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

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 1, 2), 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
``````

Ocentric and ographic latitudes do not refer to a positive East vs. positive West distinction.

Ocentric and ographic instead refer to where one measures latitude from. Direct from the proj docs:

The geodetic (or geographic) latitude (also called planetographic latitude in the context of non-Earth bodies) is the angle between the equatorial plane and the normal (vertical) to the ellipsoid surface at the considered point. The geodetic latitude is what is normally used everywhere in PROJ when angular coordinates are expected or produced.

The geocentric latitude (also called planetocentric latitude in the context of non-Earth bodies) is the angle between the equatorial plane and a line joining the body centre to the considered point.

https://proj.org/operations/conversions/geoc.html

The only time that these values will differ is when the body is non-spherical. In the planetary case, the Moon is defined as a sphere and ographic lat = ocentric lat. For non-spherical definitions of Mars ocentric lat != ographic lat.

• Oh, so ographic is simply synonymous with geographic/planetographic and odetic with geodetic/planetodetic? Dec 10, 2022 at 16:03
• Correct. Geocentric is planetocentric and ographic is planetographic. Similar to geoid and aeroid (Martian geoid), the naming convention is to attempt to remove (for better or worse) the 'geo' root from the terms. We see the same thing when talking about coordinate that are in the 'Earth Centered Earth Fixed' (ECEF) reference frame vs. the 'Body Centered Body Fixed' (BCBF) reference frame. Hopefully the other examples do not muddy the clarity of the answer! Dec 10, 2022 at 16:21

I just read a page from USGS (https://planetarynames.wr.usgs.gov/Page/MARS/system) which distinguishes "planetographic" from "planetocentric". They drop the "planet-” prefix for brevity and explain that 'ographic means longitude increases westward (as we do in earth for western hemisphere) whereas 'ocentric means longitude increases eastwards, as in terrestrial eastern hemisphere--and as in standard Cartesian coordinate systems. My guess is that Americans started assigning 'ographic coordinates on Mars missions in the 20th century, and now we all are transitioning to 'ocentric coordinate reference systems.