Map Projections

The best way to represent earth is a globe. Earth is close to a sphere (or an ellipsoid) but it’s not actually one. But globes are hard to carry in your backpack, you can only see one side of the globe, it’s hard to measure distances and they’re just not as convenient as paper maps.

What is a Map Projection?

A map projection is a systematic transformation of the latitudes and longitudes of locations on the surface of a sphere or an ellipsoid into locations on a plane. Map projections are necessary for creating maps.

Carl Friedrich Gauss in Theorema Egregium proved that a sphere’s surface cannot be represented on a plane without distortion. The same applies to other reference surfaces used as models for the Earth. Since any map projection is a representation of one of those surfaces on a plane, all map projections distort. Every distinct map projection distorts in a distinct way. Depending on the purpose of the map, some distortions are acceptable and others are not; therefore, different map projections exist in order to preserve some properties of the sphere-like body at the expense of other properties. There is no limit to the number of possible map projections.

Tissot’s Indicatrix

The classical way of showing the distortion inherent in a projection is to use Tissot’s indicatrix. It is the geometry that results from projecting a circle of infinitesimal radius from a curved geometric model, such as a globe, onto a map. Tissot proved that the resulting diagram is an ellipse whose axes indicate the two principal directions along which scale is maximal and minimal at that point on the map.

A single indicatrix describes the distortion at a single point. Because distortion varies across a map, generally Tissot’s indicatrices are placed across a map to illustrate the spatial change in distortion. A common scheme places them at each intersection of displayed meridians and parallels. These schematics are important in the study of map projections, both to illustrate distortion and to provide the basis for the calculations that represent the magnitude of distortion precisely at each point.

Tissot_mercator.png
Tissot’s Indicatrices on the Mercator’s projection on QGIS

Classification of the projections

A fundamental projection classification is based on the type of projection surface onto which the globe is conceptually projected. The projections are described in terms of placing a gigantic surface in contact with the earth, followed by an implied scaling operation. These surfaces are cylindrical (e.g. Mercator), conic (e.g. Albers), or azimuthal or plane (e.g. stereographic). Many mathematical projections, however, do not neatly fit into any of these three conceptual projection methods. Hence other peer categories have been described in the literature, such as pseudoconic, pseudocylindrical, pseudoazimuthal, retroazimuthal, and polyconic.

Another way to classify projections is according to properties of the model they preserve. Some of the more common categories are:

  • Preserving direction (azimuthal or zenithal), a trait possible only from one or two points to every other point
  • Preserving shape locally (conformal or orthomorphic)
  • Preserving area (equal-area or equiareal or equivalent or authalic)
  • Preserving distance (equidistant), a trait possible only between one or two points and every other point
  • Preserving shortest route, a trait preserved only by the gnomonic projection

Because the sphere is not a developable surface, it is impossible to construct a map projection that is both equal-area and conformal.

Some examples on QGIS

 

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