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SDW BP Glossary Issue 188: CRS and Datum

From: Little, Chris <chris.little@metoffice.gov.uk>
Date: Thu, 4 May 2017 10:00:22 +0000
To: Linda van den Brink <l.vandenbrink@geonovum.nl>, "Tandy, Jeremy" <jeremy.tandy@metoffice.gov.uk>
CC: "public-sdw-wg@w3.org" <public-sdw-wg@w3.org>
Message-ID: <3DAD8A5A545D7644A066C4F2E82072883E2CA36B@EXXCMPD1DAG4.cmpd1.metoffice.gov.uk>
1. Coordinate Reference System
ISO TC211 defines:
CRS:Coordinate System that is related to an object by a datum.
Coordinate System: collection of coordinate Tuples related to the same coordinate reference system.
Tuple: ordered list of values.
Coordinate: one of a sequence of n numbers designating the position of a point in n-dimensional space.
Datum: parameter or set of parameters that define the position of the origin, the scale, and the orientation of a coordinate system.

2. Conflating all these gives us a circular mess of CRS : collection of sequences of n numbers designating the position of a point in n-dimensional space that are related to an object by a parameter or set of parameters that define the position of the origin, the scale, and the orientation of a coordinate system.

3. ISO19111 Spatial Referencing by Coordinates and OGC Abstract Specification Topic 5 defines:
Coordinate Reference System: coordinate system that is related to the real world by a datum.

4. We also have:
SRS: system for identifying position in the real world.
Presumably, the wavelength of a signal in the electromagnetic spectrum is part of a CRS but not a SRS.

5. ISO TC211 does not actually say that, usually in our case, the object is the Earth, giving a Geo-Spatial Reference System.

6. 'Dimension' is not ISO TC211 defined, but the usual definition is the inverse of a coordinate definition.

7. I also think that 'collection' implies a specific mathematical meaning, allowing an infinite number of elements, but is not defined. It is also implied that the tuples are all consistent and same 'type'. 

8. I propose the following which is consistent with ISO:

"Coordinate Reference System (CRS): A coordinate system to locate entities of interest with respect to an object using a datum. [[http://registry.it.csiro.au/sandbox/iso-tc211/terms/703|ISO TC211]] If the entities of interest and the object and datum are in the real world, the CRS is a Spatial Reference System (SRS). If the object is the Earth, the SRS is a Geo-Spatial Reference System (GRS). A GRS may be local, regional or global in scope. An example of a CRS that is not a SRS is the wavelength of a signal in the electromagnetic spectrum."


9. Then we need to add:
"Datum: parameter or set of parameters that define the position of the origin, the scale, and the orientation of a coordinate system [[http://registry.it.csiro.au/sandbox/iso-tc211/terms/703|ISO TC211]]."

Chris Little
Co-Chair, OGC Meteorology & Oceanography Domain Working Group

IT Fellow - Operational Infrastructures
Met Office  FitzRoy Road  Exeter  Devon  EX1 3PB  United Kingdom
Tel: +44(0)1392 886278  Fax: +44(0)1392 885681  Mobile: +44(0)7753 880514
E-mail: chris.little@metoffice.gov.uk  http://www.metoffice.gov.uk


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