A map is a representation of a geographic area, usually a portion of the earth's surface. It may be shown in many different ways, from a traditional map printed on paper to a digital map built pixel by pixel on the screen of a computer. Maps can show almost anything, from the electric supply grid of your community to the terrain of the Himalayas to the depths of the ocean floor. A map can be practical, directing travelers from one point to another through confusing terrain, or explaining the world by attaching specific types of information to geography. But maps can also entertain and invite exploration. For example, a colorful map of the Marquesas Islands with exotic-sounding ports such as Hakapehi on Nuku Hiva might have a beckoning appeal to some. Similarly, a detailed map of the many features of Athens or Bangkok might entice others to travel to these sites. A map can even be created for the surface of Mars, based on data transmitted to Earth from computer-controlled spacecraft, showing places that most people will never visit.
Maps can be drawn in many different styles, each showing different faces of the same subject and allowing us to visualize the world in a convenient, informative, or stimulating way. To use maps effectively, just learn the few simple skills described here. In addition, be aware of these important facts:
(1) No map is perfect. People make maps from data they collect with certain tools. Even computer-generated maps depend on programs designed by people and on data collected by human-designed machines. People make mistakes and machines are never totally accurate all the time, nor can any device record every detail of a landscape. Therefore, maps can contain errors and inaccuracies. Because of data errors or cartographic errors, a certain village may not be exactly where the map shows it or a mountain peak may not be exactly as high as it appears on the map.
Cartographers using traditional tools such as the recording of ground data by hand or the use of high-altitude photography are limited by how many objects and how small an object they can record. Very small features may not be accurately placed on the map or they may not appear at all. Modern tools such as high-resolution satellite photography can record details to a resolution of several meters. Most surface objects of practical importance can be recorded with such imagery and translated into highly accurate maps or photographs, but they are still subject to interpretation and data error. Cartographers sometimes purposely limit the details they present in a map in order to make the map useful and less confusing.
(2) Maps grow old. The world is constantly changing both physically and culturally, so maps can become outdated, no longer showing the world accurately. Modern technology has provided a partial solution, computers have made it possible to renew maps easily without redrawing them. However, appropriate information reflecting changes in the world must still be collected periodically and used to revise the maps' databases.
(3) Maps are biased. Because maps generally do not show every single feature of a chosen geographic area, every tree, house, and roads the cartographer must decide the projection and scale for the map and decide how much detail to present. The purpose of the map as well as the cultural background of the cartographer often dictates this process, called generalization. Information on the map and how it may be distorted can influence what people think about the world and what they do.
The first question to ask about a map is what its theme is. The theme is the particular aspect of the world that the map attempts to show, such as roads, borders, vegetation, or statistical data. Maps can be divided by theme into three categories. The first, general maps, are those that contain many themes and give a broad picture. General maps are often practical, showing the world in a way that allows people to get from one point to another without getting lost, or allows them to learn about the overall layout of an unfamiliar place without having to go there. An example of a general map is a road map of a country showing major cities, mountains, rivers, landmarks, etc. The second category is thematic maps, which contain one or a few themes and show in-depth information. Thematic maps can show almost any kind of information that varies from place to place, such as a country's population or income level by state, province, or county, with each division colored differently to indicate the relative level of population or income. The third category of map is charts, which are accurate maps of routes of travel used for ocean and air navigation. They must be updated frequently so that captains and pilots know of current dangers along their route.
Maps are made in many different forms. The first maps made by people were probably lines drawn in sand or small pebbles and sticks arranged on the ground. Modern maps are published for the long-term use of many people. Printed maps are the simplest forms. They show the world as flatâ€”that is, in two dimensions. On a printed map, relief, mountains, valleys, and other terrain is shown with special symbols to make up for the lack of depth, which is the third dimension. Relief maps are rigid flat maps with actual bumps and depressions added to indicate elevated landforms and low areas. They are usually made of clay or molded plastic, and the relief is usually exaggerated to give a greater impression of depth.
In between the effects created by flat maps and relief maps is the visual experience created by stereograms, which are flat maps or aerial photographs positioned in slightly differing pairs. Viewed through special 3-D lenses that fool the eyes, stereograms give the effect of viewing actual relief. Globes are spherical models of the Earth, the Moon, and other planets. They give a more realistic impression of features on a curved surface.
Computer maps are the most versatile. A mapping program can dynamically show many different views of the same subject, allow changes in scale, and incorporate animation, pictures, sound, and Internet links to sources of supplementary information. A person can update a computer-generated map with new information by simply supplementing the map's database, allowing the map to grow over time to present more geographic detail and more themes. Having a powerful digital map is like having dozens of printed thematic maps overlaid on a particular area, each electronically connected to an immense library of information on the main theme and on many related ones.
How people use a map depends on the type of map they have and what sort of information they want from it. In the case of simple maps, only one or two types of information may be available and few or no map skills are required to use it. For example, a sketch of a neighborhood may only show what relationship a particular house has to the street corner or whether it is farther from there to the market or to the school. Even those who cannot read the local language can use such maps. But complex maps can indicate actual distance, the exact location of many important land features, elevation, vegetation, political divisions, and many other aspects of the world. To interpret such a complex map, some basic map skills are required.
Most maps, including the majority of maps of the earth, share a number of basic features. They assume a certain projection and scale, they usually express location in terms of coordinates, and they have a legend.
The surface of the earth is curved and maps are flat, whether they're printed maps or computer screen pictures. This means that all maps except for globes and pictures of globes are distortions of how the earth really looks. For small areas, the distortion is insignificant because small areas on the globe look like a flat surface. But for large areas or for purposes demanding high accuracy, such distortion can be very important. Why do map distortions occur? A simple explanation of this can be found in the case of the orange peel. When the curved outer surface of an orange is removed and laid flat, the peel spreads out in separate pieces. Cartographers face the same problem when mapping the surface of the earth. They have to remove the pieces of geography in a certain way and stretch or stitch the pieces together again in order to make a continuous flat map.
The way the geography of the earth is taken from the globe and reassembled on a flat surface is called the map's projection. Another way of thinking of projection is this: Every point on the globe can be projected by a straight line onto a transparent form wrapped around the globe. The shape of the form and how the points are spread onto it determine the type of projection. Some common forms are cylinders, cones, ellipses, and flat planes, giving rise to cylindrical, conic, elliptical, and orthographic projections. There are many types of projection, each distorting the spherical surface of the earth in a different way and each with its practical advantages and disadvantages.
The size of a map in relation to the earth is its scale, which is usually stated as a fraction or ratio. The numerator, at the top of the fraction, is one unit on the map and the denominator, at the bottom of the fraction, is the number of the same units that are represented in the real world. For example, a scale of 1/10,000 means that one centimeter on the map is equivalent to 10,000 centimeters on the ground. As a ratio, this scale would be shown as 1:10,000. The larger the denominator and the smaller the fraction, the more of the earth is represented on a single map. Therefore, small-scale maps show a large piece of the earth, and large-scale maps show a relatively small piece. Another way to think of map scale is that items in small-scale maps appear small, whereas the same items in large-scale maps appear large.
Computer maps may have a varying scale that changes according to the zoom¯ level of the view. The more zoomed in, or closer you are to the earth, the larger the depicted scale.
The surface of the globe is divided into a spherical grid for the convenience of finding certain points. The grid consists of imaginary lines called latitude and longitude. Latitude is a series of concentric circles paralleling the Equator and extending to both poles. Longitude is a series of meridians, or longitudinal lines drawn between the poles at regular intervals that pass perpendicularly through the Equator. Where a particular latitude crosses a particular longitude, a pair of numbers, or coordinates, can be assigned. Every point on the earth has a set of coordinates that indicate its position relative to every other point.
Latitude is measured from zero at the Equator to 90 degrees north and south at the poles. Longitude is measured from zero to 180 degrees west and east. The reference lines for counting are the Equator, for latitude, and a line drawn through Greenwich in England, the prime meridian, for longitude. These are the zero lines. A degree of latitude is equivalent to about 112 kilometers (about 70 miles). Because longitudinal lines converge toward the poles, degrees of longitude vary according to the position on the earth. At the equator, one degree of longitude is the same length as one degree of latitude, and at the north and south poles, the distance between degrees of longitude is zero.
Degrees are divided into 60 minutes, and each minute is divided into 60 seconds. For example, the Eiffel Tower in Paris has the following coordinates: latitude 48° 51' 32' north and 2° 17' 35' east. Sometimes, coordinates are expressed in decimal minutes instead of minutes and seconds, so the coordinates of the Eiffel Tower can also be written as 48° 51.5333 north latitude and 2° 17.5833 east longitude. Most official maps indicate latitude and longitude, so viewers know exactly what part of the earth the map represents.
Some maps have other special-purpose coordinate systems, such as the State Plane Coordinate System used on maps in the United States or the Universal Trans-Mercator (UTM) system used on many military maps.
Maps use sets of symbols to indicate the placement of real objects. The legend is a block of text or a window in which the symbols used on the map are explained. Legend symbols can include icons to represent buildings, different colors to indicate elevation, different types of lines to indicate borders or roads of varying size, and dots and circles to show the relative population of towns and cities. If the details of a map look unfamiliar, take a moment to study the legend before proceeding further.
Direction: Which Way Is Up?
Most maps give a reference point to indicate how a direction on the map corresponds to a direction in the real world. This is crucial when using the map to travel between points. A good map indicates a cardinal direction for such orientation, usually by an arrow pointing north. Maps from past centuries used various cardinal directions. Some older European maps placed East at the top, pointing to the area then known as the Orient, leading to the term orientation. Old Muslim maps put South facing upward. Modern maps usually adopt the convention that the top of the map corresponds to North, the bottom to South, the left edge to West, and the right edge to East. Direction can also be determined from coordinates, if they are shown. All views are oriented with North as up except when the map is centered on the North Pole or South Pole.
The poles representing the rotational axis of the earth do not correspond to the magnetic poles, the direction a compass points. This is because the magnetic poles constantly change position or wander. The north-pointing arrow on many accurate maps is divided into two parts, one indicating polar and one indicating magnetic north. The angular difference between these is known as the map's magnetic declination. For example, according to a 1987 map of Moscow, the compass points to magnetic north at 7° 46' to the right of true polar north, so the magnetic declination according to this map is 7Â° 46' east. The declination changes with location on the globe and it also changes with time as the magnetic poles wander. Some localities have a change in magnetic declination of several minutes per year. Lines of longitude are oriented toward the rotational axis of the earth. Digital maps are made in reference to this axis and usually ignore magnetic north.
The Ups and Downs of Maps: Elevation
Topography adds a third dimension to the flat-map picture of the world. Cartographers use different techniques to indicate topography, which means the hills and valleys of the surface of the earth. Early maps used bars, or lines of overlapping triangles to show hills or mountain ranges. A few ancient maps, including a Buddhist map from 14th century Japan, show mountains as artistic, three-dimensional figures. Symbols such as hatched or spoked symbols were also used on some European maps. Modern maps show mountains in shaded relief, called hill shading. Traditional topographic maps use concentric lines, called hypsographic lines, to indicate elevation. Each line is assigned a height above sea level. Corresponding lines indicating ocean depth are called hydrographic lines.
Instead of concentric lines, color maps often use a standardized color scale to indicate elevation: Sea level is blue, low land elevations are shades of green, higher elevations range from tan to brown, and the highest peaks are shown in white, suggesting snow. Deeper shades of blue correspond to deeper parts of lakes or oceans.
Learning to read maps is easy and intuitive. A person can use the map skills discussed in this Sidebar to solve navigation problems, plan future activities, or go on a virtual pleasure trip anywhere in the world. 11 October 2010Comment