Enhance geospatial CRS descriptions for separate datums

Improved clarity in the description of geospatial coordinate reference systems and possible datums. Added examples for geographic and projected CRSs.

Author: chris-little

standard/clause_specification_text.adoc

@@ -329,8 +329,8 @@ Reference system objects are used to provide information about how to interpret
 
 [[geospatial_coordinate_reference_systems]]
 //### 6.1. Geospatial Coordinate Reference Systems
-==== Geospatial Coordinate Reference Systems
-Geospatial coordinate reference systems (CRSs) link coordinate values to the Earth.
+==== Geospatial Coordinate Reference Systemn
+Geospatial coordinate reference systems (CRSs) link coordinate values to the Earth.  The fixed, stable, position on the Earth for anchoring the CRS is called a `datum`. A CRS definition may have a specific datum included, so that the CRS is complete. Other CRS definitions may rely on a separate datum definition that has to be specified explicitly. For example, if long term precision and accuracy of coordinates are required, the datum may be defined as fixed to a specific continental tectonic plate.
 
 //#### 6.1.1 Geographic Coordinate Reference Systems
 ===== Geographic Coordinate Reference Systems
@@ -339,6 +339,7 @@ Geographic CRSs anchor coordinate values to an ellipsoidal approximation of the
 
  - The value of the `"type"` member MUST be "GeographicCRS"
  - The object MAY have an `"id"` member, whose value MUST be a string and SHOULD be a common identifier for the reference system.
+ - The object MAY have a `"datum"` member if the identified reference system definition does not specify a datum. 
  - The object MAY have a `"description"` member, where the value MUST be an i18n object, but no standardized content is interpreted from this description.
 
 Note that sometimes (e.g. for numerical model data) the exact CRS may not be known or may be undefined. In this case the `"id"` may be omitted, but the `"type"` still indicates that this is a geographic CRS. Therefore clients can still use geodetic longitude, geodetic latitude (and maybe height) axes, even if they cannot accurately georeference the information.
@@ -364,6 +365,16 @@ Example of a three-dimensional geographic CRS (latitude-longitude-height):
   "id": "http://www.opengis.net/def/crs/EPSG/0/4979"
 }
 ```
+Example of a two-dimensional geographic CRS (longitude-latitude) with an different datum:
+
+[%unnumbered%]
+```json
+{
+  "type": "GeographicCRS",
+  "id": "http://www.opengis.net/def/crs/OGC/1.3/CRS84",
+  "datum": "http://www.opengis.net/def/crs/OGC/1.3/CRS84"
+}
+```
 
 //#### 6.1.2 Projected Coordinate Reference Systems
 ===== Projected Coordinate Reference Systems
@@ -372,6 +383,7 @@ Projected CRSs use two coordinates to denote positions on a Cartesian plane, whi
 
  - The value of the `"type"` member MUST be "ProjectedCRS"
  - The object MAY have an `"id"` member, whose value MUST be a string and SHOULD be a common identifier for the reference system.
+ - The object MAY have a `"datum"` member if the identified reference system definition does not specify a datum. 
  - The object MAY have a `"description"` member, where the value MUST be an i18n object, but no standardized content is interpreted from this description.
 
 If a Coverage conforms to one of the defined <<common_domain_types,domain types>> then the coordinate identifier `"x"` is used to denote easting and `"y"` is used for northing.
@@ -385,6 +397,17 @@ Example of a projected CRS using the http://spatialreference.org/ref/epsg/osgb-1
   "id": "http://www.opengis.net/def/crs/EPSG/0/27700"
 }
 ```
+Example of a projected CRS using ta separate datum:
+
+[%unnumbered%]
+```json
+{
+  "type": "ProjectedCRS",
+  "id": "http://www.opengis.net/def/crs/EPSG/0/27700",
+  "datum": "http://www.opengis.net/def/crs/OGC/1.3/CRS84"
+}
+```
+
 
 //#### 6.1.3 Vertical Coordinate Reference Systems
 ===== Vertical Coordinate Reference Systems