Map Projection Series Introduction (four)----projected coordinate system

4.4 Common map projection in China

 Basic scale topographic map of China (1:1 million, 1:500,000, 1:250,000, 1:100,000, 1:50,000, 1:25,000, 1:10,000, 1:5,000) except 1:1 million, using Gauss-gram gauss–krüger Gauss-kruger projection (Horizontal isometric cylindrical projection, also called transverse Mercator Transverse Mercator projection), is the geographic basis.
The 1:100 topographic map uses the Lambert Lambert projection (positive shaft conformal cut conic projection), and its principle of distribution is consistent with the international one out of 10,000 atlas projection used by the International Geographic Society for global Unification.
 Topographic maps with less than 500,000 at sea are multi-use Mercator Mercator projection (positive axis conformal cylindrical projection).
Most of the provinces and regions of China and most of this scale map also use Lambert projection and belong to the same projection system Albers projection (positive axis, such as cone projection).

        1)   Gaussian-gram gauss–krüger Gauss-kruger projection (horizontal isometric cylindrical projection)
           – Our provisions 1:10,000, 1:25,000, 1:50,000, 1:100,000, 1:250,000, 1: The topographic map of the 500,000 scale uses Gauss G Gauss–krüger projection.
           – This projection is called transverse Mercator projection in countries such as the UK and US
           – transverse isometric cylindrical Projection
                    the farther away from the central meridian, the greater the distortion. br>                    Equator is a straight line, the parallels that leave the equator are arcs, convex equator
      &NB Sp             No angle distortion
                    Very small deformation of length and area

– Projection of Beijing 54 and Xi ' an 80 projection coordinate systems
– Gaussian projection features:
Central Meridian length deformation ratio is 1
On the same meridian, the length deformation increases with the decrease of latitude, and at the equator is the largest
On the same parallel line, the length distortion increases with the increase of the warp, and the speed increases rapidly.
In the 6 belt range, the maximum deformation of the length is no more than 0.14%

  control the deformation by means of a band:
–6° Belt
for 1:25,000 ~1:50 scale map
starting at the beginning of the Meridian (Greenwich), according to the difference of 6 degrees to a projection band from the west to the east, a total of 60 projection belts. The scope of our country can be divided into 11 6 degree bands.
–3° Belt
for larger than 1:10,000 scale maps
From the longitude 1°30′, according to the difference of 3 degrees for a projection belt from the west to the East Division, a total of 120 projection belts worldwide. The scope of our country can be divided into 22 three degree bands.
– The coordinate system origin is the central meridian of each projection band and the equator intersection point

In order to facilitate the surveying operation of topographic map, a planar Cartesian coordinate system is arranged in the Gaussian-G Gauss–krüger projection Band, the method is that the central meridian is the x-axis, the equator is the y-axis, the central meridian and the equator intersection are the coordinates origin, the X-value is positive in the northern hemisphere, negative in the southern hemisphere, The west of the central meridian is negative. Since our territory is in the northern hemisphere, the X value is positive, in order to avoid negative value of y, it is stipulated that the coordinate longitudinal axis of each projection belt is shifted westward by 500km, and the original horizontal value of the central meridian is changed from 0 to 500km. In order to facilitate the distinction between the points of the band, you can add the revving number before the Y-value of each point position.

2)Lambert Lambert projection(Positive shaft conformal cut conic projection)
– for maps that are less than 1:1 million (including 1:1 million).
– A projection that works best for mid-latitude. It is similar to the Albers projection, except that its shape is more accurate than the area depicted.
– Since our country is located in the mid-latitude region, China maps and sub-provincial maps often use cut conic projections (Lambert or Albers projections):
the central meridian of the map of China is usually 105 degrees east longitude.
Two standard parallels are 25 degrees north latitude and 47 degrees north latitude
– The parameters of each province can be determined based on the location and shape of the contour. For example, the parameters of Gansu Province are:
the central meridian is 101 degrees east longitude.
Two standard parallels are 34 and 41 degrees north latitude, respectively .

– Projection method:
a conic projection is usually based on two standard parallels, making it a cut projection. Latitude spacing beyond the standard parallels will increase. This is the only common conic projection that represents the poles as a single point.
You can also use a single standard parallel and scale factor definition. If the scale factor is not equal to 1.0, the projection will actually become a cut projection.

3) Abrams Albers projection (positive axis and other cone projection)
– also known as "double standard parallels, such as cone projection," for Abrams (Albers). The area of the projection area remains equal to the field.
– The most suitable for the east-west distribution of the continental plate, not suitable for North-south distribution of the continental plate.
– Albers projection is often used to maintain the equal area characteristics when processing national data showing 4 million or 1 million.

4) Mercator Mercator projection (positive axis conformal cylindrical projection)
– Designed specifically for nautical purposes by Mercator in 1569.
– The design idea is to make a cylinder that is aligned with the axis of the Earth cut or cut on the globe, projecting the graticule on the sphere onto the cylindrical surface in an isometric condition, and then carrying the cylindrical face along a bus cut to a plane.
– widely used in navigation , important projection of aviation .

5. Projection coordinate system

A map projection is a mathematical transformation that transforms a map from a spherical (geodetic plane) to a plane. The coordinate system determined by this is generally referred to as the projected coordinate system .

The projection coordinate system is a planar coordinate system which is determined by the datum plane and the map projection two sets of parameters.

(If someone says: This point Beijing 54 coordinates value is x=4231898,y=21655933, actually refers to the Beijing 54 datum plane projection coordinates, namely the Beijing 54 datum plane's latitude and longitude coordinates in the right angle plane coordinates projection result. )

To correctly determine the projected coordinate system, we must first understand the basic concepts of the Earth ellipsoid (ellipsoid), The Geodetic Datum (Datum) and the map projection (Projection) and the relationship between them.

6. Summary

Through the previous series of introductory content, I hope readers can understand and grasp the content:

① Description of the Earth space model

– Earth's natural surface, geoid, earth ellipsoid model

Establishment of geographic coordinate system of ②

– Reference ellipsoid, geodetic plane, geographic coordinate system

– China's common geographic coordinate system, elevation system

③ Map projection

– Projection material, projection deformation, projection classification

– Common map Projections in China: Beijing54, Xian80, CGCS2000, WGS 1984

④ Projected coordinate system

– Earth plane + map projection

–arcgis the projected coordinate system definition

Map Projection Series Introduction (four)----projected coordinate system