Nautical Unit Converter
Convert distance, speed, depth and angular units between International Nautical, UK Admiralty and standard systems.
Use converterNautical Tools & Historical Navigation
NautiScale
Nautical unit converter, maritime reference and historical coordinate tools for navigation, old charts and shipwreck research.
Convert between International Nautical, UK Admiralty and standard measurement systems, and learn why historical coordinates may not match modern maps when they were recorded using Madrid, Paris, Ferro or other prime meridians.
Historical Meridian Converter
Convert longitude values between Greenwich, Madrid, Paris and Ferro to interpret old navigation records more accurately.
Explore meridiansMaritime Reference Guides
Learn how nautical miles, knots, fathoms and cables developed from practical navigation at sea.
Read guidesWhy nautical measurement is different
Nautical units are closely connected to the shape of the Earth. A nautical mile was historically linked to one minute of latitude, making it useful for navigation across charts and open water.
Modern navigation uses the international nautical mile of exactly 1,852 meters, but older British Admiralty charts used a slightly longer nautical mile of 6,080 feet. That difference is small over one mile, but it can become significant over long distances or when interpreting historical data.
Historical meridians and old coordinates
Today, longitude is normally measured from Greenwich. But historical maps and navigation records did not always use Greenwich as their zero meridian.
Spanish, French and other cartographic traditions used different reference meridians, including Madrid, Paris and Ferro. If these coordinates are read as Greenwich coordinates without conversion, the resulting position may be wrong by hundreds of kilometers.
Historical Coordinate Tools
Prime Meridians Before Greenwich
Greenwich became the international reference in the late 19th century, but many older charts and navigation records used different prime meridians. NautiScale helps identify and convert those references.
Madrid Meridian
Used in Spanish cartography and navigation records. Essential when interpreting some historical Spanish maritime sources.
Read about MadridFerro / El Hierro
One of the most influential historical reference meridians in early European cartography.
Explore FerroParis Meridian
A major French reference meridian used before Greenwich became the dominant international standard.
Learn about ParisGreenwich
The modern global reference for longitude, adopted internationally after the 1884 Prime Meridian Conference.
Compare meridiansWhy it matters
A longitude recorded as 10° west from Madrid, Paris or Ferro does not point to the same place as 10° west from Greenwich. Without conversion, historical shipwreck positions can appear displaced by hundreds of kilometers.
Open Meridian ConverterRecommended reading
What is a Nautical Mile?
Learn why the nautical mile is tied to latitude and navigation.
What is a Knot?
Understand maritime speed and its link to nautical miles.
What is a Fathom?
Discover how sailors measured depth before modern instruments.
Why Historical Coordinates Are Often Wrong
See how old prime meridians can shift recorded positions.
Ferro / El Hierro Meridian
Explore one of the most important historical reference meridians.
Madrid Meridian and Spanish Navigation
Learn how Spanish cartography used its own reference systems.
Frequently Asked Questions
What is NautiScale?
NautiScale is a nautical conversion and maritime reference site focused on navigation units, historical measurement systems and old coordinate interpretation.
Why are there different nautical miles?
The international nautical mile is exactly 1,852 meters, while the older UK Admiralty nautical mile was 6,080 feet, or 1,853.184 meters.
Why do historical coordinates differ from modern maps?
Many historical coordinates were measured from meridians other than Greenwich, such as Madrid, Paris or Ferro. They must be converted before comparing them with modern maps.
Is this useful for shipwreck research?
Yes. Historical shipwreck locations may appear incorrect if the original chart used a different prime meridian or an older nautical measurement standard.