ArcGIS geographic coordinate system and projected coordinate system

geographic coordinates: spherical coordinates. The reference plane is an ellipsoid , coordinate unit : latitude and longitude

        geodetic coordinates: plane coordinates. The reference plane is   horizontal

the process of converting geographic coordinates to geodetic coordinates can be understood as projection . (projection : converting irregular earth surfaces to planes)
in the ArcGIS two sets of coordinate systems are pre-defined in: Geographic coordinate system ( Geographic coordinate system ) Projected coordinate system ( Projected coordinate system )

1 , first understand the geographic coordinate system (geographic coordinate system), the geographic coordinate system literal translation is the geographical coordinate systems, is the latitude and longitude as a map of the storage units. Obviously, geographic coordinate syst em is a spherical coordinate system. We want to store the digital information on the earth on the spherical coordinate system, how to operate it? The earth is an irregular ellipsoid, how to store data information in a scientific way to the ellipsoid? This inevitably requires us to find such an ellipsoid. Such an ellipsoid has characteristics: it can be quantified and calculated. Has a long half axis, short half axis, eccentricity. The following lines are the krasovsky_1940 ellipsoid and its corresponding parameters. spheroid:krasovsky_1940
Semimajor axis:6378245.000000000000000000
Semiminor axis:6356863.018773047300000000
Inverse flattening (flat rate): 298.300000000000010000

However, with this ellipsoid it is not enough, and a geodetic datum is needed to locate the ellipsoid. In the coordinate system description, you can see a line like this:

datum:d_beijing_1954 says the geodetic plane is d_beijing_1954.

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With the spheroid and datum two basic conditions, the geographic coordinate system can be used.
Full parameters:

Alias:
Abbreviation:
Remarks:
Angular Unit:degree (0.017453292519943299)
Prime Meridian (starting longitude): Greenwich (0.000000000000000000)
Datum (earth plane): d_beijing_1954
Spheroid (Reference ellipsoid): krasovsky_1940
Semimajor axis:6378245.000000000000000000
Semiminor axis:6356863.018773047300000000
Inverse flattening:298.300000000000010000

2 , the next is Projection coordinate system (projection coordinate system), first look at some parameters in the projection coordinate system.

Projection:gauss_kruger
Parameters:
false_easting:500000.000000
false_northing:0.000000
central_meridian:117.000000
scale_factor:1.000000
latitude_of_origin:0.000000
Linear Unit:meter (1.000000)
Geographic coordinate System:
name:gcs_beijing_1954

Alias:
Abbreviation:
Remarks:
Angular Unit:degree (0.017453292519943299)
Prime Meridian:greenwich (0.000000000000000000)
datum:d_beijing_1954
spheroid:krasovsky_1940
Semimajor axis:6378245.000000000000000000
Semiminor axis:6356863.018773047300000000
Inverse flattening:298.300000000000010000

As can be seen from the parameters, each projected coordinate system is bound to have geographic coordinate systems.

The projection coordinate system is essentially a planar coordinate system whose map units are usually meters.

So why are the parameters of the coordinate system present in the projection coordinate system?

At this point, it is also important to illustrate the significance of projection: the process of converting spherical coordinates to planar coordinates is called projection.

Well, the projection condition comes out:

A, spherical coordinates

b, conversion process (i.e. algorithm)

In other words, to get the projected coordinates you have to have a "take" projection of the spherical coordinates, and then use the algorithm to the projection!

geographic Coordinate System parameters.
about Beijing 54 and Xian 80 is our most used coordinate system           First introduce the basic knowledge of Gauss-G gauss–krüger projection, understand directly skip, China's large and medium-scale maps are used Gaussian-gram gauss–krüger projection, which is usually 6 degrees and 3 degrees with the projection, 1:25,000 -1:50-scale topographic map using the difference of 6 degrees, 1:10,000 scale topographic map using the difference of 3 degrees. The specific zoning method is: 6 degrees from the Prime meridian beginning, according to the difference of 6 degrees for a projection belt from west to East division, the world is divided into 60 projection belt, with the number of the 1-60;3 is from the longitude of 1 degrees 30 seconds longitude, according to the difference of 3 degrees for a projection belt from the west to the east, a total of 120 projection

         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 , 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 of the Y-value at each point, such as the coordinates of a point within 20 band can be represented as ya=20 745921.8m.

         systems\projected in coordinate Coordinate Systems\gauss kruger\beijing 1954 directory, we can see four different naming methods:
          Beijing 1954 3 degree GK CM 75e.prj
          Beijing 1954 3 degree GK Zone 25.prj
         Beijing 1954 GK Zone 13.PRJ
         Beijing 1954 GK Zone 13n.prj

          The descriptions for them are as follows:
          Three degree band method of the Beijing 54 coordinate system, the central meridian in the East 75 degrees of the belt coordinates, the horizontal axis without the band number
          Three degrees of the Beijing 54 coordinate system, the central meridian in the East 75 degrees of the sub-belt coordinates, horizontal axis before the band number
          Six Degrees of the Beijing 54 coordinate system, the band number is 13, the horizontal axis of the front plus the number
          Six-degree belt method of the Beijing 54 coordinate system, with a band number of 13, the horizontal axis before the band number

          in coordinate systems\ Projected coordinate Systems\gauss Kruger\xian 1980 directory, the file naming method has changed:
          Xian 1980 3 degree GK CM 75e.prj
          Xian 1980 3 degree GK Zone 25.prj
         Xian 1980 GK CM 75E.PRJ
         Xian 1980 GK Zone 13.prj

XI ' an coordinate file naming method, meaning and the first two coordinates of Beijing, but did not appear " c11> Band number +n " This form, why not adopt a unified naming method? It's a little confusing to see.

Geodetic coordinates (GEODETICCOORDINATE): The coordinates of the Datum in Geodetic survey with reference ellipsoid. The position of the ground point P is indicated by the Earth longitude L, the Earth latitude B and the earth height H. When the point is on the reference ellipsoid, it is represented only by the longitude and latitude of the Earth. The longitude of the earth is through the angle between the meridian plane of the earth and the beginning of the Earth meridian, and the latitude of the earth is the angle between the normal of the point and the equatorial plane, and the earth height is the distance of the ground point along the normal to the reference ellipsoid.

Square Net: is a grid of two sets of parallel lines parallel to the projection axis. Because it is every kilometer to draw the coordinates of the longitudinal line and horizontal lines, so called square in the net, because the square line is also parallel to the Cartesian axis of the coordinate network cable, it is also known as rectangular coordinate network.

On the topographic map of 1:10,000--1:20, the longitude is shown directly in the form of the contour line, and the corresponding degrees are injected at the angle of the figure. In order to encrypt into a net in the diagram, inside and outside the outline is also painted encrypted Graticule encryption sub-line (the scheme is called "the" "sub-band"), if necessary, corresponding to the short-term connection can constitute an encrypted longitude network. 1:250,000 topographic map, in addition to the Neatline profile is painted on the latitude and longitude Network encryption division, the figure also has a cross-line encryption.

China's 1:500,000--1:100 topographic map, on the surface directly painted longitude nets, Neatline profile also for the encryption Longitude network encryption sub-line.

The coordinate system of a Cartesian mesh is the x-axis of the line projected by the central Meridian, and the line after the equator is the y-axis, and their intersection point is the origin of the coordinates. In this way, there are four quadrants in the coordinate system. The vertical axis from the equator to the north is positive, negative to the south, the horizontal axis from the central meridian, the east is positive, West is negative.

Although we can assume that the square is rectangular, geodetic coordinates are spherical coordinates. But we often see in a map of the square and latitude and longitude network, we are very accustomed to the longitude and latitude network for the geodetic coordinates, this time the geodetic coordinates are not spherical coordinates, she and the net projection of the square is the same (usually Gaussian projection), but also the plane coordinates.

ArcGIS geographic coordinate system and projected coordinate system