Navigators toolchest- the early chart
Background
Of all the tools
the navigator uses the chart is his most used one and where all the
position-finding data is deposited. It is the tool that describes the historic
and current position of the ship.
So how did the sea
chart or chart as it is called come about?
In an earlier
article I outlined how seafarers managed to cross great oceans without any
charts at all using that knowledge of the sea and some simple navigational
tools.
In this article I will
describe the history of the sea chart itself.
The earliest sea
chart refers to the Portolan chart in the Mediterranean as being the first sea chart
referenced between 1300 and 1500 with the Chinese using a sea chart in the
early 1400s to move westward from China to India, the Persian Gulf and as far
as Africa.
The two charts are
very different.
The Portolan chart
The first navigational chart is thought to have been European. In the Middle Ages, in the period 1300 to 1500, it was in the Mediterranean Sea.[1] The word “PORTOLAN” comes from the Italian word “portolano” which means “relates to ports and harbours”.[2] So a portolan chart is essentially a coastal map for an enclosed sea such as the Mediterranean Sea. It locates and identifies known places on the coast to assist the navigator with sailing directions between these places. It is important to note that the earth was treated as a plane surface without consideration of the spherical nature of the earth.
The charts were
most often works of art drawn by artists on vellum and richly decorated, more
like a painting than a practical navigational instrument.[3]
In addition as Campbell notes in his excellent treatise of Portolan charts, seafarers in the Mediterranean often had sight of land.[4]
The grey areas in
the image above denote areas where seafarers had sight of land and the dark
areas are sea areas where no land is in sight. So it is possible to navigate in
the Mediterranean by minimising periods when you are out of sight of land, a
sort of extended coastal pilotage approach.
Two navigational
aids were used by the medieval navigator, the first being the dependable
seasonal winds in the Mediterranean.[5]
The direction of the winds was steady in direction thus providing a known bearing from which the wind came from. The “Tramontano” wind came from the north and the “Scirocco” from the south-east so providing the seafarer with a wind compass he could use to set a course. For example, keeping the scirocco wind on the starboard quarter allowed the ship to sail in a northerly direction, say from Tunisa to Sardinia with only a short period out of sight of land.
To
further help the navigator the charts were overlaid with rhumb line emanating
from a number of points on the chart. These created directional wind roses
similar to compass roses. These constant directions were courses relative to a
particular wind and any one would lead the ship to a known coastal port or harbour.
A
noticeable absence from these charts is any hydrographic details such as water
depth or seabed condition. Apart from a black symbol indicating isolated rocks
there is little to help the navigator from the underwater information on the chart.
Although
the magnetic compass was first used by the Chinese, the Mediterranean navigator
increasingly used a magnetic compass[6] with their
Portolan charts when visibility was reduced and they were out of sight of land.
So technology helped the medieval navigator albiet without understanding of
magnetic variation and declination so that it was not a precise navigational tool
for direction. Coupled with
dead-reckoning estimates of distance travelled it nevertheless enabled ships to
trade across the Mediterranean from coast to coast.
The Mao Kun Map
In the period 1405 to 1433, the Chinese emperor Yongle commissioned a fleet of vessels to undertake seven voyages of discovery westward under the command of Admiral Zheng He.[7]
The ships were very sophisticated and large in comparison to
European ships of the period. The fleet was made up of treasure ships, horse
ships, supply ships, troop transports and war ships, a large and diverse armada
totalling as many as 300 ships of which the largest was estimated to be some
230 ft. long, much larger than their European counterpart. The purpose of the
voyages was intended to project power and diplomacy as well ass trade and seek
treasure.
The scale of the voyages was massive. The first three voyages explored the regions of Java and Sumatra, into the Indian Ocean as far as Ceylon and Calicut on the Indian Malabar coast. On the fourth voyage they extended their reach to Hormuz and on the fifth, sixth and seventh voyages they went further to the Arabian Peninsula and East Africa, all these some 50 years before the start of European Exploration with Christopher Columbus in 1492.[8]
The Mao Kun map or Zheng He’s Navigation map was first
published in 1628, some 200 years after the Treasure ship voyages.[9]
It has similarities with the Portolan chart in that it depicts coastal areas and names of bays and ports and is devoid of many sea or ocean symbols. However, it differs from the Portolan charts and voyages in that it was a rolled-up chart to be read from right to left, the right-hand side being China and the left-hand side being the furthest reach of the treasure ship voyages. Later the strip was divided into 40 separate pages.
In this sample from the complete chart, the maritime route is the dotted lines wending its way around islands on both the lower and right-hand side of the coast. So by unrolling the chart you covered the area you're sailing in. In these charts there is a little sense of distance merely a long line of coastal named places on the right-hand side until you reached your destination. For example in this sample the coast on the upper most part of this element of the chart is the coast of Ceylon (Sri Lanka and on the lower part if the chart is East Africa. The Indian Ocean being compressed into just a small space.[10]
Although the voyages were essentially coastal voyages, the
ships did venture offshore crossing the Bay of Bengal from Sumatra to Ceylon
and the Indian Ocean from Hormuz to Calicut. For this they had sailing
directions and stellar charts that enabled them to navigate at sea with compass
and stars.
The four stellar charts in the portfolio include reference to latitude as part of the height of known stars and planets so that instructions on how to cross ocean legs of the voyages were available.
Here, there is a distinct difference to the Portolan based
voyages in that stellar objects were the principal means of navigation rather
than the winds of the Mediterranean.
Still, they remained plane charts without reference to the
spherical nature of the earth and longitude remained an elusive target.
In the next article we will explore the Age of Exploration
and its influences on the sea chart.
References
‘Portolan Chart | Maritime Navigation,
Nautical Maps, Cartography | Britannica’. 20 January 2026. https://www.britannica.com/technology/portolan-chart.
‘Portolan Chart | Maritime Navigation,
Nautical Maps, Cartography | Britannica’. Accessed 8 February 2026. https://www.britannica.com/technology/portolan-chart.
‘Portolan Chart Carte Particulieres De La Mer
Mediterannée’. Accessed 3 April 2026. https://www.lyonandturnbull.com/auctions/rare-books-manuscripts-maps-and-photographs-739/lot/122.
‘Portolan Charts’ Origins Menu’. Accessed 6
February 2026. https://www.maphistory.info/PortolanOriginsMENU.html.
‘Ships of the Ming Treasure Voyages - Google
Search’. Accessed 6 April 2026..
[1]
‘Portolan
Chart | Maritime Navigation, Nautical Maps, Cartography | Britannica’, 20
January 2026, https://www.britannica.com/technology/portolan-chart.
[2]
‘Portolan
chart’, Wikipedia, 28 May 2025,
https://en.wikipedia.org/w/index.php?title=Portolan_chart&oldid=1292717927.
[3]
‘Portolan
Chart Carte Particulieres De La Mer Mediterannée’, accessed 3 April 2026,
https://www.lyonandturnbull.com/auctions/rare-books-manuscripts-maps-and-photographs-739/lot/122.
[4]
‘Portolan
Charts’ Origins Menu’, accessed 6 February 2026,
https://www.maphistory.info/PortolanOriginsMENU.html.
[5]
‘Portolan
Charts’ Origins Menu’.
[6]
‘History of
the compass’, Wikipedia, 17 March 2026,
https://en.wikipedia.org/w/index.php?title=History_of_the_compass&oldid=1344009714.
[7]
‘Ming
treasure voyages’, Wikipedia, 26 January 2026,
https://en.wikipedia.org/w/index.php?title=Ming_treasure_voyages&oldid=1334982176.
[8]
‘Age of
Exploration - Google-Søk’, accessed 6 April 2026,
https://www.google.com/search?q=age+of+exploration&oq=age+of+eplration&gs_lcrp=EgZjaHJvbWUqCQgBEAAYDRiABDIGCAAQRRg5MgkIARAAGA0YgAQyCQgCEAAYDRiABDIJCAMQABgNGIAEMgkIBBAAGA0YgAQyCAgFEAAYDRgeMggIBhAAGA0YHjIICAcQABgNGB4yCAgIEAAYDRgeMggICRAAGA0YHtIBCTExMzg1ajBqNKgCALACAQ&sourceid=chrome&ie=UTF-8.
[9]
‘Mao Kun
map’, Wikipedia, 3 October 2025,
https://en.wikipedia.org/w/index.php?title=Mao_Kun_map&oldid=1314831629.
[10]
Wikipedia, ‘Mao Kun
map’.
