The appropriateness of map projection selection directly affects the accuracy and utility of the map. The forms of latitude and longitude networks established by different projection methods vary, and their deformation properties and distribution patterns also differ. In practical applications, it is essential to minimize the deformation of map projections as much as possible. Currently, there is no projection method that can completely and without distortion represent the true surface of the Earth. When selecting a map projection, several factors should be comprehensively considered, including the scope, shape, and geographical location of the cartographic area, as well as the purpose of the map.
Factors Influencing Map Projection Selection
- Impact of cartographic area size, shape, and geographical location
Map deformation is related to the size of the cartographic area. The larger the area, the more complex the projection selection. For very small areas, the deformation is minimal regardless of the projection scheme used. Taking Xinjiang Uygur Autonomous Region, the largest in China, as an example, its size is "small" compared to the entire Earth's surface, so any projection scheme is feasible. However, for world maps, hemisphere maps, and maps of continents and oceans, the area is large, and the deformation from projection is significant, requiring consideration of various projection schemes, making the selection complex. The size of the cartographic area is determined based on the maximum deformation value achievable by the projection. Generally: when the area does not exceed 5 to 6 million square kilometers, with length deformation about 0.5% in common projections, it is called a "small" area; when the area is greater than 35 to 40 million square kilometers, with length deformation of 2-3% in projection, it is called a "medium" area; if the length deformation in projection exceeds 3%, it is a "large area".
In addition to size, the shape and geographical location of the cartographic area also determine the appropriate projection scheme. When selecting a projection, it is best to have the isodeformation lines roughly match the outline of the cartographic area. Azimuthal projections have circular isodeformation lines centered on the projection center, making them ideal for areas with circular outlines; polar regions use polar azimuthal projections, equatorial regions use transverse azimuthal projections, and mid-latitude regions use oblique azimuthal projections. For areas extending east-west in mid-latitudes, such as China and the USA, a normal conic projection is suitable. For areas near the equator or extending east-west on both sides of the equator, such as Indonesia, a normal cylindrical projection should be used. For areas extending north-south, transverse cylindrical or polyconic projections are generally used, such as for Argentina and Chile in South America; for areas extending in any direction, oblique cylindrical projections can be chosen.
In world maps, the Mercator projection (normal conformal projection) is commonly used for world route maps, world transportation maps, and world time zone maps. Arbitrary cylindrical projections are also used for time zone maps. World maps published in China often use the Equidistant Conic Projection with Differential Parallels, which better represents China's shape and its relations with neighboring countries, but deformation is larger at the edges. For east and west hemisphere maps, transverse azimuthal projections are often chosen; for north and south hemisphere maps, polar azimuthal projections are used; for land and water hemisphere maps, oblique azimuthal projections are selected.
- Purpose of the map
The purpose of the map is also a factor influencing projection selection. Different types of maps require different projection deformations. For example, administrative division maps, population density maps, and economic maps require high area accuracy, so equal-area projections should be used; conformal projections are more suitable for nautical charts, aeronautical charts, weather maps, and military topographic maps, as they correctly represent direction and maintain similarity to the ground in small areas. Educational and promotional maps do not require high deformation control in all aspects and can use arbitrary projections.
Some maps have special requirements and need specific projection types. For example, time zone maps require meridians to be parallel straight lines, so only normal cylindrical projections can be used. When drawing Chinese administrative maps, oblique azimuthal or Bonne projections are used for the South China Sea islands.
Selection of Map Projections
There are numerous map projection methods and classifications. Different map projections are used in various contexts and for different purposes. Below are common map projections for different regional scopes.
- Projections for world maps: Ensure minimal global deformation, such as polyconic projections, arbitrary pseudocylindrical projections, etc.
- Projections for hemisphere maps: Transverse equal-area (conformal) azimuthal projections for east-west hemispheres; polar equal-area (conformal, equidistant) azimuthal projections for north-south hemispheres.
- Projections for continental maps: Oblique equal-area azimuthal projections for each continent; additionally, Bonne projection for Asia and North America, normal conic projection for Europe and Oceania, Sanson projection for South America.
- Various map projections for China: National maps (various projections, Lambert projection most common), provincial maps (various projections, Gauss-Krüger projection most common), large-scale topographic maps (Gauss-Krüger projection).
Projection Schemes and Usage for China's Basic Scale Topographic Maps
The projection schemes for China's maps are specified as follows:
- China's 1:10,000 and larger-scale topographic maps use the Gauss-Krüger projection with 3-degree zones. Maximum length deformation is 0.0345%, maximum area deformation is 0.069%.
- China's 1:25,000 to 1:500,000 topographic maps use the Gauss-Krüger projection with 6-degree zones. Maximum length deformation is 0.138%, maximum area deformation is 0.276%.
- China's newly compiled 1:1,000,000 topographic maps use the Lambert projection, a normal conformal conic projection with equal absolute deformation at the edge and middle parallels. The tiling principle aligns with the internationally unified projection for global millionth-scale maps specified by the International Geographical Union.
The following table lists common map projection types and parameters.
| Map Type | Projection Used | Main Technical Parameters |
| China Full Map | Oblique equal-area azimuthal projection Oblique conformal azimuthal projection |
Projection center: φ=27°30′ λ=+105° or φ=30°30′ λ=+105° or φ=35°00′ λ=+105° |
| China Full Map (South China Sea islands as inset) |
Normal equal-area secant conic projection (Lambert projection) |
Standard parallels: φ1=25°00′, φ2=47°00′ |
| China Provincial Maps (Excluding Hainan Province) |
Normal conformal secant conic projection Normal equal-area secant conic projection |
Each provincial map uses its own standard parallels |
| China Provincial Maps (Hainan Province) |
Normal conformal cylindrical projection | |
| National Basic Scale Topographic Map Series 1:1,000,000 |
Normal conformal secant conic projection | Tiled by international 4°×6° standard, Standard parallels: φ1≈φs+35′, φ2≈φn+35′ |
| National Basic Scale Topographic Map Series 1:25,000～1:500,000 |
Gauss-Krüger projection (6° zone) |
Zone number (N): 13～23 Central meridian: λ0=(N×6-3)° |
| National Basic Scale Topographic Map Series 1:5,000～1:10,000 |
Gauss-Krüger projection (3° zone) |
Zone number (N): 24～46 Central meridian: λ0=(N×3)° |
| Urban Map Series (1:500～1:5,000) |
Urban planar local projection or Gauss projection with local urban coordinates |
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