Addition of a geodetic CRS for the spherical case.

It reuses the GeoTIFF existing definition of datum.

GeoTIFF_Standard/standard/annex-b.adoc

@@ -325,9 +325,11 @@ but if needed the size of a model sphere may be defined by specifying identical
 ===== Prime Meridian
 The coordinate axes of the system referencing points on an ellipsoid are called latitude and longitude.
 More precisely, geodetic latitude and longitude are required in this GeoTIFF standard.
+Spherical latitude and longitude may also be used for geocentric CRS.
 A discussion of the several other types of latitude and longitude is beyond the scope of this document as they are not required for conventional georeferencing.
 
 Geodetic latitude is defined to be the angle subtended with the ellipsoid's equatorial plane by a perpendicular through the surface of the ellipsoid from a point.
+Spherical latitude is defined to be the angle subtended with the same plane but by a line from the point to the ellipsoid's center.
 Latitude is positive if north of the equator, negative if south.
 
 Geodetic longitude is defined to be the angle measured about the minor (polar) axis of the ellipsoid from a prime meridian to the meridian through a point,
@@ -398,7 +400,9 @@ The following subtypes of Model coordinate reference system (CRS) are recognized
 
 * Geographic 2D;
 
-* Geocentric;
+* Geocentric 2D (spherical);
+
+* Geocentric 3D (Cartesian);
 
 * Projected ('map grid (2D)'); and
 
@@ -508,8 +512,15 @@ For a user-defined geographic 2D CRS the user is expected to provide:
 **  user-defined length unit name (through the _GeodeticCitationGeoKey_) and
     scaling from SI base unit of meter (through the _GeogLinearUnitSizeGeoKey_).
 
-===== User-defined geocentric CRS
-For a user-defined geocentric CRS the user is expected to provide:
+===== User-defined geocentric 2D (spherical) CRS
+For a user-defined geocentric CRS associated to a two-dimensional spherical coordinate system,
+the user is expected to provide the same GeoKeys than above geographic case.
+The geocentric case is distinguished from the geographic case by the
+_ModelType_ GeoKey value, which is 5 instead of 3.
+
+===== User-defined geocentric 3D (Cartesian) CRS
+For a user-defined geocentric CRS associated to a three-dimensional Cartesian coordinate system,
+the user is expected to provide:
 
 *   geocentric coordinate reference system name (through the _GeodeticCitationGeoKey_);
 

GeoTIFF_Standard/standard/clause_7_requirements.adoc

@@ -103,7 +103,9 @@ This GeoKey defines the type of Model coordinate reference system used, to which
 
 * Model CRS is a Geographic 2D CRS;
 
-* Model CRS is a Geocentric CRS;
+* Model CRS is a Geocentric 2D (spherical) CRS;
+
+* Model CRS is a Geocentric 3D (Cartesian) CRS;
 
 * Model CRS is a Projected CRS; and
 
@@ -223,6 +225,7 @@ or to indicate that the unit is user-defined.
 The *GeogAngularUnitsGeoKey* key is used to specify the angular unit for:
 
 * the axes in user-defined geographic 2D CRSs;
+* the axes in user-defined geocentric 2D (spherical) CRSs;
 * the horizontal axes in user-defined geographic 3D CRSs;
 * the longitude from the reference meridian in user-defined prime meridians; and
 * user-defined map projection parameters that are angles.

GeoTIFF_Standard/standard/requirements/Configuration_Keys/requirements_class_GTModelTypeGeoKey.adoc

@@ -20,7 +20,7 @@ _The GTModelTypeGeoKey SHALL have type = SHORT_
 {set:cellbgcolor:#FFFFFF}
 
 |Requirement 8.4 {set:cellbgcolor:#CACCCE}
-|http://www.opengis.net/spec/GeoTIFF/1.1/req/GTModelTypeGeoKey.value +
+|http://www.opengis.net/spec/GeoTIFF/1.2/req/GTModelTypeGeoKey.value +
 _The GTModelTypeGeoKey value SHALL be:_
 
 * 0 to indicate that the Model CRS in undefined or unknown; or
@@ -31,6 +31,8 @@ _The GTModelTypeGeoKey value SHALL be:_
 
 * 3 to indicate that the Model CRS is a geocentric Cartesian 3D coordinate reference system, indicated by the value of the GeodeticCRSGeoKey; or
 
+* 5 to indicate that the Model CRS is a geocentric spherical 2D coordinate reference system, indicated by the value of the GeodeticCRSGeoKey; or
+
 * 32767 to indicate that the Model CRS type is user-defined.
 {set:cellbgcolor:#FFFFFF}