Modern maps normally measure longitude from Greenwich. Historical sources, however, may use other reference meridians.
In Spanish cartographic and navigation contexts, Madrid can appear as a reference meridian. If that reference is ignored, old coordinates may seem displaced on modern maps.
The basic difference
Greenwich is the modern international reference meridian. Madrid lies west of Greenwich, so a longitude measured from Madrid must be shifted before comparison with modern Greenwich-based coordinates.
Madrid meridian ≈ 3° 41′ west of Greenwich
Decimal approximation: −3.687778°
Practical conversion
A longitude west of Madrid must be shifted farther west when converted to Greenwich.
Greenwich longitude = Madrid longitude − 3° 41′
This is a practical approximation for interpreting historical coordinates. Exact values may vary depending on the source and geodetic convention.
Why Madrid appears in historical sources
Before the universal adoption of Greenwich, different scientific and cartographic traditions used different reference meridians. National observatories and capitals often served as reference points.
Spanish maritime, administrative or cartographic records may therefore require checking whether their longitudes were referenced to Greenwich, Madrid, Ferro or another meridian.
Greenwich and the modern standard
Greenwich became the dominant international reference after the 1884 International Meridian Conference. Its adoption was helped by its widespread use in nautical charts and maritime navigation.
Modern GPS, digital maps and most geographic databases use Greenwich-based longitude, which is why historical coordinates often require conversion before comparison.
Common mistake
The most common error is assuming that every old longitude is already measured from Greenwich. In historical research, that assumption can shift a location by several degrees.
For shipwreck research, old charts and maritime archives, the reference meridian should always be checked before interpreting a coordinate as inaccurate.