There are many kinds of map projections, which are generally classified according to two criteria: one is the classification according to the deformation nature of the projection, and the other is the classification according to the composition of the projection.
Classification according to the nature of projective deformation
According to the deformation properties of projection, it can be divided into the following categories: equiangular projection, equiareal projection and arbitrary projection.
- Isometric projection
The similarity of infinitesimal figures can be maintained. The length ratio at the same point is the same everywhere-deformation circle, the radius of deformation circle at different points is different, and the projection figure is not completely similar to the actual shape of the ground in a wide range. Because this kind of projection has no angle deformation, it is convenient to measure the direction/angle on the map, so it is often used in maps with high requirements for real angle and direction, such as navigation, ocean current and wind direction. Because the deformation of this kind of projection area is very large, Measure Area cannot be used.
- Equal area projection
Equal area projection is equal area projection, which is convenient for area comparison and measurement. It is often used for natural and economic maps with high requirements for area accuracy, such as geological, soil, Land Use, administrative divisions and other maps.
- Arbitrary projection
Any projection is neither equal in angle nor equal in area, and the deformation exists in all aspects, but it is moderate. In any projection, there is a special kind of projection called equidistant projection, which satisfies that the ratio of meridian length is 1 in normal projection and the ratio of vertical circle length is 1 in oblique or transverse projection. Arbitrary projection is often used in teaching maps, scientific Reference maps and general World maps.
Classification by way of projection composition
According to the structure of projection, it can be divided into two categories: geometric projection and analytic projection.
Geometric projection is a kind of projection obtained by projecting the longitude and latitude network on the ellipsoidal surface directly or with some additional conditions to the geometric bearing surface, and then expanding the geometric surface into a plane, including azimuthal projection, conical projection and cylindrical projection. According to the position relationship between the projection plane and the spherical surface, it can be divided into orthographic projection, transverse projection and oblique projection. As shown in the following figure:
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Azimuthal projection: a projection formed by projecting a spherical longitude and latitude network onto a plane by taking the plane as the geometric bearing surface and making the plane tangent or secant to the ellipsoidal surface. There is no deformation at the tangent point or secant line, and the farther away from the tangent point or secant line, the greater the deformation.
Conic projection: a projection formed by taking a cone as the geometric bearing surface, making the cone tangent or secant to the ellipsoidal surface, and projecting the spherical longitude and latitude network onto the conical surface. This projection is suitable for the map of the area extending along the latitude line in the middle latitude zone, and this projection is mostly used in the map of our country.
Cylindrical projection: a projection formed by taking a cylinder as the geometric bearing surface, making the cylinder tangent or secant to the ellipsoidal surface, and projecting the spherical longitude and latitude network onto the cylindrical surface. This Projection Type is generally applicable to the compilation of maps near the equator and World Map.
Analytic projection is a kind of projection which can get the longitude and latitude network directly by analytic method without the help of auxiliary geometric surface. It mainly includes: pseudo azimuth projection, pseudo conic projection, pseudo cylindrical projection and polyconic projection. It will not be repeated here.
Pseudo-Azimuth Projection: Modified from Azimuth Projection. In the case of the positive axis, the latitude lines are still concentric circles. Except that the Center Meridian is a straight line, the other longitude lines are changed to the curve of the Center Meridian and intersect at the center of the latitude line.
Pseudo-cylindrical projection: modified from cylindrical projection. On the basis of orthographic cylindrical projection, the latitude lines are still required to be parallel straight lines. Except that the Center Meridian is a straight line, the rest of the meridians are changed to curves symmetrical to the Center Meridian.
Pseudo-conic projection: Modified from conic projection. On the basis of the normal conic projection, the meridians are required to be concentric arcs. Except that the Center Meridian is a straight line, the rest of the meridians are changed to curves symmetrical to the Center Meridian.
Polyconic projection: This is a projection designed by assuming that multiple conical surfaces are tangent to the sphere. The latitude line is a coaxial arc, the center of which is located on the Center Meridian, the Center Meridian is a straight line, and the rest of the longitude lines are curves symmetrical to the Center Meridian.
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| Figure: Schematic diagram of the shape of the longitude and latitude lines of the pseudo-azimuth projection (quoted from the network) | Figure: Schematic diagram of the shape of the longitude and latitude lines of the pseudo-cylindrical projection (quoted from the network) |
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| Figure: Schematic diagram of the shape of the longitude and latitude lines of the pseudo-conic projection (quoted from the network) | Figure: Schematic diagram of the shape of the longitude and latitude lines of the polyconic projection (quoted from the network) |
- Geometric projection
- Analytic projection
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