The Perspective from Space

Article excerpt

CARTOGGRAPHY is a very ancient art. As long ago as the time of the Pharaohs, surveyors were drawing up plans of plots of cultivated land in order to re-establish property boundaries or replace cornerstones buried under the silt brought down by the floodwaters of the Nile.

The great voyages of discovery of the fifteenth century le to the trnasformation of world maps. Returning from their voyages, the great navigators of those days recounted their experiences, which cartogrpahers working quietly in their homes incorporated into more accurage maps.

The emergence of modern cargrography, however, dates from the eighteenth century, when scientific expeditions such as the French expeditions to Lapland and Peru finally confirmed Newton's hypothesis that that world was shaped like a sphere slightly flattened at the poles.

It was throught these expeditions that cartography acquired the mathematical basis it needed to become the instrument for the accurate, graphic, scale representation of our knowledge of the Earth that it is today.

The starting-point of all cartography is the science of geodesy--the determination of the size and shape of the Earth and the exact position of a series of reference points on its surface. By means of successive readings taken in turn from each of these points, the co-ordinates of these points on the surface of the Earth are determined and then transposed to a flat surface.

Today, methods and instruments have completely changed. Using a small calculator linked to an aerial, geodesists "listen" to the specialized satellites of the Global Positioning System, which give them their precise position immediately. During the previous three centuries, however, they would have had to take readings with theodolites of the various topograhical features around them. On the basis of these reference points, topographers would have made a detailed examination of the terrain so as to locate the roads, houses, forests, rivers, etc., that they had observes.

From about 1930, aerial photography, a technique which had been perfected during World War I, radically modified this painstaking work and signalled the end of direct accumulation of data by observation on the ground. Aeroplanes are flown in a series of straight lines and photographs are taken at fairly close intervals. Each section of terrain flown over is photographed twice, from different angles. In this way the aerial photographs provide the binocular effect of human vision, which makes possible the appreciation of distance and relief.

The cartographer makes use of this stereoscopic effect which, used in conjuction with extremely precise measurement techniques, gave rise to the science of photogrammetry (the measurement of photographs applied to surveying and mapping, as well as for engineering and other purposes). Through the twin eyepieces of their photogrammetrical reconstituion equipment, each of which is focused on one of the two successive aerial photographs, the stereo-operators get a true picture of the terrain in relief, with its heights, its valleys, houses and trees. All that then has to be done is to trace the relief with a cursor, whilst operating certain controls, for the coordinates of the various features to be recorded.

However, a certain amount of fieldwork remains necessary to check the correct identification of detail, to fill in details not visible in areas in shadow, or to add information such as place-names or administrative boundaries that aerial photographs cannot supply.

The documentation thus produced is corrected by the map editor and then printed. The process calls for great care and precision and is quite lengthy. About two years should be allowed from the taking of the aerial photographs to the publication of the map.

The computer invasion

Computers came upon the cartographical scene in the late 1950s. They were first used in geodetic calculations. …