IAU Planetary Projections

Well-Known Text coordinate reference systems for planetary bodies based on IAU standards

Overview

The IAU2000 WKT system provides a comprehensive set of coordinate reference systems (CRS) for planetary bodies, similar to EPSG codes for Earth. These projections implement IAU (International Astronomical Union) standards for planetary cartography and can be used directly with GDAL tools.

These projections enable standardized spatial reference systems across planetary science applications, from Mars rovers to outer planet moons.

Available Versions

IAU2000

Based on the 2002 IAU/IAG report (Seidelmann et al.)

IAU2009

Based on the 2011 IAU report (Archinal et al.)

IAU2015

Based on the 2018 IAU report (Archinal et al.) - Most current

Each version reflects updated planetary body parameters (radii, rotation elements) from the respective IAU Working Group reports.

Code Structure

Projection codes follow a systematic pattern:

[NAIF_ID * 100] + [PROJECTION_TYPE]

Examples

Code	Body	Projection
49900	Mars	Geographic (ocentric)
49901	Mars	Geographic (ographic)
49910	Mars	Equirectangular, ocentric, clon=0
49918	Mars	North Polar Stereographic
30100	Moon	Geographic (ocentric)
30110	Moon	Equirectangular, ocentric, clon=0

Projection Code Suffixes

Suffix	Projection Type	Notes
00, 01	Geographic	00=ocentric, 01=ographic
10, 11	Equirectangular, clon=0	Even=ocentric, odd=ographic
12, 13	Equirectangular, clon=180	Even=ocentric, odd=ographic
14, 15	Sinusoidal, clon=0
16, 17	Sinusoidal, clon=180
18, 19	North Pole Stereographic
20, 21	South Pole Stereographic
22-29	Mollweide, Robinson	Various central meridians
60-83	AUTO projections	Dynamic central meridian

Ocentric vs. Ographic:

• Ocentric: Planetocentric latitudes (angle from center)
• Ographic: Planetographic latitudes (angle from surface normal)

Using Projections from GitHub

GDAL can read projections directly from the raw GitHub repository using curl:

Reproject with gdalwarp

gdalwarp -t_srs \
  "http://raw.githubusercontent.com/USGS-Astrogeology/GDAL_scripts/master/OGC_IAU2000_WKT_v2/IAU2000_prjs/Moon%202000.prj" \
  input_moonMap.img output_moonMap_degrees.tif

Inspect Projection

gdalsrsinfo http://raw.githubusercontent.com/USGS-Astrogeology/GDAL_scripts/master/OGC_IAU2000_WKT_v2/IAU2015_prjs/Mars%202015.prj

URL Encoding: Spaces in filenames must be encoded as %20 in URLs.

Use HTTPS

Both http and https protocols work:

gdalwarp -t_srs \
  "https://raw.githubusercontent.com/USGS-Astrogeology/GDAL_scripts/master/OGC_IAU2000_WKT_v2/IAU2015_prjs/Mars%202015.prj" \
  input.tif output.tif

Generating Custom WKT Files

The create_IAU2000_wkt_v3.py script generates WKT projection definitions from CSV radius tables.

Usage

python create_IAU2000_wkt_v3.py naifcodes_radii_m_wAsteroids_IAU2015.csv output.wkt

Parameters

input.csv · string · required

CSV file with NAIF IDs and planetary radii. Available versions:

• naifcodes_radii_m_wAsteroids_IAU2000.csv
• naifcodes_radii_m_wAsteroids_IAU2009.csv
• naifcodes_radii_m_wAsteroids_IAU2015.csv

output.wkt · string

Output WKT file. If omitted, writes to stdout.

Input CSV Format

Naif_id,Body,IAU2015_Mean,IAU2015_Semimajor,IAU2015_Axisb,IAU2015_Semiminor
199,Mercury,2439700.00,2439700.00,2439700.00,2439700.00
299,Venus,6051800.00,6051800.00,6051800.00,6051800.00
499,Mars,3389500.00,3396190.00,3396190.00,3376200.00
301,Moon,1737400.00,1737400.00,1737400.00,1737400.00
401,Phobos,11100.00,13400.00,11200.00,9200.00

Example Output WKT

49900,GEOGCS["Mars 2015",DATUM["D_Mars_2015",SPHEROID["Mars_2015_IAU",3396190.0,169.8944472236118]],PRIMEM["Reference_Meridian",0],UNIT["Degree",0.0174532925199433],AUTHORITY["IAU2015","49900"]]

Supported Planetary Bodies

The system includes projections for:

Terrestrial Planets

• Mercury (199)
• Venus (299)
• Earth (399)
• Mars (499)

Moons

• Moon/Luna (301)
• Phobos (401), Deimos (402)
• Jovian moons: Io, Europa, Ganymede, Callisto, Amalthea, Thebe, Adrastea, Metis
• Saturnian moons: Titan, Rhea, Iapetus, Dione, Tethys, Enceladus, Mimas, Hyperion, Phoebe, Janus, Epimetheus, and more
• Uranian moons: Titania, Oberon, Umbriel, Ariel, Miranda
• Neptunian moons: Triton, Proteus, Nereid
• Plutonian moons: Charon

Asteroids & Comets

• Ceres, Vesta, Pallas, Lutetia, Ida, Gaspra, Eros, Mathilde, Steins
• Comets: Halley, Borrelly, Tempel 1, Hartley 2, Wild 2

Gas Giants

• Jupiter (599)
• Saturn (699)
• Uranus (799)
• Neptune (899)
• Pluto (999)

Coordinate Reference System Details

Datum Definition

Each planetary body uses a triaxial ellipsoid (or sphere) defined by:

• Semimajor axis (a): Equatorial radius
• Semiminor axis (c): Polar radius
• Flattening: f = (a - c) / a or 1/f for SPHEROID definition

Triaxial Bodies

For bodies where a ≠ b ≠ c, the mean radius is used:

if theA != theB and theA != theC:
    theA = theMean  # Use mean radius
    theC = theMean

Projection Types

Static Projections (10-29)

Fixed parameters, specific central meridians:

• Equirectangular: Simple cylindrical, lat/lon = x/y
• Sinusoidal: Equal-area, good for global mosaics
• Stereographic: Polar projections (North/South)
• Mollweide: Equal-area elliptical
• Robinson: Compromise projection

AUTO Projections (60-83)

Dynamic parameters that can be adjusted:

• Sinusoidal AUTO (60, 61)
• Stereographic AUTO (62, 63)
• Transverse Mercator AUTO (64, 65)
• Orthographic AUTO (66, 67)
• Equidistant Cylindrical AUTO (68, 69)
• Lambert Conformal Conic AUTO (70, 71)
• Lambert Azimuthal Equal Area AUTO (72, 73)
• Mercator AUTO (74, 75)
• Albers AUTO (76, 77)
• Oblique Cylindrical Equal Area AUTO (78, 79)
• Mollweide AUTO (80, 81)
• Robinson AUTO (82, 83)

AUTO projections allow custom central meridians and standard parallels while maintaining body-specific datum parameters.

Integration with GDAL

Assign Projection to Unprojected Data

gdal_edit.py -a_srs "IAU2015:49910" mars_image.tif

Reproject Between Systems

gdalwarp -s_srs "IAU2015:49910" -t_srs "IAU2015:49918" \
  mars_equi.tif mars_polar.tif

Extract Projection Info

gdalsrsinfo -o wkt "IAU2015:49900"

Script Location

~/workspace/source/OGC_IAU2000_WKT_v2/Source_Python/create_IAU2000_wkt_v3.py

Pre-generated Files

The repository includes pre-generated WKT files:

~/workspace/source/OGC_IAU2000_WKT_v2/IAU2000_v3.wkt
~/workspace/source/OGC_IAU2000_WKT_v2/IAU2009_v3.wkt  
~/workspace/source/OGC_IAU2000_WKT_v2/IAU2015_v3.wkt

And individual .prj files for each body:

~/workspace/source/OGC_IAU2000_WKT_v2/IAU2015_prjs/Mars 2015.prj
~/workspace/source/OGC_IAU2000_WKT_v2/IAU2015_prjs/Moon 2015.prj

PostGIS Integration

The repository includes experimental PostGIS scripts:

~/workspace/source/OGC_IAU2000_WKT_v2/Source_Python/postGIS_untested/

These generate SQL INSERT statements for loading IAU projections into PostGIS spatial_ref_sys table.

Untested: PostGIS scripts are experimental and may require modification for your database version.

IAU Standards References

IAU2000 (Seidelmann et al., 2002)

Seidelmann, P.K., et al. (2002). “Report of the IAU/IAG Working Group on Cartographic Coordinates and Rotational Elements of the Planets and Satellites: 2000,” Celestial Mechanics and Dynamical Astronomy, v.82, Issue 1, pp. 83-111.

IAU2009 (Archinal et al., 2011)

Archinal, B.A., et al. (2011). “Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements of the Planets and Satellites: 2009,” Celestial Mechanics and Dynamical Astronomy, v.109, Issue 2, pp. 101-135.

IAU2015 (Archinal et al., 2018)

Archinal, B.A., et al. (2018). “Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements of the Planets and Satellites: 2015,” Celestial Mechanics and Dynamical Astronomy, 130: 22. https://doi.org/10.1007/s10569-017-9805-5

Best Practices

1. Choose IAU Version

   Use IAU2015 for most current radii and parameters. Use older versions only for compatibility with existing datasets.

2. Select Projection

   Choose based on application:

   • Global mosaics: Sinusoidal or Mollweide (equal-area)
   • Polar regions: Stereographic (18-21)
   • Regional mapping: Equirectangular (10-13) or AUTO projections
   • Web maps: Equirectangular at equator
3. Ocentric vs. Ographic

   • Ocentric: Standard for most planetary work, consistent with SPICE kernels
   • Ographic: Traditional for mapping, matches surface tangent
4. Document Metadata

   Always record which IAU version and projection code you use in dataset metadata.

Contact

Trent Hare
Astrogeology, U.S. Geological Survey
[e-mail]

Additional Resources

• USGS Astrogeology GDAL Scripts Repository
• IAU Working Group on Cartographic Coordinates
• NAIF SPICE System
• GDAL Spatial Reference System Tutorial