Mars' Prime Meridian, such as: Longitude "0" (1 - Original NASA/MGS/MSSS CTX b/w Frame)

On Earth, the Longitude of the Royal Observatory in Greenwich, England is defined as the "Prime Meridian" or the 0 point of Longitude. Locations on Earth are measured in degrees East or West from this position.
The Prime Meridian was defined by an International Agreement in 1884 as the position of the Transit Circle: a large telescope located in the Observatory's Meridian Building. The Transit Circle was built by Sir George Biddell Airy, the 7th Astronomer Royal, in 1850. While visual observations with Transit were the basis of navigation until the space age, it is interesting to note that the current definition of the Prime Meridian is in reference to orbiting satellites and Very Long Baseline Interferometry (VLBI) measurements of distant radio sources such as quasars.

However, said "International Reference Meridian" is now about 100 mt East of the Transit Circle at Greenwich.

For Mars, the Prime Meridian was first defined by German astronomers W. Beer and J. H. Mädler in 1830-32.