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How to Use IAU Identifiers in PROJ

The Well-Known Text (WKT) format is a text markup language for representing coordinate reference systems (CRS). Here we will decode two examples of WKT: one for a Geographic Coordinate Reference System (GEOGCRS) and one for a Projected Coordinate Reference System (PROJCRS).

Example 1: GEOGCRS for Mars (2015) - Sphere

                GEOGCRS["Mars (2015) - Sphere ",
                    DATUM["Mars (2015) - Sphere",
                        ELLIPSOID["Mars (2015) - Sphere", 3396190, 0,
                            LENGTHUNIT["metre", 1, ID["EPSG", 9001]]],
                        ANCHOR["Viking 1 lander : 47.95137 W"]],
                    PRIMEM["Reference Meridian", 0,
                        ANGLEUNIT["degree", 0.0174532925199433, ID["EPSG", 9122]]],
                    CS[ellipsoidal, 2],
                        AXIS["geodetic latitude (Lat)", north,
                            ORDER[1],
                            ANGLEUNIT["degree", 0.0174532925199433]],
                        AXIS["geodetic longitude (Lon)", east,
                            ORDER[2],
                            ANGLEUNIT["degree", 0.0174532925199433]],
                    ID["IAU", 49900, 2015],
                    REMARK["Use semi-major radius as sphere radius for interoperability. Source of IAU Coordinate systems: doi:10.1007/s10569-017-9805-5"]]

Explanation:

  • GEOGCRS["Mars (2015) - Sphere "]: This specifies a geographic coordinate reference system named "Mars (2015) - Sphere".
  • DATUM["Mars (2015) - Sphere"]: The datum used is "Mars (2015) - Sphere".
    • ELLIPSOID["Mars (2015) - Sphere", 3396190, 0, LENGTHUNIT["metre", 1, ID["EPSG", 9001]]]: The ellipsoid for Mars has a semi-major axis of 3,396,190 meters and an eccentricity of 0 (indicating a sphere).
    • ANCHOR["Viking 1 lander : 47.95137 W"]: The anchor point is the Viking 1 lander located at 47.95137 degrees west.
  • PRIMEM["Reference Meridian", 0, ANGLEUNIT["degree", 0.0174532925199433, ID["EPSG", 9122]]]: The prime meridian is the reference meridian at 0 degrees.
  • CS[ellipsoidal, 2]: The coordinate system is ellipsoidal with 2 axes.
    • AXIS["geodetic latitude (Lat)", north, ORDER[1], ANGLEUNIT["degree", 0.0174532925199433]]: The first axis is geodetic latitude, pointing north, with an angle unit in degrees.
    • AXIS["geodetic longitude (Lon)", east, ORDER[2], ANGLEUNIT["degree", 0.0174532925199433]]: The second axis is geodetic longitude, pointing east, with an angle unit in degrees.
  • ID["IAU", 49900, 2015]: The identifier is IAU 49900, defined in 2015.
  • REMARK: Additional information about the coordinate system source.

Example 2: PROJCRS for Mars (2015) - Sphere / Sinusoidal, clon = 0

                PROJCRS["Mars (2015) - Sphere / Sinusoidal, clon = 0",
                    BASEGEOGCRS["Mars (2015) - Sphere",
                        DATUM["Mars (2015) - Sphere",
                        ELLIPSOID["Mars (2015) - Sphere", 3396190, 0,
                            LENGTHUNIT["metre", 1, ID["EPSG", 9001]]],
                        ANCHOR["Viking 1 lander : 47.95137 W"]],
                        PRIMEM["Reference Meridian", 0,
                            ANGLEUNIT["degree", 0.0174532925199433, ID["EPSG", 9122]]],
                        ID["IAU", 49900, 2015]],
                    CONVERSION["Sinusoidal, clon = 0",
                        METHOD["Sinusoidal",
                            ID["PROJ", "SINUSOIDAL"]],
                        PARAMETER["Longitude of natural origin", 0,
                            ANGLEUNIT["degree",0.0174532925199433,ID["EPSG", 9122]],
                            ID["EPSG", 8802]],
                        PARAMETER["False easting", 0,
                            LENGTHUNIT["metre",1,ID["EPSG", 9001]],
                            ID["EPSG", 8806]],
                        PARAMETER["False northing", 0,
                            LENGTHUNIT["metre",1,ID["EPSG", 9001]],
                            ID["EPSG", 8807]]],
                    CS[Cartesian, 2],
                        AXIS["Easting (E)", east,
                            ORDER[1],
                            LENGTHUNIT["metre", 1]],
                        AXIS["Northing (N)", north,
                            ORDER[2],
                            LENGTHUNIT["metre", 1]],
                    ID["IAU", 49920, 2015]]

Explanation:

  • PROJCRS["Mars (2015) - Sphere / Sinusoidal, clon = 0"]: This specifies a projected coordinate reference system named "Mars (2015) - Sphere / Sinusoidal, clon = 0".
  • BASEGEOGCRS["Mars (2015) - Sphere"]: The base geographic CRS is "Mars (2015) - Sphere".
    • DATUM["Mars (2015) - Sphere"]: The datum used is "Mars (2015) - Sphere".
      • ELLIPSOID["Mars (2015) - Sphere", 3396190, 0, LENGTHUNIT["metre", 1, ID["EPSG", 9001]]]: The ellipsoid for Mars has a semi-major axis of 3,396,190 meters and an eccentricity of 0 (indicating a sphere).
      • ANCHOR["Viking 1 lander : 47.95137 W"]: The anchor point is the Viking 1 lander located at 47.95137 degrees west.
    • PRIMEM["Reference Meridian", 0, ANGLEUNIT["degree", 0.0174532925199433, ID["EPSG", 9122]]]: The prime meridian is the reference meridian at 0 degrees.
    • ID["IAU", 49900, 2015]: The identifier is IAU 49900, defined in 2015.
  • CONVERSION["Sinusoidal, clon = 0"]: The conversion method is "Sinusoidal" with a central longitude (clon) of 0.
    • METHOD["Sinusoidal", ID["PROJ", "SINUSOIDAL"]]: The method used is "Sinusoidal".
    • PARAMETER["Longitude of natural origin", 0, ANGLEUNIT["degree", 0.0174532925199433, ID["EPSG", 9122]], ID["EPSG", 8802]]: The longitude of natural origin is 0 degrees.
    • PARAMETER["False easting", 0, LENGTHUNIT["metre", 1, ID["EPSG", 9001]], ID["EPSG", 8806]]: The false easting is 0 meters.
    • PARAMETER["False northing", 0, LENGTHUNIT["metre", 1, ID["EPSG", 9001]], ID["EPSG", 8807]]: The false northing is 0 meters.
  • CS[Cartesian, 2]: The coordinate system is Cartesian with 2 axes.
    • AXIS["Easting (E)", east, ORDER[1], LENGTHUNIT["metre", 1]]: The first axis is easting, pointing east, with a length unit in meters.
    • AXIS["Northing (N)", north, ORDER[2], LENGTHUNIT["metre", 1]]: The second axis is northing, pointing north, with a length unit in meters.
  • ID["IAU", 49920, 2015]: The identifier is IAU 49920, defined in 2015.

For more details on Well-Known Text (WKT) representation, refer to the OGC WKT CRS standard.