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The default ellipsoid is the Clarke 1866 ellipsoid. For a datum that uses another ellipsoid, either use the "ellps=abbr" option string for a recognized ellipsoid or define a custom ellipsoid. The following table lists the abbreviations for the recognized ellipsoids:
| Abbreviation | Ellipsoid Name | Length of Major Axis (a) | Reciprocal of Flattening (1/f) |
|---|---|---|---|
| airy | Airy 1830 | 6377563.396000m | 299.32497530 |
| andrae | Andrae 1876 | 6377104.430000m | 300.00000000 |
| aust_SA | Australian National & South American 1969 | 6378160.000000m | 298.25000000 |
| APL4.9 | Applied Physics Laboratory 1965 | 6378137.000000m | 298.25000000 |
| ATS77 | Average Terrestrial System 1977* | 6378135.000000m | 298.25700000 |
| bessel | Bessel 1841 | 6377397.155000m | 299.15281285 |
| bess_mod | Bessel 1841 (Modified) | 6377492.017600m | 299.15281000 |
| bess_nam | Bessel 1841 (Namibia) | 6377483.865000m | 299.15281285 |
| clrk58 | Clarke 1858 | 6378293.639000m | 294.26068000 |
| clrk66 | Clarke 1866 | 6378206.400000m | 294.97869820 |
| clrk66mi | Clarke 1866 (Michigan) | 6378693.704000m | 294.97868450 |
| clrk80 | Clarke 1880 Modified (Arc) | 6378249.145326m | 293.46630760 |
| clrk80ben | Clarke 1880 Modified (Benoit) | 6378300.790000m | 293.46623450 |
| clrk80IGN | Clarke 1880 Modified (IGN) | 6378249.200000m | 293.46602130 |
| clrk80o | Clarke 1880 | 6378249.136000m | 293.46631540 |
| clrk80RGS | Clarke 1880 Modified (RGS) | 6378249.145000m | 293.46500000 |
| clrk80SGA | Clarke 1880 Modified (SGA 1922) | 6378249.200000m | 293.46598000 |
| CPM | Commission des Poids et Mésures 1799 | 6375738.700000m | 334.29000000 |
| danish | Danish National Grid* | 6377103.965000m | 300.00000000 |
| delmbr | Delambre 1810 | 6376428.000000m | 311.50000000 |
| engelis | Engelis 1985 | 6378136.050000m | 298.25660000 |
| evrst30 | Everest 1830 | 6377276.345800m | 300.80170000 |
| evrst48 | Everest 1948 | 6377304.063000m | 300.80170000 |
| evrst56 | Everest 1956 | 6377301.243000m | 300.80174000 |
| evrst69 | Everest 1969 | 6377295.664000m | 300.80170000 |
| evrstP | Everest (Pakistan)* | 6377309.613000m | 300.80170000 |
| evrstSS | Everest (Sabah & Sarawak) | 6377298.556000m | 300.80170000 |
| fschr60 | Fischer (Mercury Datum) 1960 | 6378166.000000m | 298.30000000 |
| fschr60m | Modified Fischer 1960 | 6378155.000000m | 298.30000000 |
| fschr68 | Fischer 1968 | 6378150.000000m | 298.30000000 |
| fschr68mmd | Fischer 1968 (MMD)* | 6378160.000000m | 298.24716740 |
| GRS67 | Geodetic Reference System 1967 or International Astronomical Union 1968 | 6378160.000000m | 298.24716743 |
| GRS80 | Geodetic Reference System 1980 or New International | 6378137.000000m | 298.25722210 |
| helmert | Helmert 1906 | 6378200.000000m | 298.30000000 |
| hough | Hough 1960 | 6378270.000000m | 297.00000000 |
| IAU76 | International Astronomical Union 1976 | 6378140.000000m | 298.25722200 |
| indon | Indonesian National Spheroid | 6378160.000000m | 298.24700000 |
| intl | International or Hayford | 6378388.000000m | 297.00000000 |
| kaula | Kaula 1961 | 6378163.000000m | 298.24000000 |
| krass | Krassovsky 1942 (1940) | 6378245.000000m | 298.30000000 |
| lerch | Lerch et al. 1979 | 6378139.000000m | 298.25700000 |
| MERIT | MERIT 1983 | 6378137.000000m | 298.25700000 |
| mod_airy | Modified Airy 1849 | 6377340.189000m | 299.32493740 |
| mprts | Maupertius 1738 | 6397300.000000m | 191.00000000 |
| new_intl | New International 1967 | 6378157.500000m | 298.24961540 |
| NWL9D | Naval Weapons Laboratory 1965 | 6378145.000000m | 298.25000000 |
| OSU86F | OSU86 Gravity Potential Model | 6378136.200000m | 298.25722000 |
| OSU91A | OSU91 Gravity Potential Model | 6378136.300000m | 298.25722000 |
| plessis | Plessis 1817 | 6376523.000000m | 308.64099710 |
| seasia | Southeast Asia | 6378155.000000m | 298.30000720 |
| SGS85 | Soviet Geodetic System 1985 | 6378136.000000m | 298.25700000 |
| shdanov | Shdanov 1893 | 6377717.000000m | 299.70000000 |
| struve | Struve 1860 | 6378297.000000m | 294.73000000 |
| walbeck | Walbeck 1817 | 6376896.000000m | 302.78000450 |
| war_off | War Office | 6378300.583000m | 296.00000000 |
| WGS60 | World Geodetic System 1960 | 6378165.000000m | 298.30000000 |
| WGS66 | World Geodetic System 1966 | 6378145.000000m | 298.25000000 |
| WGS72 | World Geodetic System 1972 | 6378135.000000m | 298.26000000 |
| WGS84 | World Geodetic System 1984 | 6378137.000000m | 298.25722356 |
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