<Li> Quasi-Zenith Satellite System (QZSS) (Japan) </Li> <Li> Legenda (satellite system) </Li> <Tr> <Th> Standards (History) </Th> </Tr> <Tr> <Td> <Table> <Tr> <Td> NGVD 29 </Td> <Td> Sea Level Datum 1929 </Td> </Tr> <Tr> <Td> OSGB36 </Td> <Td> Ordnance Survey Great Britain 1936 </Td> </Tr> <Tr> <Td> SK - 42 </Td> <Td> Systema Koordinat 1942 goda </Td> </Tr> <Tr> <Td> ED50 </Td> <Td> European Datum 1950 </Td> </Tr> <Tr> <Td> SAD69 </Td> <Td> South American Datum 1969 </Td> </Tr> <Tr> <Td> GRS 80 </Td> <Td> Geodetic Reference System 1980 </Td> </Tr> <Tr> <Td> NAD 83 </Td> <Td> North American Datum 1983 </Td> </Tr> <Tr> <Td> WGS 84 </Td> <Td> World Geodetic System 1984 </Td> </Tr> <Tr> <Td> NAVD 88 </Td> <Td> N. American Vertical Datum 1988 </Td> </Tr> <Tr> <Td> ETRS89 </Td> <Td> European Terrestrial Reference System 1989 </Td> </Tr> <Tr> <Td> GCJ - 02 </Td> <Td> Chinese encrypted datum 2002 </Td> </Tr> </Table> <Ul> <Li> International Terrestrial Reference System </Li> <Li> Spatial Reference System Identifier (SRID) </Li> <Li> Universal Transverse Mercator (UTM) </Li> </Ul> </Td> </Tr>

Explain with a suitable diagram how longitudes are marked on the surface of the earth