Whether the choice of map projection is appropriate or not directly affects the accuracy and value of the map. The forms of the longitude and latitude networks established by different projection methods are different, and their deformation properties and deformation distribution laws are also different. In practical applications, the distortion of the map projection should be minimized as much as possible. At present, there is no Projection Transformation method that can represent the real surface of the earth completely and without deformation. Select Map projection should take into account several factors such as the extent, shape, and geographic location of the mapping area, as well as the purpose of the map.

Factors Affecting the Choice of Map Projection

• Impact of size, shape and geographic location of mapping area

Map deformation is related to the size of the mapping area, and the larger the mapping area is, the more complex the projection selection is. For a very small area, no matter what projection scheme is used, the deformation is very small. Taking Xinjiang Uygur Autonomous Region, the largest region in China, as an example, the size of its regional scope is "small" for the whole earth's surface, so it is feasible to design any projection scheme for this area. However, for example, World Map, hemispheric map, maps of continents and oceans, etc., their regional scope is very large, and the deformation caused by projection is also very large, so there are many projection schemes to be considered, which makes the selection of projection more complicated. The size of the mapping area is determined according to the maximum deformation value that can be achieved by the projection. The general rule is that when the size of the mapped area does not exceed 5 million to 6 million square kilometres, that is, when the length deformation is about 0.5% in Common Projections, it is called a "small" area; When the area of the mapping area is more than 35 million to 40 million kilometers, that is, when the length deformation in the projection is 2 to 3%, it is called a "medium" area; if the length deformation of the mapping area in the projection is more than 3%, it is called a "large area".

In addition to the size of the mapping area affecting the choice of projection, the shape and geographical location of the mapping area also determine the projection scheme applicable to a certain area. When selecting the projection, it is better to make the contour line of equal deformation basically consistent with the contour shape of the mapping area. The shape of the deformation line of the azimuthal projection is a circle with the center of the projection as the center, so it is most suitable for representing the area with a circular outline; the normal axis azimuthal projection is used for the polar and nearby areas, the horizontal axis azimuthal projection is used for the area with the equator as the center, and the oblique axis azimuthal projection is used for the middle latitude area. When the mapping area extends along the east-west direction and is located in the middle latitude area, the normal conic projection should be used, such as China, the United States, etc. When the mapping area is near the equator or extends in the east-west direction on both sides of the equator, the normal cylindrical projection should be used, such as Indonesia. When the mapping area extends along the north-south direction, the horizontal axis cylindrical projection and polyconic projection are generally used, such as Argentina and Chile in South America; for the area extending in any direction, the oblique axis cylindrical projection can be used.

In World Map, Mercator projection (isometric projection) is often used to draw world route map, world traffic map, world time zone map, and arbitrary cylindrical projection is also used to draw time zone map. The World Map published in China mostly adopts the equal-difference latitude line multi-conic projection, which is good for expressing the shape of China and the contrast relationship with its neighbors, but the deformation of the projection edge area is large. For the eastern and western hemispheres, the horizontal azimuth projection is often used, for the southern and northern hemispheres, the normal azimuth projection is used, and for the water and land hemispheres, the oblique azimuth projection is used.

• The purpose of the map

The purpose of the map is also a factor affecting the choice of projection, and maps for different purposes require different projection deformations. For example, administrative division maps, population density maps, economic maps, etc. Require high accuracy of area, so equal area projection should be selected; isometric projection is more suitable for navigation maps, aeronautical maps, weather maps, military topographic maps, etc., which can correctly show the direction and keep the map similar to the field in a small area. Teaching drawings and publicity drawings do not require high deformation in all aspects, and any projection can be selected.

For some maps with special requirements, a specific Projection Type is selected. For example, the time zone diagram requires the meridians to be parallel straight lines, so only the normal cylindrical projection can be selected. When drawing the map of China's administrative regions, the oblique azimuth projection or Penner projection is used for the South China Sea islands.

Choice of map projection

There are many map projection methods and classifications, and different map projections are used in different occasions and uses. The following lists the map projections commonly used in different areas.

Projection of World Map: ensure that the global overall deformation is not large, multi-conic projection, arbitrary pseudo-cylindrical projection, etc.

The projection of hemispherical map: the east and west hemispheres have horizontal axis equal area (equiangular) azimuth projection; the north and south hemispheres have normal axis equal area (equiangular, equidistant) azimuth projection.

Projections of the maps of the continents: Oblique equal-area azimuthal projections were used in all States, in addition to Asia and North America (Penner projection), Europe and Oceania (normal equal-area conic projection), and South America (Sanson projection).

Various map projections in China: national map (various projections, mostly Lambert projection), provincial map (various projections, most common Gauss-Kr Krüger projection), large-scale topographic map (Gauss-Kr Krüger projection).

Projection Scheme and Application of Basic Scale Topographic Map in China

China's map projection scheme is as follows:

• In China, the 3-degree zone Gauss-Kruger projection is used for topographic maps with a scale of 1: 10,000 and larger than 1: 10,000. The maximum length deformation of the projection is 0.0345%, and the maximum area deformation is 0.069%.
• The 6-degree zone Gauss-Kruger projection is used for 1: 25,000 to 1: 500,000 topographic maps in China. The maximum length deformation of the projection is 0.138%, and the maximum area deformation is 0.276%.
• The Lambert projection is used in the new 1: 1,000,000 topographic map of China, which is a normal equiangular conic projection with the same absolute value of the deformation of the marginal latitude and the median latitude. Its principle of framing is consistent with the international one-millionth map projection stipulated by the International Geographical Society for global unified use.

The following table lists the common projection types and Parameters of Map.

Map Type	Projection used	Main technical parameters
The whole map of China	Oblique Equal Area Azimuthal Projection Oblique Equal Angle Azimuthal Projection	Center of projection: φ = 27 ° 30 ′ λ = + 105 ° or φ = 30 ° 30 ′ λ = + 105 ° or φ = 35 ° 00 ′ λ = + 105 °
Full Map of China (Illustrated by South China Sea Islands)	Normal Equal-Area Secant Conic Projection (Lambert Projection)	Standard parallel: φ 1 = 25 ° 00 ′, φ 2 = 47 ° 00 ′
Map of China by province (region) (except Hainan Province)	Normal Equal Angle Secant Conic Projection Normal Equal Area Secant Conic Projection	Each province (region) map adopts its own standard latitude line.
China Province (Region) Map (Hainan Province)	Orthographic equiangular cylindrical projection
National Basic Scale Topographic Map Series 1: 1,000,000	Normal isometric secant conic projection	According to the international unified 4 ° × 6 ° frame, standard latitude: φ1 ≈ φs + 35 ′, φ2≈φn+35′
National Basic Scale Topographic Map Series 1: 25,000 ~ 1: 500,000	Gauss-Kruger projection (6 ° zoning)	Projection band number (N): 13 ~ 23 Center Meridian: λ0 = (N × 6-3) °
National Basic Scale Topographic Map Series 1:5000 ~ 1: 10,000	Gauss-Kruger projection (3 ° zoning)	Projection band number (N): 24 ~ 46 Center Meridian: λ0 = (N × 3) °
City Map Series (1:500 ~ 1:5000)	Local projection of city plane or Gaussian projection of city local coordinate