Tourist map of Stavanger, Norway A map is a representation of space that allows us to see the relationship between places, objects or themes. The space shown on a map may be on the Earth’s surface, beneath its surface or oceans, in outer space—or entirely imaginary. Most maps are two-dimensional (folded sheets and atlases), but some are three-dimensional (globes and models) or digital (on satnavs or websites). We think of modern maps as being accurate, but in fact all maps contain inaccuracies and omissions, most of them deliberate. Map-makers, called cartographers, carefully choose what information to include and what to leave out of a map, depending on the map’s intended use and theme. In pictorial maps, often created for tourists, pictures of landmarks, neither accurate nor to scale, nevertheless allow the user to recognize them quickly—which is the intention of the map.
Types of maps
Among the most commonly used maps are transport maps, which help the user to navigate. Transport maps include: road, bicycle, hiking and train or bus route maps, and nautical and aeronautical charts for navigation at sea or in the air respectively. Details helpful to the user, such as road types, tolls and petrol stations for a car driver, are included, while other, unnecessary details are left out. Many city maps, for use with a range of modes of transport, offer wide-ranging information about streets, building types and public transport. Plans of buildings, such as museums, allow the visitor to navigate a small area.
Smaller-scale maps, which show a larger area, may represent a country, region or the world. Such maps may be political, with the main purpose of showing boundaries of countries, states and counties, or physical, depicting landforms and relief. Many general-purpose maps are combinations of the two.
Topographic maps show the surface physical features of an area, while geological maps show the rocks and fault lines. Bathymetric maps show the terrain beneath the surface of oceans or other water. Extraterrestrial maps include star charts for use by astronomers.
A thematic map is one that focuses on a specific subject, for example weather or population density. The map's coastlines, boundaries and places act only as points of reference in a thematic map. Thematic maps may show the geographical variations in the natural world, for example, climate, weather, soils, vegetation and mineral resources. Alternatively, they may cover human characterstics such as population density, languages, religions or farming types.
Historical maps, ranging from small-scale maps showing the changing areas of kingdoms and empires, to large-scale plans featuring the movement of troops in battle, are types of thematic maps.
Many maps are drawn to scale: a distance on the map represents an actual distance on the ground. Scale is often expressed in the form of a ratio. For example, 1:10,000 means that 1 centimetre on the map is the equivalent of 10,000 centimetres on the ground. A map of this scale is what is known as a large-scale map, showing a relatively small area, such as a town. Small-scale maps show larger areas, such as countries or continents, and may have a scale ratio of, say, 1:5,000,000. A bar scale, showing the map's scale as a line with marked intervals, is usually included in the key.
Most maps do not keep to their scale with complete accuracy. For example, a road map draws roads wider than they should be, so they can be seen clearly. Topological maps do not keep to a scale at all, although the relationships between features on the map are still true to reality. Common examples of topological maps are train and bus route maps, where the distance between stations or stops is stretched or shrunk to present the information simply.
A map’s orientation is the way in which the top of the map relates to compass directions in reality. The most common orientation for a map is north—the top of the map corresponds to north. However, many maps have different orientations: for example, “south-up” maps are sometimes made in the southern hemisphere, and vehicle satnav maps always keep the direction in which the vehicle is moving at the top. A map’s orientation is often shown by a compass "rose" or pointer marked on the map.
Some maps feature a grid, imaginary lines criss-crossing the map area, which helps the user to follow both orientation and scale. Grid squares may relate to lines of latitude and longitude, or an area devised by the cartographer. Geographic coordinates, given numerically as grid references or in degrees of latitude and longitude, can locate a point precisely on a map.
In other maps, the grid is marked with letters and numbers. With the aid of an index or key, the user can locate a street or feature within a grid square using grid references in the form “B6”, for example.
Mercator (left) and Mollweide projections The Dymaxion map projectionSince the Earth is a sphere, a key issue for cartographers is how to represent its three dimensions on a flat surface. Over the centuries, cartographers have come up with many projections: systems of representing the sphere as a flat plane. All projections are distortions to some extent, particularly in small-scale maps, which show a large area of the Earth's surface. Some or all of the distances, areas, directions and shapes must be stretched or shrunk.
In the past, a commonly used projection was the Mercator, which greatly enlarged the area of regions closer to the Poles, making Greenland look larger than Australia, which is in fact three and half times bigger than Greenland. In contrast, the Mollweide projection keeps true area but distorts shape.
Labels, symbols and keys
Important features on maps are usually labelled, with different styles to show the importance or type of feature: for example, capital letters for country names and italics for rivers. A system of symbols and colours is used to mark features, road types and land use. For example, a cross is often used to mark a church, a barred line to show a railway, and green for a park or forest. These symbols and markings are usually explained in a key at the edge of the map or at the front of an atlas. In a digital interactive map, the user can decide which features will be shown or labelled, such as restaurants or car parks.
Representing three dimensions
Topographic (left) and relief maps Mountains, valleys and buildings are three-dimensional. Some maps try to give the user a sense of the three-dimensional space represented on the map. Topographic maps show relief (height) using contour lines. Contour lines link points with the same height. They are usually drawn at the same height intervals, so that when they lie close together, they indicate a steep slope. Relief maps often include layer colours between contours and additional shading to give a visual impression of "hilliness" in upland areas.
Axonometric maps use ground and aerial photography, along with building plans, to create digital “models” of cities. Axonometric maps are often used in navigation apps and can be interactive, so the user feels as if they are “walking around” particular buildings.