
The Mapmakers: An Essay in Four PartsMapmaking: 16th Century Mapmaking16th CenturyMaps (of land surfaces) and charts (of sea coasts) are scaled down representations of the earth's surface. For this reason they are ideal documents to prove that a discovery has taken place and provide the means for the exploration to be repeated by others. Maps are made up of three measurable elements: location, direction and distance. On some maps symbols and elevation are added. Symbols give more meaning to locations while elevation adds altitudinal distance. The precision of these elements and their exact placement on maps relative to each other, is what separates accurate maps from poor ones. Accuracy is dependent on:
By the 16th century there was a general agreement that position be recorded by latitude and longitude. Due to the unvarying relationship between the earth's axis and the sun and stars, latitude (the angle between a place, the centre of the earth and the equator) could be easily calculated. This was done either by measuring the height of the sun at noon above the horizon and correcting that observation for the day of the year (sun's declination); or, by measuring the height of the North Star (Polaris) and compensating slightly for the difference between the position of Polaris and the geographic pole, since the two do not exactly coincide. To do these tasks two instruments could be used; the astrolabe, mainly used for measurements on land, and the crossstaff (also called Jacob's Staff) for observations at sea.
Sixteenth century measurements of latitude such as Jacques Cartier's were accurate to about onequarter to onehalf of a degree (one degree latitude equalling about 111 km). Longitude, the angle between a place, the earth's axis and a prime meridian (today the prime meridian is the longitude of Greenwich, England), was impossible to calculate accurately until John Harrison invented the marine chronometer, (a large pocket watch set on Greenwich mean time) in 1773. Since the ancient Greeks, geographers had known that longitude could best be determined by calculating the difference in solar time between two places. Since the earth is 360° in circumference and rotates on its axis every 24 hours, one hour of time equals 15 degrees longitude. One degree therefore, equals four minutes of time and about 111 km at the equator. Since timepieces were not generally available until late in the 18th century, longitude had to be obtained by estimating eastwest distances from a place of departure to a destination. On land, distances were estimated by travel time  for example the distance an average man could walk in an hour (one league or about five kilometres). The French called this the "lieu d'une heure de chemin." Similarly, at sea, the estimated speed of a ship was converted into distance. This was called "dead reckoning." A navigator kept very careful note of all his speeds, course changes, encounters with currents, etc. in a log book. At the end of the day, he would convert all his observations into distances and plot them on his chart according to his compass observations.
By the 16th century, the mariner's compass was in general use. It was divided into 32 "points" or "winds", rather than degrees. Each point was equal to 11°15'. Compasses were not accurate enough to sail by degrees. Since a compass points to the magnetic pole and maps are on the geographic pole (true north) compasses had to be corrected for this difference (magnetic declination). In the 16th century, few mariners knew how to do that, or considered it to be unimportant. Nor did many know that magnetic declination varied across the earth's surface and that it changed over time (variation). A result of all this confusion was that compass bearings on 16th century maps tended not to be very accurate. Most were in fact magnetic bearings, giving these maps a peculiar orientation to modern readers. Due to the twin problems of measuring direction and distance over the open sea, most 16th century navigators preferred to minimize guesswork through "parallel" (or "latitude") sailing. A captain would sail along the coast of Europe until he reached the latitude of the place he wanted to go to. He would then depart the European coast and use the one instrument he trusted, his crossstaff, to stay on that latitude until he got to the other side. On this journey he would then have to estimate his distance along a relatively straight course. This distance would then become the distance between Europe and his destination on his map along the one line of latitude he had sailed. By means of a table calculated by mathematicians for every line of latitude (parallels), the navigator could now mark off his lines of longitude (meridians). The more often he travelled over a route, the better his observations got. Once he reached his destination, he would sail within sight of the coast, taking compass bearings of the coastline and of prominent features, estimating distances and, weather permitting, calculating the latitudes of places. Bays, river mouths, hills, etc. were sketched on the chart as the ship sailed past them. These rough reconnaissance surveys formed the bases of most 16th century maps. Another method for calculating distance sailed was the rule 'to raise or lay a degree of latitude'. This was an early form of 'plane sailing' (using rightangled triangles) wherein a navigator would lay out a course with his compass. When he had crossed one degree of latitude by observation with his crossstaff (the adjacent side of his triangle) he could look up the distance he had sailed (hypotenuse of his triangle), and longitudinal distance traversed (side opposite his course angle), in a set of tables calculated by mathematicians. The invention of trigonometry made these tables redundant. It was not until the early 17th century, motivated by the search for harbours and locations for settlement, that more accurate maps were produced with better instruments. 