The problem of a round Earth
A navigator who sails by compass holds a steady bearing. On a globe, a course that crosses every meridian at the same angle is called a rhumb line, or loxodrome, and it spirals towards the poles. On a flat chart, the question is how to draw the globe so that such a line appears straight, because a straight line can be laid out with a ruler and measured with a pair of dividers.
Earlier charts did not solve this. The plain charts used by many sailors in the 16th century treated degrees of latitude and longitude as equal squares. That works near the equator and fails further north, where meridians converge and a degree of longitude covers less distance. A pilot who used such a chart on a long course could find his reckoning going wrong in ways that were hard to diagnose. The Portuguese mathematician Pedro Nunes had discussed the rhumb line in the 1530s, and the problem was well known to scholars.
Mercator’s chart of 1569
Gerardus Mercator, a Flemish geographer, instrument maker and engraver working in Duisburg, published a large world map in 1569. Its long title says that it was a new and enlarged description of the world, suitably adapted for use in navigation. On this map, meridians are drawn parallel and the spacing of the parallels of latitude increases towards the poles, so that a rhumb line appears as a straight line. The scale increases with latitude, which is why regions near the poles look much larger than they are, and why Greenland looks so enormous in comparison with Africa on a Mercator map.
Mercator did not explain in detail how he derived the spacing of the parallels. Historians describe his method as most likely a graphical or approximate one. What he produced was in effect a mathematical solution, although it was not presented as one.
Wright and the tables
The mathematics was set out later. The English mathematician Edward Wright published “Certaine Errors in Navigation” in 1599, in which he explained the principle and gave tables of what are called meridional parts, so that a chart maker could construct a Mercator chart accurately and a navigator could solve problems of course and distance by calculation. Wright had sailed on a voyage himself, and he was concerned with practical improvement of the art.
Even then, adoption was slow. Many pilots distrusted a chart that appeared to distort the world, and plain charts continued to be used for some time. By the 17th century, the Mercator chart began to appear on Dutch and English charts, and over the 18th century it became the standard for ocean navigation. Today the Mercator projection remains the basis of most nautical charts, with a note that distances must be measured against the latitude scale at the side, not against a single scale for the whole sheet.
Pilot books and printed charts
Charts were only part of the story. Lucas Janszoon Waghenaer, a Dutch pilot, published his Spieghel der Zeevaerdt (“Mariner’s Mirror”) in 1584, a collection of charts and sailing directions for the coasts of western Europe. It was so influential that English editions in the late 1580s gave rise to the word “waggoner” for a sea atlas of this type. Dutch chart makers such as the Blaeu family built a large trade in printed sea atlases in the 17th century, serving the merchant fleets of the East Indiaman era and others.
Printing made charts cheaper and more uniform, but it also fixed errors. A mistake on a copper plate could be repeated in many copies, and chart makers often copied from earlier work with little new survey.
The age of survey
The next improvement came from systematic measurement. As navigators gained better instruments for latitude and, from the late 18th century, a reliable way to find longitude using the marine chronometer, coastlines could be placed more accurately. Soundings taken with a lead line recorded the depth of the water, and the nature of the bottom, such as sand, shell or mud, was noted for anchorage.
National organisations took charge. The British Admiralty established a hydrographic office in 1795, with Alexander Dalrymple as its first hydrographer, and gradually built a library of surveyed charts that were sold to merchant ships as well as naval ones. The Beagle survey voyages and the voyages of Captain Cook are among the best-known expeditions that charted coasts in detail. Other maritime nations created their own offices, and over the 19th century, survey ships mapped coasts around the world.
From paper to display
Charts developed new features during the 19th and 20th centuries, including standardised symbols, depth contours, light characteristics, buoys and hazards, and notes about magnetic variation. Corrections were issued to captains as notices to mariners, so that a paper chart could be kept up to date by hand.
In recent decades, electronic chart displays that use the same underlying projection have replaced paper on many commercial ships. The surveyed data is stored digitally, and the ship’s position from satellite navigation appears on it. Yet the Mercator principle of a chart on which a steady bearing is a straight line has outlasted nearly five centuries of change.