A map projection is a mathematical formula used to convert the three-dimensional surface of the earth to a two-dimensional surface, such as a map. Because the earth's surface is curved, it must be squeezed or stretched to fit into the same area on a map's flat surface. The projection process always distorts shape, area, distance, and/or direction.
There are many map projections and each one is good at representing one or more spatial properties, but no projection can preserve all four properties. Because spatial properties are often used to make decisions, anyone who creates or uses maps should know which projections distort which properties and to what extent. For example, an Equal-Area Cylindrical projection gives you accurate area calculations but inaccurate shapes, a Mercator projection maintains true direction but sacrifices accuracy of area and distance, and a Robinson projection is a compromise of all the properties. The projection you choose significantly affects the properties of a small-scale map, but has less effect on the properties of a large-scale map.
With Equal-area projections, the areas of all regions are shown in the same proportion to their true areas. If a coin is placed on any area of such a map, it will cover as much of the area on the surface of the sphere as it would if it were placed elsewhere on the map. Shapes, angles, and scale must be distorted on most parts of such a map.
Equidistant projections maintains constant scale along all great circles (shortest distance between any two points) from one or two points. It is not possible to preserve distances (scale) correctly throughout a map projection. Additionally, no flat map can be both equidistant and equal-area.
In Conformal projections, at any point the scale is constant in every direction. Meridians and parallels intersect at right angles, just as they do on the earth. The shapes of very small areas and angles with very short sides are preserved. No map can be both equal-area and conformal.
Note: Information for this section was taken from U.S. Geological Survey (USGS), Datums And Projections: A Brief Guide.
The following brief explanation of some common map projections is not a comprehensive review and is intended to be used only as a guide. Deciding which map projection to use is determined largely by the specific properties of the projections, as well as what is being mapped and where it is on the earth. For example, although the Mercator projection is often used for world mapping, it does not preclude it from being used for regions on or close to the equator with a predominately east-west extent.
MapStudio does not support all of the following projections. Some are mentioned only for their unique characteristics and potential usefulness in desktop mapping software such as ESRI's ArcView. Projections available in MapStudio are noted in bold.
Conic, equal-area. Available in MapStudio. Used by the USGS to map the 48 contiguous states or other large areas of the United States. Good for mapping large countries or areas with an east–west extent and that require equal-area representation. Similar in appearance to the Lambert Conformal Conic; however, the spacing of the graticule (system of latitude and longitude lines) is different.
Scale: True only along the chosen standard parallels (standard parallels are those parallels free of distortion).
Distortion: Free of distortion along either of the two chosen parallels. As an equal-area projection, all areas are proportional to the same areas on the earth.
Usage: Commonly used for maintaining an equal-area representation of large countries or regions with a mainly east–west extent. Also used for many thematic maps due to its equal-area characteristics.
Conic, neither conformal or equal-area. Available in MapStudio. This projection is a compromise between the Albers Equal-Area and Lambert Conformal Conic, and as such is neither conformal, equal-area, nor perspective. A good projection to use when mapping a region within a few degrees of latitude and entire area is on one side of the equator.
Scale: True only along the chosen standard parallels and along all meridians. Standard parallels are those free of distortion.
Distortion: Free of distortion along either of the two standard parallels, but increases further away. Distortion is a compromise between equal-area and conformal.
Usage: Commonly used for mapping areas in the middle latitudes and for small countries.
Azimuthal, conformal, perspective. Available in MapStudio. Suited for mapping areas that extend equally in all directions from a center point. Examples include Asia, Pacific Ocean, polar regions, and other areas where equal-area representation is desired and that are approximately circular in nature. Usually only one hemisphere is portrayed. Can be represented in three aspects: polar, equatorial, and oblique.
Scale: True scale only at the center point and increases further away from the center; directions are true only from the center point; any straight line drawn through the center is a great circle.
Distortion: Only the center is free from distortion; distortion of areas and shapes increases away from the center point.
Usage: Commonly used for polar regions and hemispheres requiring equal-area representation. The equatorial aspect is used for mapping either the Eastern or Western Hemisphere. The oblique aspect can be used to map continents and oceans.
Conic, conformal. Available in MapStudio. Probably the most widely used map projection in the United States. It is similar to Albers Equal-Area Conic; however, the spacing of the graticule is different. Used by the USGS as its base projection for the 7.5-minute and 15-minute topographic map series, as well as the U.S. State Base Map series for the 48 contiguous states.
Scale: True only along the chosen standard parallels. Standard parallels are those free of distortion.
Distortion: Free of distortion along either of the two chosen parallels. Distortion of shapes and areas is minimal surrounding the standard parallels, but increases further away.
Usage: Used extensively for large-scale mapping, for example USGS topographic and State Base Map series. Also useful for mapping countries or regions with a mainly east–west extent.
Cylindrical, conformal. Available in MapStudio. May be the most commonly (and incorrectly) used projection for world maps. An excellent projection for equatorial regions, otherwise the Mercator projection is primarily intended for navigation. Extreme area distortion in the polar regions.
Scale: True scale along the equator, scale increases with distance from the equator to infinity at the poles (poles cannot be shown), scale is constant along any given parallel.
Distortion: Very minimal area distortion along the equator, great area distortion in the polar regions.
Usage: Designed and recommended for navigation because of straight rhumb lines. Standard for marine charts and recommended and used for regions predominantly bordering the equator. Often and incorrectly used for world atlases and wall maps.
Note: All rhumb lines (lines of constant compass bearing) are straight lines.
Cylindrical, neither conformal nor equal-area. Available in MapStudio. Rectangular map projection used to represent the world in atlases and wall maps. Similar in appearance to the Mercator projection but not useful for navigation and does show the poles. Sometimes viewed as a compromise projection to the Mercator when in need of a rectangular world map.
Scale: True scale only along the equator.
Distortion: No distortion at the equator; shape, area, and scale distortion increases away from the equator, becoming extreme in high latitudes; distortion in polar regions not as severe as the Mercator projection.
Usage: Popular in atlases and world maps requiring a rectangular projection with less distortion than the Mercator, especially near the polar regions.
Note: The Mercator and Miller projections are almost identical near the equator.
Cylindrical, conformal. Oblique aspect of the Mercator projection. The Oblique Mercator projection is based on a cylinder placed tangent to the globe on any great circle other than the equator or a meridian. This projection can be used to show a straight line (such as a flight path), the shortest distance between two points along a great circle selected by the mapmaker.
Scale: True scale only along the great circle (line of tangency) or along two lines parallel to it. Distances, directions, areas, and shapes are fairly accurate within 15 degrees of the great circle.
Distortion: Increases rapidly outside the 15 degree band of tangency (great circle) selected for the projection.
Usage: Useful for regions that have a general extent that is oblique to the equator, neither east–west nor north–south, such as Hawaii; also used for plotting linear paths that are situated along a line oblique to earth's equator, such as NASA satellite tracking maps.
Note: Rhumb lines (lines of constant compass bearing) are curved.
Azimuthal, perspective; neither conformal nor equal-area. Popular projection to represent the earth as it would appear from space. Widely used for perspective views of earth, moon and other planets. Can be represented in three aspects: polar, equatorial, and oblique.
Scale: Decreases along all lines radiating from the center point of the projection. Any straight line through the center point is a great circle. Directions are true only from the center point of the projection.
Distortion: Only the center is free of distortion, while distortion increases away from the center and becomes extreme along the edges.
Usage: Especially useful when requiring perspective views of the earth and other planets from space. Used by the USGS in the National Atlas of the United States.
Pseudocylindrical, neither conformal nor equal-area. Available in MapStudio. Popular projection for world maps, uses tabular coordinates rather than mathematical formulas to make the map look right. Has become the de facto world projection for many atlases and wall maps.
Scale: True scale along latitudes 38 degrees north and south; scale is constant along any given latitude.
Distortion: No point is completely free of distortion, but distortion is very low along the equator and within about 45 degrees of the center; distortion is greatest near the poles.
Usage: Widely used in numerous atlases and wall maps; used by Rand McNally's Goode's World Atlas for thematic world maps and adopted by National Geographic in 1988 for its world maps.
Pseudocylindrical, equal-area. Has been used as a world equal-area projection to show distribution patterns. Also used to map very large areas with a mainly north–south extent. Widely used in the interrupted version.
Scale: True along the central meridian and all parallels.
Distortion: Free of distortion along the central meridian and equator, becoming quite severe in the outer meridians and near the poles. Interrupting the projection and using several central meridians can significantly reduce distortion.
Usage: Can be used to map Africa, South America, and other large areas with a predominately north–south extent. Also occasionally used to show world distribution patterns.
Azimuthal, conformal, perspective. Available in MapStudio. Most widely used azimuthal projection, mainly for portraying large, continent-size areas of similar extent in all directions. Usually only one hemisphere is portrayed. Can be represented in three aspects: polar, equatorial, and oblique.
Scale: True scale only at the center point and increases further away from the center; direction is true only from the center point; any straight line through the center point is a great circle.
Distortion: Only the center is free from distortion; distortion of areas and large shapes increases away from the center point.
Usage: Commonly used in the polar aspect for topographic maps of polar regions and navigation above 80 degrees latitude. Oblique aspects are used to show paths of solar eclipses. Recommended for regions approximately circular in shape (i.e., polar regions). Used by the USGS for maps of Antarctica.
Cylindrical, conformal. Available in MapStudio. Transverse aspect of the Mercator projection. The Transverse Mercator projection is tangent along a meridian (as opposed to the equator for the Mercator projection). Used as the main projection by the USGS's topographic map series. Also useful where north–south extent of a region is greater than the east–west extent.
Scale: True scale only along the central meridian (line of tangency) selected by the mapmaker, or else along two lines parallel to it; all distances, directions, shapes, and areas are reasonably accurate within 15 degrees of the central meridian.
Distortion: Increases rapidly outside the 15-degree band along the central meridian (line of tangency); shapes and angles within any small area are essentially true (conformality).
Usage: Recommended for mapping of regions with a predominantly north–south extent; used as the main base projection for USGS topographic maps; basis for the Universal Transverse Mercator (UTM) projection and the State Plane Coordinate System in states having a north–south extent.
Note: Unlike the Mercator projection, rhumb lines (lines of constant compass bearing) generally are not straight lines on the Transverse Mercator.
World: Mercator, Miller, and Robinson
Hemisphere: Lambert Azimuthal Equal-Area and Stereographic
Continent, ocean, and region: Albers Equal-Area Conic, Equidistant Conic (Simple Conic), Lambert Azimuthal Equal-Area, Lambert Conformal Conic, Stereographic, and Transverse Mercator
Medium-scale and large-scale: Lambert Conformal Conic, Sterographic, and Transverse Mercator