[PROJ] Geodetic vs Geocentric

[Martin Desruisseaux]

Le 2023-10-18 à 20 h 46, Javier Jimenez Shaw via PROJ a écrit :

> Do you mean that a geocentric system is using the geodetic latitude 
> and the geodetic system is using a geocentric latitude? Interesting
>
No. There is no explicit GeocentricCRS type in ISO 19111. The addition 
of a GeocentricCRS type has been debated during OGC meetings and 
excluded for reasons that I do not fully understood, but my remembrance 
is that it was indeed related to semantic issues. There is an explicit 
GeocentricCRS type in GeoAPI, but it is inherited from OGC 01-009 (a 
specification that predates ISO 19111), and PROJ 5 took inspiration from 
that as well.

In ISO 19111, GeodeticCRS is a base type which can have either geodetic 
or geocentric latitude, depending on their coordinate system:

 1. GeodeticCRS associated to SphericaCS use geocentric latitudes. ISO
    19111 rather call that "spherical latitudes".
 2. GeodeticCRS associated to EllipsoidalCS use geodetic latitudes.

GeographicCRS is a specialization of GeodeticCRS restricted to case 2 
above. As said above, the introduction of a GeocentricCRS as a 
specialization of GeodeticCRS for case 1 has been debated but rejected 
at OGC, so for now we have to use the generic GeodeticCRS base class. 
However it does not mean that ISO 19111 will not revisit this issue in 
some future version.

     Martin

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