Projection System Data

1	Emphasis on map projection knowledge
	Concept of projection In mathematics, projection refers to establishing the correspondence between two surface point sets. In cartography, map projection is to establish a one-to-one correspondence between the surface and the projection plane, that is, to study how to express the earth surface to the map plane. Importance-correct map projection is the basis for combining/overlays data from different sources. Poor understanding of map projection, it is impossible to use GIS to solve practical problems!
	Author: LionGG reply to this speech

2	Reply: emphasis on map projection knowledge
	Approximate approximation of the Earth-earth elliptical body/Earth reference point The natural surface of the earth is a complex surface with mountain, Valley, river, lake, ocean, and other fluctuations. It is an irregular surface that cannot be expressed using a simple digital model. In order to solve the geometric problem arising from measurement, we usually choose a shape and size which are close to the earth, and its mathematical computation is convenient to represent the shape of the earth.
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3	Reply: emphasis on map projection knowledge
	Spheroid or Ellipsoid) It can be imagined that the earth sphere rotates around the short axis (the ground axis) to form a smooth sphere, I .e., the rotating elliptical body (the double axis elliptical body ). Generally referred to as the earth elliptical body, which is widely used by countries around the world. Three elements of the earth's elliptical body: long half axis a, short half axis B, flat rate f = (a-B)/. Geodetic Datum) A datum is defined by a spheroid, which approximates the shape of the Earth, and the spheroid's position relative to the center of the Earth.
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4	Reply: emphasis on map projection knowledge
	China earth Coordinate System In the 1954 Beijing coordinate system, the coordinate origin was located in Pulkovo, the western Soviet Union, and the krasowski elliptical body was used. The coordinate system of Xi'an in 1980 is located in the north torrent village, Yongle town, jiangyang city, north of Xi'an. It adopts the earth elliptical body recommended by the international survey and geophysical Federation (IUGG/IAG) in 1975. China elevation Series In 1956, the elevation of the Yellow Sea was 72.289 meters, and the elevation of the Qingdao level origin (located in the Guanxiang mountain cave in Qingdao. According to the new tide survey data, the elevation value of the Level origin was revised to 1985 meters in 72.260.
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5	Reply: emphasis on map projection knowledge
	Domestic Earth elliptical body Before 1952, International 1909 (Hayford) 1953-1978, kraovsky 1940 After January 1978, IUGG/IGA 1975
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6	Reply: emphasis on map projection knowledge
	Custom Method for medium ground punctuality in GIS software The world coordinate system-WGS84 is determined by satellite positioning. The earth's elliptical body is WGS84, and the Earth's Datum point (Datum) is the center of the sphere. At present, the conversion between any two Datum can be realized through the forward and inverse conversion between each Datum and the WGS84 in the world coordinate system. The conversion from each Datum to WGS84 (geocentric model) usually uses the Bursa-Wolf parameter: <dx, dy, dz> {rx, ry, rz, ds}. Among them, <Dx, dy, dz> indicates the offset (in meters) from the elliptical origin of the current Datum to the WGS84 elliptical origin of the world coordinate system ). <Rx, ry, rz> indicates the rotation in three directions (in arc-seconds ). <Rs> in parts-per-million ).
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7	Reply: emphasis on map projection knowledge
	Map Projection Method-(classification by map projection) Azimuthal Projections: uses a plane as the secondary projection plane to make the sphere tangent or cut to the plane, and projects the mesh on the sphere surface to the plane. Tangent-tangent cut-secant Cylindrical Projections: uses the Cylindrical decent as the secondary projection plane to make the sphere tangent or cut to the Cylindrical surface, and projects the mesh on the sphere surface to the Cylindrical surface, then the cylindrical surface is scaled along the bus into a plane. Conical Projections: uses a decent cone as an auxiliary projection plane to cut or cut the sphere and the cone surface, and projects the mesh on the sphere surface to the cone surface, then the cone is scaled along the bus into a plane.
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8	Reply: emphasis on map projection knowledge
	The projection can be divided: -Normal aspect) -Horizontal projection (the transverse aspect) -Oblique axis projection (the oblique aspect)
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9

Reply: emphasis on map projection knowledge

Map Projection Method-(classified by Map Projection deformation properties) Conformality (Conformal Projection): A Projection that has the same angle as the corresponding angle on the earth elliptical decent. It is also called Orthomorphic ). During projection, keep the length deformation ratio m on the line equal to the length deformation ratio n on the weft, that is, m = n. Equivalence, Equal-area Projection, Equivalent Projection. the area of any image on the Projection plane is Equal to the area of the corresponding image on the earth elliptical decent. During projection, adjust the meridian length so that m x n = 1 to keep the area unchanged. Any projection. All projection except equality and product projection have deformation of length, angle, and area at the same time. Equidistance (Equidistance, Equidistant Projection) is common. The same Projection does not have non-existent length deformation. It only keeps the main direction length ratio of the deformation ellipse 1 (a = 1 or B = 1 ).