
to the audience, which I felt was also an important consideration in
this context. Also, it makes the point, without belaboring it, that
our current version of CAP is a starting point for moving
onward.
Carl also uploaded the OGC CRS Registry Prototype to the TC
documents section, and if I can get permission to publish it, I will
include it in the materials for the newsletter, so, Carl, if you
notice this, please let me know if I can have permission to publish
this document and, if so, what needs to be said or included, such as
copyrights or disclaimers, since it incorporates a Galdos corporate
logo?
A Coordinate Reference System Registry as a Useful Addition to
the OASIS Common Alerting Protocol
by Carl Reed,
Executive Director, Open GIS Consortium,
Chair OASIS Emergency Management, GIS Subcommittee
As we seek to improve and
refine CAP, one significant enhancement from the realm of GIS may be
to add a new element (field) to the CAP specification that allows an
application to specifiy a Coordinate Reference System (CRS) other
than WGS 84, Latitude/Longitude. This is, at least in part, a response
to a US-specific requirement to provide for the use of previously
mandanted standard, USNG-NAD83.
One possible approach is
to follow the OGC and ISO standards as to how to specify a
CRS as an XML encoding. Within this context, the OGC Geography
Markup Language, an XML encoding for spatial information, can be
used. However, in order for an applications using CAP to
"understand" a specific CRS codings, access to a CRS
registry is required. The OGC and OGC member Galdos have collaborated
to standup a prototype CRS registry.
More specifically, in GML
a geometric coordinate is interpreted by reference to a CRS
definition. For example:
<gml:Point srsName =
"<http://crs.opengis.org/crsportal/>http://crs.opengis.org/crsportal/.. /#ep4326">
<gml:pos>200
300</gml:pos>
</gml:Point>
The attribute srsName
points to the CRS definition. This could be in a flat file or managed
by a registry. The CRS definition is written in GML (this
conforms to ISO 19111 and the CRS WG of OGC). This way we can
have ANY NUMBER of CRS. One can have standard CRS definitions
via the registry with different organizations being the authority for
the registry contents - e.g. one could "put up" a registry
for the EPSG database definitions, for Ordnance Survey (UK)
etc.
It is a recommended best
practice to encode the srsName attribute as a URN and there are
specific recommendations for how this URN is to be encoded. Note
that the URN reference is intended to be an identifier reference - it
is not the intention that CRS information be encoded in the reference
- however such non-opaque identifiers (as long as they remain
identifiers) can be defined by whoever hosts the particular registry.
This is consistent with the URN specification.
The current CRS registry
hosted at
<http://crs.opengis.org/crsportal>http://crs.opengis.org/crsportal is an early version of the above CRS registry in several
respects. In particular:
It is based on the OGC Web Registry Service from OWS
1.2. The Galdos implementation is based on the OASIS
ebRIM. This work has been incorporated into the draft OGC Catalog 2.0
Specification.
It is based on the EPSG (content) v6.1
It is based on an almost
final version of the CRS definitions in GML (i.e. just before GML 3.0
passed)
Ciao,
Rex
Tel: 510-849-2309
Fax: By Request
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