What is an "ographic" latitude (such as in IAU projection 39916)?

Q/A
#1

(This is a duplicate/cross-port of my question I asked at GIS Stack Exchange, where it was suggested I ask this question here again — I apologise if this is not the right community)

In pyproj, I defined/loaded projection IAU 39916. This has the name “Earth (2015) / Ographic / Equirectangular, clon = 180”. (I was looking for an equirectangular projection for representing an area on both hemispheres around the antimeridian.)

What does “ographic” mean here? In this context, it cannot mean orthographic, because the projection is equirectangular and not orthographic. Google Search gives me results on converting between “ographic” and “ocentric” (example from this site), but I have not found what those actually are. Is it related to geocentric and geodetic latitudes in any way?

In [292]: from pyproj import CRS

In [293]: crs = CRS.from_authority("IAU_2015", 39916)

In [294]: print(crs.name)
Earth (2015) / Ographic / Equirectangular, clon = 180
#2

The IAU 2015 codes included in PROJ are targetted for extraterrestrial use (non-Earth bodies). Earth is included as it is part of the IAU WGCCRE report they are generated from.

Here is a good figure showing the difference and the meaning of text “Ographic” as you are seeing in the code’s title: Geocentric Latitude — PROJ 9.0.0 documentation (essentially it is tagging the latitude system used).

The implication for defining “Ocentric” or “Ographic” impacts latitude values (on elliptical-defined bodies). Note on a body defined as a sphere, ocentric and ographic latitude systems are the same. Fortunately, most planetary bodies are defined as a sphere (e.g., the Moon) or defined as a triaxial for which the IAU recommends using a spherical definition (see IAU report above).

So again, bodies defined as an ellipse (e.g., Earth or Mars), the use of ocentric and ographic will need to be carefully applied. Most Earth-centric mapping applications will actually assume the use of ographic latitudes. So much so, using ocentric within OGC standards can still be sometimes problematic (an issue we are working with the OGC to correct).

Let’s start there, but there is a lot I can try to unravel if you need more information.