Geographic methods. Map location and measurement

The map is the distinctive data bank of the geographer. Since geography deals particularly with locations, distributions, areal associations, and interrelationships of phenomena in space, accurate observation and measurement of the surface of the Earth and the recording and displaying of location on maps are of prime importance.

Latitude and longitude are commonly utilized for plotting locations on the surface of the globe. Fairly accurate measurements of latitude were made in antiquity by Greek scholars. Measurements of longitude remained rough¹, however, because of the difficulty in measuring differences in solar time (the Sun “moves” westward at the mean rate of one degree each four minutes). The perfection of the chronometer solved this problem, but for long each country had its own system for numbering the meridians. Finally, by an international agreement reached in 1884, an imaginary line from pole to pole through Greenwich, near London, was recognized as the prime meridian. Measurement of direction, was aided considerably by use of the magnetic compass, but as Christopher Columbus noted in crossing the Atlantic, the direction in which the compass pointed varied with longitude.

The measurement of distances overland could be counted in days of journey on foot, by camel, by horse, or by other means. More accurate measurements of short distances were obtained by using a chain², and the chain as a unit of length (66 feet) is still a traditional surveying measure in English-speaking countries. Later, the chain itself was replaced by a steel tape, and still later electronic instruments came into use. A practical measurement of distances at sea was developed in the 16^th century: a log³ was thrown overboard and the amount of time it took the stationary log to play out a certain distance on a line marked off with knots⁴ was measured. Navigation by means of satellites is now available, but a ship’s speed is still measured in knots and records are kept in a logbook⁵. After the adoption of the metre as a standard unit in France in the late 18^th century, it gradually replaced older local and national measures of distance over much of the world during the 19^th and 20^th centuries.

Maps of small areas – topographic maps, for example – can be made by a method called triangulation⁶. A base-line is measured with chains or other devices, and by using this base as one side of a triangle the other sides are calculated from the angles at the two ends of the base-line. Angles can be measured more easily and accurately than distances, and from the points on the corners of the original triangle, a network of points joined by triangles can be established. Triangulation was known to the ancient Egyptians and Greeks; with improved instruments, especially the theodolite, this method was utilized in the great national surveys of Europe and America from the 18^th to the 20^th century. How to represent the entire, spherical Earth or large areas of it on maps remained a problem. In 1492 the German navigator and geographer Martin Behaim completed the construction of a terrestrial globe. Ships following straight lines on flat maps, however, did not arrive at expected points. Mercator devised a map projection – which became known as the Mercator projection – on which ships following straight lines would arrive at the plotted points⁷. Modern geographers use maps drawn in what is called the equal-area projection, but even this projection distorts shapes or distances, particularly toward the edges of the map.

With the inceasing specialization of knowledge, measuring the shape of the Earth developed into the discipline of geodesy. Plotting land positions for detailed maps became the province of surveying, and constructing numerous types of maps with appropriate projections grew into the field of cartography. Maps have remained as the basic tools in geography for plotting and analyzing a vast range of physical, biologic, historical, economic, political, and social data.

The Round Earth on Flat Paper