How aerial photographs are used to make maps
Modern map-makers rely on a method that was used by some of the earliest cartographers. They go to a high place to get a picture of the terrain. In bygone centuries the cartographer would climb to the top of a hill, carrying instruments and drawing equipment with him, whereas today photographers go aloft in an aeroplane.
The first aerial photographs for mapping were taken in 1851 by a Frenchman, Aime Laussedat, who floated across the French countryside in a hot-air balloon. Photographs taken from military aircraft were used to map the trenches during the First World War.
For aerial surveying, the aeroplane flies at a height dictated by the scale of photograph needed for the map. If the scale is to be 1:50,000 and the camera has a focal length of 6in (150mm), the flying height would have to be 25,000ft (7500m). The photographs are taken vertically downwards as the plane flies in strips back and forth across the land to be mapped. Each photograph overlaps the previous one by about 60 per cent, and the adjacent strips overlap by about 30 per cent. This ensures that all parts of the ground are photographed at least twice. An aeroplane flying at 25,000ft would need to take at least 12,700 photographs to cover an area the size of France.
Pairs of adjoining photographs are viewed through an instrument called a stereo-plotter which reveals a single three-dimensional image of the ground. This image is adjusted to fit a network of points whose exact position on the ground is known. The stereo-plotter can then be operated to measure, record and plot the position and height of all map details at the required scale.
The fixed points might have been established by surveyors for earlier surveys or created especially for the purpose. These points – like every other spot on Earth – have a latitude (the distance north or south of the Equator) and a longitude (the distance east or west of the Greenwich meridian).
To ‘capture’ the ground detail, the stereo-plotter operator guides a spot of light over each feature in the photograph, automatically recording the information as digital data on magnetic tape. The ‘captured’ map detail can be simultaneously displayed on a video screen or plotter for verification.
The taped information is then fed into a computer, together with other requirements such as the area to be covered by the map, the style of the map and its scale. Drawing machines run by the computer can produce both preliminary maps for checking and finished maps for printing.
All the information that has been gathered and analysed must eventually be turned into a map designed for a particular purpose, such as a road map for motorists or a land-use map showing urban areas, farmland, woodlands and marshes.
The area to be included in the map may be at a very large scale (covering only a small area of ground). The landscape will then be shown in great detail, including individual buildings and ponds. Maps like this are used by town planners to set out new roads, for example.
To produce a map at a smaller scale (and so show a greater area), the cartographer reduces several of the large-scale maps into one. But as the scale becomes smaller the amount of detail has to be reduced and symbols have to be used. For example, a village or small town which started as a collection of individual buildings is amalgamated into a single shape.
As the scale gets smaller still, villages are omitted altogether and the towns are shown as dots or squares. Finally, on maps of whole continents or the world, only major cities can be included, marked by a spot.
The height of the land is usually shown by contour lines, which are lines joining points of equal heights. The closer together the lines (labelled in feet or metres) the steeper the slope. The contour lines may be combined with colours – called contour layer tinting – to illustrate the range from sea level (usually green) to high mountains (usually brown or purple). Hill-shading, which makes hills stand out from valleys, gives the map a 3-D effect. It may supplement layer tinting, or can be used alone.
A major problem which a cartographer must solve is how to represent the curved surface of the Earth on a map which is flat.
To do this without some distortion is impossible, like trying to flatten half a tennis ball without splitting it.
The answer is to use one of the many mathematically devised map projections. These arrange the latitude and longitude lines and other details to minimise the distortion for a particular purpose. This will always be at the expense of accuracy in some less important respects. For example, the familiar Mercator’s Projection is used to plot straight navigation courses, but they distort scale so that countries far away from the Equator, such as Greenland, look much larger than they are.
A projection designed to show countries as close as possible to their correct relative sizes and positions will distort distances and be unusable for navigation.
The carefully designed map is then drawn, either by draughtsmen or, increasingly, by computers. Hand-drawn maps are done on layers of film, each one showing different features such as roads, rivers, contours and areas of colour. There may be 20 or more layers which are put together and combined photographically to produce a film for each of the four or six colours that are used to print most maps. Some computer-drawn maps eliminate the need for this complicated process, and produce the printing film directly.
Some computer-generated maps are never printed at all, but are sent electronically to computer screens in aeroplanes or ships for navigation.
Maps in the future may bypass the present system of aerial photography, ground surveys and conventional printing. Satellites, orbiting the planet, may beam images directly to computers, which would print out maps or send them as electronic signals to screens in planes, ships, or cars.
The scale of a map
One of the most important factors in map-making and map-reading is the scale. A motoring map might be at a scale of 1:250,000, meaning that every unit (inch, millimetre or centimetre) on the map represents 250,000 of those units on the ground. So the scale of the map could also be expressed as ‘1in represents 4 miles’ or `1cm represents 2.5 kilometres’. A map of the world in an atlas might be at a scale of 1:60,000,000 (1in to 947 miles, or lcm to 600 kilometres).
Maps at different scales are used for different purposes. It would be impossible to plan a motoring route from a world map scale, but a world map at 1:200,000 would be enormous – about 220yds (200m) wide.