portolans

Mariners during the 15th century relied on charts called "portolans" to assist them on their voyages. 'Portolan' comes from the Italian word 'portolani', which were medieval pilot books. The portolans contained maps of coastlines, locations of harbors, river mouths, and man-made features visible from the sea. They were a compilation of centuries of seafarer observations. As sailors' skills improved and the use of the compass was more widespread, portolans improved in accuracy. Also Columbus used these portolans on his journeys. Portuguese chartmakers added the meridian line, a point useful for latitude sailing as well as for navigating solely by compass. A geographic feature could now be located through the use of its distance in degrees of latitude from a ship's point of departure. Note that the use of latitude and longitude was understood since the time of Ptolemy, the second century CE.

During the fifteenth century Portugal led the European world in sea exploration. The golden age of discovery for Portugal lasted almost a century until the Dutch eventually seized their trade routes from them.

Useful Information

Parallels: Circles parallel to the equator, ranging from 0° to 90° N or S. Only the equator is a great circle.
Meridians: half-Circles conversing at the poles, ranging from 0° to 180° E or W. Each pair of meridians forms a great circle.
Prime Meridian: 0° or the Greenwich meridian which divides the Western and Eastern hemispheres.
Great Circle: The intersection of a sphere and a plane through its center.
Small Circle: The intersection of a sphere that does not pass though its center.
Time Zones: By convention 24 zones, each 15° longitude wide. Hence, noon at greenwich gives midnight at 180° E.
GMT: Greenwich Mean Time or UTC or Zulu, and is the time in Greenwich. Antonym: Local time. Berlin's local time = GMT + 1.
Date Line: The meridian which extents from the prime meridian. Here, not only the hour changes, but also the date.
Latitude: Position property defined by the number of degrees North or South of the equator, varies from 0° to 90°.
Longitude: Position property defined by the number of degrees East or West of the prime meridian, varies from 0° to 180°.
Position: Latitudes first and Longitudes second. For example: Nijmegen (NED) = 51° 50',1 N , 05° 52',0 E.
Nautical Mile: One nm is one minute (') on the chart. 1' equals 1.852 meters.
Knots: Nautical miles per hour.