Planetary bodies

The Moon and Mars break almost every assumption the rest of this reference makes. This page states exactly how, because getting it wrong produces data that looks fine and is silently, systematically displaced.

The core problem

RFC 7946 §4 says GeoJSON coordinates are WGS 84 — a datum defined by an ellipsoid fitted to Earth — and removed the crs member that would let a file say otherwise.

Mars is not Earth. Its coordinates are not WGS 84 and cannot be. But a conforming GeoJSON file has no way to say so.

Planetary files here are RFC 7946-shaped, not RFC 7946-conformant

This is a deliberate, documented deviation from the standard, not an oversight.

A Mars file in this repository is syntactically valid GeoJSON that any parser will read. Its coordinates are degrees. But they are degrees on Mars, and the file itself cannot tell you that. A consumer that assumes WGS 84 — which the specification entitles it to do — will place Olympus Mons somewhere in the Pacific.

The authoritative CRS declaration therefore lives in manifest.json, and reading it is mandatory before using planetary data. There is no in-band alternative.

Coordinate reference systems

Planetary coordinates use body-fixed frames from the IAU/IAG Working Group on Cartographic Coordinates and Rotational Elements, whose 2015 report is the current reference. These are registered in PROJ under the IAU_2015 authority.

Body	Frame	Reference radius
Moon	IAU_2015:30100	1 737 400 m (sphere)
Mars	IAU_2015:49900	3 396 190 m equatorial

In a manifest:

"crs": {
  "authority": "IAU_2015",
  "code": "49900",
  "body": "mars",
  "proj": "+proj=longlat +R=3396190 +no_defs",
  "note": "Planetocentric latitude, east-positive longitude, -180..180"
}

The proj string is included so consumers can hand it directly to GDAL, PROJ, pyproj or rasterio without looking anything up.

The two errors everyone makes

Planetocentric versus planetographic latitude

Two different definitions of "latitude" are in active use for Mars:

• Planetocentric — the angle at the body's centre. What this project uses.
• Planetographic — the angle of the surface normal to the equatorial plane.

On a perfect sphere they are identical. Mars is oblate enough that they diverge by up to about 0.3°, which is roughly 18 km at the equator. That is large enough to put a feature in the wrong crater and small enough that nothing looks obviously broken.

This project stores planetocentric latitude. Datasets that arrive planetographic must be converted before merging, and the manifest records which convention applies.

East-positive versus west-positive longitude

Historical Mars maps used west-positive longitude. Modern practice, and the IAU recommendation for Mars, is east-positive. Lunar data is east-positive too.

Mixing the two mirrors your map about the prime meridian. Because many planetary features are roughly symmetric in distribution, this often does not look wrong at a glance.

This project stores east-positive longitude in the −180…180 domain, for consistency with the Earth data and because most web mapping libraries assume it. Sources using the 0…360 domain must be converted.

There is no ISO 3166 for other worlds

No registry of "countries" exists for the Moon or Mars, and under the Outer Space Treaty none is likely to. So the naming convention substitutes:

• Body code in place of a country code — MOON, MARS. These are chosen to be four characters so they can never collide with a three-character ISO 3166-1 alpha-3, present or future.
• QUAD in place of an admin level, for the USGS quadrangle schemes that are the closest thing to a systematic subdivision of a planetary surface.
• Feature names from the IAU Gazetteer of Planetary Nomenclature, which is the authoritative registry of approved names for surface features. Do not invent names, and do not use informal mission nicknames.

data/moon/MOON/MOON_QUAD.geojson
data/mars/MARS/MARS_QUAD.geojson

shapeGroup carries the body code; shapeType is QUAD.

Basemaps

There are no Web Mercator tile services for planetary bodies. USGS Astrogeology publishes equirectangular WMS endpoints, so preview maps for these bodies use EPSG:4326-style plate carrée rather than the Web Mercator that the Earth maps use.

This is why geojson-map.js reads a data-body attribute: it selects both the basemap and the map projection from it.

Status

Nothing planetary has been added yet. This page is written first, on purpose. The conventions above are the ones that are expensive to change once files exist — particularly the latitude convention, which cannot be detected from the data itself after the fact.