# ISSUE-188: Glossary Issue: CRS and Datum

## Glossary Issue: CRS and Datum

State:
PENDING REVIEW
Product:
best practices
Raised by:
Chris Little
Opened on:
2017-05-04
Description:
1. Coordinate Reference System
ISO TC211 defines:
CRS:Coordinate System that is related to an object by a datum...
CS: 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.

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.

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

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.

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

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

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'.

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."

"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]]."

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