2 Maps, Scale, and Map Projections

Map types and purposes

Maps are designed representations of places. They simplify the world and use symbols and spatial relationships to help people locate features, compare patterns, and communicate geographic information. A map is useful when its type, scale, symbols, and projection suit the question being asked.

• Reference maps show where features are located, including roads, rivers, place names, and boundaries. Street maps and many general-purpose maps are examples.

• Thematic maps emphasize a particular subject or pattern, such as population density, rainfall, or land use. Their symbols can show categories, quantities, or values across space.

• Topographic maps represent landforms and elevation, commonly with contour lines, alongside features such as roads and water.

• Navigation charts support travel by water or air and emphasize information needed for navigation.

A map can combine purposes: a topographic map may also serve as a reference map. Its intended use determines which information should be prominent.

Elements that help readers interpret maps

Map elements give readers the information they need to interpret a map. Common elements include:

• Title: identifies the mapped place or subject.

• Legend (key): explains symbols, colors, and line styles.

• Scale: relates map distance to ground distance.

• Orientation: indicates direction, often with a north arrow or compass rose. North may refer to true north, grid north, or magnetic north; these are not always identical.

• Coordinate grid or graticule: helps locate places using a grid or latitude and longitude.

• Date and data sources: indicate when the map or its data were created and where the information came from.

• Labels and symbols: identify places and represent features that may be too small or complex to draw realistically.

Not every map needs every element. For example, a north arrow may be unnecessary when the map clearly states its orientation. The goal is to provide enough information for readers to use the map correctly.

Scale and map detail

Map scale relates a distance measured on the map to the corresponding distance on the ground. It can be expressed in three common ways:

• Representative fraction: a ratio such as $1:50{,}000$, meaning that one unit on the map represents $50{,}000$ of the same units on the ground. Thus, $2$ centimeters on a $1:50{,}000$ map represents $100{,}000$ centimeters, or $1$ kilometer, on the ground.

• Verbal scale: a written statement such as “$1$ centimeter represents $500$ meters.”

• Graphic scale: a marked bar that represents ground distance.

Scale and level of detail

A large-scale map has a smaller denominator in its representative fraction and usually shows a smaller area in greater detail. A small-scale map has a larger denominator and generally shows a larger area with less detail. For example, a neighborhood map at $1:10{,}000$ is larger-scale than a regional map at $1:1{,}000{,}000$. These descriptions concern the map’s ratio, not the physical size of its paper or screen.

A graphic scale is especially useful if a printed map is resized: its bar changes size along with the map, while a printed ratio or verbal scale may no longer be accurate. On digital maps, zooming changes the scale and often the amount of detail displayed.

Projections and spatial distortion

Earth’s surface is curved, while a flat map is two-dimensional. A map projection is the mathematical method used to represent locations from the curved Earth on a flat surface. Flattening a curved surface necessarily distorts at least one spatial property. Depending on the projection, distortion affects area, shape, distance, or direction; no flat world map preserves all four everywhere.

Properties different projections prioritize

• Conformal projections preserve local angles and shapes of small features, but can distort area. The Mercator projection is useful for some navigation tasks because constant compass bearings appear as straight lines. However, it greatly enlarges high-latitude areas: Greenland, for instance, appears much larger relative to equatorial regions than it is.

• Equal-area projections preserve relative area, making them useful for maps that compare quantities across regions. The Albers Equal Area Conic projection is often used for maps of the contiguous United States. Shapes may be distorted, especially over large areas.

• Equidistant projections preserve distance only from particular points or along specified lines, not every distance between every pair of places. They can be useful when distances from a chosen center matter.

• Compromise projections balance several distortions without perfectly preserving area, shape, distance, or direction. Robinson is one example, often chosen for general world maps.

Projections are also commonly grouped by geometric construction, such as cylindrical, conic, or azimuthal, and by orientation. These categories describe how a projection is built, but do not by themselves tell you which spatial properties it preserves.

Choosing and interpreting a projection

Choose a projection according to the map’s purpose, geographic extent, and location. A world map must manage distortion over a very large area, while a local map may have little noticeable distortion at its working scale. A projection suited to a north–south region may not be the best choice for an east–west region.

Consider what the map will be used to measure or compare. An equal-area projection is appropriate for comparing regional areas, while a conformal projection may suit navigation or detailed local mapping. Projection choices affect how spatial information appears and how measurements should be interpreted: a projection that enlarges some regions can mislead readers comparing their apparent sizes, and a distance-preserving property limited to one line or center does not make all map distances reliable.

In GIS, data displayed in different coordinate systems may need to be transformed into a common system for consistent overlay and analysis. Check the projection and its intended use before comparing areas, shapes, or distances.