Before the Greenwich meridian became the modern reference for longitude, many European maps used other prime meridians. One of the most influential was the Ferro meridian, associated with El Hierro, the westernmost of the Canary Islands.
For historical navigation, old maps and shipwreck research, this matters because coordinates measured from Ferro cannot be read directly as Greenwich coordinates.
Why El Hierro became a reference point
In classical and early modern geography, mapmakers often wanted a prime meridian placed at the western edge of the known world. This had one practical advantage: longitudes could be written eastward as positive values, avoiding negative longitudes.
El Hierro, known historically as Ferro, was treated by European geographers as a symbolic western limit. This made it attractive as a zero meridian for maps covering Europe, Africa and the Atlantic world.
The 1634 French decision
In 1634, under Louis XIII and Cardinal Richelieu, France promoted the use of the Ferro meridian as a cartographic reference. In practice, the meridian was often treated as exactly 20 degrees west of Paris rather than measured from a precise physical point on El Hierro.
This distinction is important. The “Ferro meridian” used on maps was not always the exact longitude of a monument or observatory on the island. It became a conventional mathematical reference.
Ferro and Greenwich
In many practical conversions, the Ferro meridian is treated as approximately 17° 40′ west of Greenwich. Some historical and geodetic sources give slightly different values, such as 17° 39′ 46″ west of Greenwich, depending on the convention used.
For a historical coordinate converter, this means Ferro should be handled as an explicit reference system. A longitude “from Ferro” is not the same as a longitude “from Greenwich.”
Approximate practical conversion:
Greenwich longitude = Ferro longitude − 17° 40′
Why this matters for old maps and shipwrecks
A common error in historical research is to copy an old longitude and plot it directly on a modern map. If the original source used Ferro, the position may be displaced by more than seventeen degrees of longitude.
At sea, that error can represent hundreds or even more than a thousand kilometers, depending on latitude. For shipwreck research, treasure hunting, naval history or maritime archaeology, identifying the original meridian is essential.
Ferro after Greenwich
The International Meridian Conference of 1884 selected Greenwich as the international prime meridian. Even after that, older maps, atlases and regional geodetic systems continued to preserve older references for some time.
This is why Ferro remains relevant today: not as the modern zero meridian, but as a key to understanding historical maps and coordinate systems.