Conformal map projection

In cartography, a conformal map projection is one in which every angle between two curves that cross each other on Earth (a sphere or an ellipsoid) is preserved in the image of the projection, i.e. the projection is a conformal map in the mathematical sense. For example, if two roads cross each other at a 39° angle, then their images on a map with a conformal projection cross at a 39° angle.

Properties

A conformal projection can be defined as one that is locally conformal at every point on the Earth. Thus, every small figure on the earth is nearly similar to its image on the map. The projection preserves the ratio of two lengths in the small domain. All Tissot's indicatrices of the projections are circles.

Conformal projections preserve only small figures. Large figures are distorted by even conformal projections.

In a conformal projection, any small figure is similar to the image, but the ratio of similarity (scale) varies by location, which explains the distortion of the conformal projection.

In a conformal projection, parallels and meridians cross rectangularly on the map. The converse is not necessarily true. The counterexamples are equirectangular and equal-area cylindrical projections (of normal aspects). These projections expand meridian-wise and parallel-wise by different ratios respectively. Thus, parallels and meridians cross rectangularly on the map, but these projections do not preserve other angles; i.e. these projections are not conformal.

List of conformal projections

Applications

Large scale

Many large-scale maps use conformal projections because figures in large-scale maps can be regarded as small enough. The figures on the maps are nearly similar to their physical counterparts.

A non-conformal projection can be used in a limited domain such that the projection is locally conformal. Glueing many maps together restores roundness. To make a new sheet from many maps or to change the center, the body must be re-projected.

Seamless online maps can be very large Mercator projections, so that any place can become the map's center, then the map remains conformal. However, it is difficult to compare lengths or areas of two far-off figures using such a projection.

The Universal Transverse Mercator coordinate system and the Lambert system in France are projections that support the trade-off between seamlessness and scale variability.

For small scale
A contour chart of scale factors of GS50 projection

Maps reflecting directions, such as a nautical chart or an aeronautical chart, are projected by conformal projections. Maps treating values whose gradients are important, such as a weather map with atmospheric pressure, are also projected by conformal projections.

Small scale maps have large scale variations in a conformal projection, so recent world maps use other projections. Historically, many world maps are drawn by conformal projections, such as Mercator maps or hemisphere maps by stereographic projection.

Conformal maps containing large regions vary scales by locations, so it is difficult to compare lengths or areas. However, some techniques require that a length of 1 degree on a meridian = 111 km = 60 nautical miles. In non-conformal maps, such techniques are not available because the same lengths at a point vary the lengths on the map.

In Mercator or stereographic projections, scales vary by latitude, so bar scales by latitudes are often appended. In complex projections such as of oblique aspect. Contour charts of scale factors are sometimes appended.

Sources
  • Snyder, John P. (1989). An Album of Map Projections, Professional Paper 1453 (PDF). US Geological Survey.

References
  1. "Miller Oblated Stereographic Projection".
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