Modern maps normally measure longitude from Greenwich, but historical navigation did not always use that reference. Spanish cartography and navigation records may refer to Madrid or to other older references such as Ferro.
For shipwreck research, old charts and maritime archives, understanding the original reference meridian is essential before plotting any historical coordinate on a modern map.
Why Madrid matters
Spain was one of the leading maritime powers of the early modern period. Spanish pilots, cosmographers and cartographers produced a large body of navigational knowledge connected with Atlantic routes, American waters and later Pacific navigation.
Because of this, many historical maritime documents connected with Spanish administration, exploration or navigation may require careful interpretation before their coordinates are compared with modern Greenwich-based maps.
The Madrid meridian
In practical historical-coordinate work, the Madrid meridian is commonly treated as being approximately 3° 41′ west of Greenwich. In decimal form, this is about −3.687°.
This means that a longitude measured from Madrid must be shifted before being compared with a modern longitude measured from Greenwich.
Approximate practical conversion:
Greenwich longitude = Madrid longitude − 3° 41′
Madrid, Ferro and Spanish sources
Historical Spanish material may not always use Madrid. Earlier cartographic traditions also used Ferro, associated with El Hierro in the Canary Islands, as a western prime meridian.
This makes source criticism important. A chart, logbook or later summary may preserve a longitude value while losing the original reference system. When that happens, the position can be misunderstood.
Why this matters for shipwreck research
Many shipwreck positions have passed through several hands: original observations, copied charts, archive summaries, printed books and modern databases. Each step may preserve the number but remove the context.
Before rejecting an old position as inaccurate, it is worth asking three questions:
- Which meridian did the source use?
- Was the coordinate copied from an older chart?
- Was the position later converted to Greenwich, or left unchanged?
Spanish cartography and later European navigation
Early English, Dutch and French navigation in the Americas developed in a world where Spanish exploration and cartography had already accumulated extensive geographic knowledge.
This does not mean every later chart was Spanish in origin, but it does mean that Spanish geographic information formed part of the wider European navigational inheritance. When studying early modern maritime routes, that context matters.