Charts and Longitude

History of Maps and Cartography

Course 1 · Chapter 7 · Charts, Longitude and the Sea

A map for the land shows where things are, and a chart for the sea shows where it is safe to go. Charts carry soundings, rocks, tides and lights, and a mistake can wreck a ship. Their history is tied to a famous problem, how to know your east-west position at sea, whose solution depended on clocks as much as on stars. Names and dates are from general knowledge and flagged "check".

What a Chart Shows

Depths

Soundings, measured with a lead line, give the depth at points along the coast, and contour lines for the sea floor came later (check).

Dangers

Rocks, wrecks, shoals and tidal races are marked, along with safe channels and anchorages.

Aids

Lighthouses, buoys, landmarks and compass information help a navigator to fix a position and steer.

The Longitude Problem

Item (check each)	Detail
Latitude	Found from the height of the sun or the Pole Star above the horizon, using an instrument such as a quadrant or sextant.
Longitude	Needs the difference between local time and the time at a reference place. Without a clock that kept good time at sea, sailors estimated it by dead reckoning, using speed, heading and time.
Disaster, 1707	A Royal Navy fleet under Sir Cloudesley Shovell was wrecked on the Isles of Scilly, with great loss of life, an event often linked to the longitude problem (check the causes).
Longitude Act, 1714	Parliament offered large prizes for a practical method (check the amounts and conditions).
Two answers	The lunar-distance method, using the moon's position among the stars, and the marine chronometer (check).
John Harrison	A clockmaker from Yorkshire who built a series of timekeepers, H1 to H4, tested in the 1730s to 1760s. His dealings with the Board of Longitude were long and difficult (check the story).

Not a single invention The popular story presents Harrison as a lone genius against the establishment. Historians stress that the solution came from several lines of work, astronomical tables for the lunar method, clockmaking, and the practical trials at sea. Chronometers did not come into general use at once, because they were costly, and lunar distances remained in use for decades (check). The honest summary is that longitude was solved by both astronomy and clockmaking, over many years.

Surveying the Seas

Hydrography. The science of measuring and charting the waters.
The Admiralty. The Royal Navy established a hydrographic office in 1795, with Alexander Dalrymple as its first hydrographer, and it became a source of charts for the navy and for merchant ships (check the dates).
Captain James Cook surveyed Newfoundland in the 1760s, and charted large parts of the Pacific on later voyages (check).
Francis Beaufort, Hydrographer from 1829, expanded survey work and is also known for the wind scale (check).
Today. The UK Hydrographic Office, based in Taunton, continues the work, and charts are now mostly electronic (check).

Land map	Sea chart (check)
Shows buildings and roads	Shows depths, rocks and channels
Features stay in place	Sandbanks shift, wrecks appear, so charts need constant corrections
Users can stop and look around	Users depend on the chart in poor visibility
Errors cause inconvenience	Errors can cost lives

A British comparison The Royal Observatory at Greenwich, founded in 1675 to improve navigation, gave its name to the prime meridian, adopted as the world's reference at a conference in Washington in 1884 (check). A visit shows the meridian line in the courtyard, and some of Harrison's timekeepers are on display at the Royal Museums Greenwich (check what is currently shown).

Check the details Numbers for prizes, the date and cause of the Scilly disaster, and the sequence of Harrison's clocks vary between popular accounts. Check them against a scholarly source such as the Royal Museums Greenwich.

Exercises

Exercise 1

Explain why knowing the time at a reference place gives your longitude, using the idea that the Earth turns 360 degrees in 24 hours.

→ Solution

Exercise 2

Write a balanced two-paragraph account of the longitude story that credits both astronomy and clockmaking.

→ Solution

Exercise 3

Look at a sea chart for a harbour you know, from an online source or from a boating guide, and list ten symbols you can identify.

→ Solution

Quick Reference

Latitude: sun or star height
Longitude: time difference, 15 degrees per hour
Solutions: lunar distances and chronometers
Charts: depths, dangers, aids; kept up to date
Britain: Admiralty hydrography, Greenwich