Maps, Scale and Projections

The One Thing

Maps are selective, scaled and symbolically represented models of the Earth rather than exact copies of reality. Their usefulness depends on choosing an appropriate scale and projection while recognising unavoidable distortions of distance, direction, area or shape.

Who and What

• Map: A scaled and generalized representation of the Earth's surface, or part of it, on a flat surface. Maps are essential for locating places, understanding spatial relationships and analysing geographical patterns.
• Globe: A three-dimensional model of the Earth that represents its spherical shape with relatively little distortion.
• Sketch: A rough drawing of an area made without accurate scale or precise measurements.
• Plan: A detailed drawing of a small area, usually prepared on a large scale.
• Map scale: The ratio between a distance on a map and the corresponding distance on the ground. The basic formula is:

Scale = Distance on map / Corresponding distance on ground.

• Representative Fraction: A numerical scale written as a ratio, such as 1:50,000, meaning one unit on the map represents 50,000 of the same units on the ground.
• Statement scale: A scale expressed in words, such as 1 centimetre represents 1 kilometre.
• Graphic or linear scale: A graduated line or bar showing ground distances. It remains useful when a map is enlarged or reduced proportionally.
• Large-scale map: A map showing a small area in considerable detail, such as a town plan or cadastral map. A scale such as 1:10,000 is large scale.
• Small-scale map: A map showing a large area with less detail, such as a map of a country or the world. A scale such as 1:1,000,000 is small scale.
• Map projection: A mathematical or geometrical method of representing the curved Earth on a flat surface.
• Globe network: The system of parallels and meridians drawn on a globe or transferred to a map through a projection.
• Latitude: The angular distance of a place north or south of the Equator, measured in degrees. The Equator is 0° latitude, and latitude increases to 90° north and 90° south.
• Longitude: The angular distance of a place east or west of the Prime Meridian, measured in degrees. The Prime Meridian is 0° longitude, and longitude extends to 180° east and 180° west.
• Parallels of latitude: Imaginary circles running east-west that remain parallel to the Equator.
• Meridians of longitude: Imaginary semicircles joining the North Pole and South Pole. They converge at the poles.
• Cylindrical projection: A projection in which the Earth's surface is transferred onto a cylinder, commonly useful for equatorial and world maps.
• Conical projection: A projection in which the Earth's surface is transferred onto a cone, generally useful for middle-latitude regions.
• Zenithal or azimuthal projection: A projection in which the Earth's surface is transferred onto a plane, often useful for polar areas or maps centred on a particular place.
• Equal-area projection: A projection preserving the relative area of regions, although shape or distance may be distorted. It is useful for comparing the size of regions.
• Conformal projection: A projection preserving local angles and shapes over small areas, but it may greatly distort area. It is useful for navigation and some large-scale mapping.
• Equidistant projection: A projection preserving distances accurately from one or more selected points or lines, but not from every location.
• True-direction projection: A projection preserving direction accurately from a selected centre or along particular lines.
• General-purpose map: A map showing several physical and cultural features, such as relief, rivers, roads, settlements and boundaries.
• Thematic map: A map showing the distribution or pattern of one particular subject, such as rainfall, population or crops. It may use dots, lines, graduated symbols, colours, isolines or choropleth shading.
• Topographic map: A detailed map showing natural and cultural features, including relief, drainage, settlements, transport routes and land use.
• Conventional signs: Standard symbols representing features such as roads, rivers, settlements and boundaries.
• Map legend: A key explaining the symbols, colours and patterns used on a map.
• Grid: A network of intersecting lines used to locate places and measure positions on a map.
• Generalisation: The reduction of detail and selection of important features when preparing a map at a smaller scale.
• Distortion: The unavoidable change in the shape, area, distance or direction of Earth's features when the curved surface is shown on a plane.

Causes and Consequences

• Because the Earth is nearly spherical, no flat map can preserve area, shape, distance and direction perfectly everywhere. Every map therefore involves distortion.
• Because maps are selective models rather than exact copies of reality, their content depends on purpose, scale and the information considered important by the map-maker.
• Because a larger-scale map represents a smaller area, it can show greater detail. A smaller-scale map covers a larger area but requires greater generalisation.
• Because scale describes the reduction of distance, it must be distinguished from projection, which describes how the curved Earth is transferred to a flat surface.
• For a scale of 1:50,000, 1 centimetre on the map represents 50,000 centimetres, or 500 metres, on the ground. Ground distance can be calculated using:

Ground distance = Map distance / Representative fraction denominator, when both distances use the same units.

• Unit conversion is necessary when interpreting scale: 100 centimetres = 1 metre, 1,000 metres = 1 kilometre, and therefore 100,000 centimetres = 1 kilometre.
• Because different regions and purposes require different properties to be preserved, no single projection is best for all purposes. Projection choice depends on the location, extent and shape of the mapped region and on whether area, shape, distance or direction is most important.
• Cylindrical projections are commonly suitable for areas near the Equator, conical projections for middle latitudes, and zenithal or azimuthal projections for polar or centred regions.
• Equal-area projections preserve relative area and therefore support comparisons of regional size, but they may distort shape or distance.
• Conformal projections preserve local angles and shapes, which supports navigation and some large-scale mapping, but they may exaggerate area.
• Equidistant and true-direction projections preserve selected distances or directions rather than preserving them accurately from every location.
• Because map users need to interpret both information and limitations, a map should normally include a title, scale, direction or north arrow, legend, grid or location reference, and source where appropriate.
• Maps classified by scale, purpose, content and method of representation support navigation, planning, resource management, disaster response, environmental study and regional comparison.
• Accurate map reading requires understanding the scale, symbols, orientation, grid, projection and limitations of the map.

What Gets Asked

• Compare a globe, map, sketch and plan in terms of dimensionality, accuracy, scale and purpose.
• Explain the difference between large-scale and small-scale maps, including the relationship between scale, area covered, detail and generalisation.
• Calculate ground distance using a Representative Fraction, including the scale 1:50,000, and apply the conversions between centimetres, metres and kilometres.
• Distinguish map scale from map projection and explain why transferring the spherical Earth to a flat surface produces distortion.
• Compare cylindrical, conical and zenithal or azimuthal projections in relation to the regions for which they are most suitable.
• Evaluate equal-area, conformal, equidistant and true-direction projections according to the properties they preserve and distort.
• Explain how conventional signs, a map legend, grid, direction or north arrow, scale and source contribute to accurate map interpretation.
• Distinguish general-purpose, thematic and topographic maps, including the use of dots, lines, graduated symbols, colours, isolines and choropleth shading on thematic maps.