Map projection

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Claudius Ptolemy- The World

USGS map Albers conic tall

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Tissot mercator

Usgs map miller cylindrical

Usgs map traverse mercator

Usgs map space oblique mercator

Map projection refers to the systematic transformation of the latitudes and longitudes of locations on the surface of a sphere or an ellipsoid into locations on a plane. The process is fundamental in the field of cartography, as the Earth is roughly spherical in shape and maps are typically made on flat surfaces. Map projections are used to represent the curved surface of the Earth on the flat surface of a map. Each map projection distorts the surface in some way and choosing a projection involves balancing those distortions given the purpose of the map.

Types of Map Projections

Map projections can be classified into several types based on the projection surface used (planar, cylindrical, and conical) or the property of the map (conformal, equal-area, equidistant, and azimuthal).

Cylindrical Projections

In cylindrical projections, the surface of the Earth is projected onto a cylinder. When the cylinder is unrolled, it becomes a map. A well-known example is the Mercator projection, which is conformal, preserving angles and shapes but distorting size, especially near the poles.

Conical Projections

Conical projections involve projecting the Earth's surface onto a conical surface, which is then unrolled into a flat map. These projections are often used for mapping small to medium-sized countries in the mid-latitudes. The Albers projection is an example of an equal-area conical projection.

Azimuthal Projections

Azimuthal or planar projections project the Earth's surface onto a flat plane. They are often used for polar maps. The distance or direction from the central point on the map to any other point is preserved.

Equal-Area Projections

Equal-area projections, also known as equivalent or authalic projections, maintain area across the map. This means that the size of any area on the map is in true proportion to its size on the Earth. The Lambert Azimuthal Equal-Area projection is an example.

Conformal Projections

Conformal projections preserve angles, making them useful for navigational purposes. However, they cannot preserve scale everywhere. The Mercator projection is a well-known conformal projection.

Equidistant Projections

Equidistant projections maintain scale along specific lines or from a specific point, making distances true only from or along those lines or points. The Equirectangular projection is an example where distances along the equator and meridians are true to scale.

Compromise Projections

Compromise projections attempt to minimize errors without fully preserving any particular property. The Robinson projection is an example, widely used for world maps because it offers a good balance between size and shape.

Choosing a Map Projection

The choice of a map projection depends on the purpose of the map. For example, navigational maps often use conformal projections to preserve angles, while thematic maps that display phenomena like population density may use equal-area projections to show true sizes.

Distortions in Map Projections

All map projections introduce some form of distortion because it is impossible to flatten a sphere or ellipsoid without stretching, compressing, or tearing it. The type of distortion varies with the projection used and includes distortions of shape, area, distance, and direction.

Conclusion

Map projections are a crucial aspect of cartography, enabling the representation of the Earth's three-dimensional surface on two-dimensional maps. While no map projection can perfectly preserve all geographic properties, the variety of projections available allows cartographers to choose the most appropriate one for their specific needs.