# coordinate system definition and projection transformation method in ArcGIS

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Coordinate system is the important mathematical foundation of GIS data, it is used to represent the reference system of geographical features, images and observations, and the definition of coordinate system can ensure that geographic data display its position, direction and distance correctly in software, and the lack of GIS data of coordinate system is imperfect. It is therefore important to properly define the coordinate system and perform the projection transformation in the ArcGIS software.

1. 1. Coordinate systems in ArcGIS

Two sets of coordinate systems are predefined in ArcGIS, the geographic coordinate system (geographic coordinate system), and the projected system (Projectedcoordinate system).

1.1 Geographic coordinate system

Geographic coordinate system (GCS) uses three-dimensional spheres to define the position on the earth. Important parameters in GCs include angular units of measure, Prime Meridian, and Datum plane (based on a spheroid). Latitude and longitude are used in geographic coordinate systems to determine the point on a spherical surface, and latitude is the angle of measurement from the center of the earth to a point on the globe. The horizontal lines in the spherical system are equal latitude lines or parallels, and the vertical line is the longitude line or meridian. These lines envelop the earth and form a grid network called the Graticule.

The longitude and latitude values in GCs are measured in decimal units or in degrees, minutes, and seconds (DMS). Latitude values are measured in relation to the equator, ranging from -90° (Antarctic point) to +90° (North Pole). Longitude values are measured relative to the prime meridian. The range is -180° (when traveling westward) to 180° (when traveling eastward).

In ArcGIS, the coordinate system commonly used in China is gcs_beijing_1954 (krasovsky_1940), gcs_xian_1980 (iag_75), gcs_wgs_1984 (wgs_1984), gcs_cn_2000 (CN _2000).

2. 1.2 Projected coordinate system

The process of converting spherical coordinates to planar coordinates is called projection. The nature of the projected coordinate system is the planar coordinate systems, in which the map units are usually meters. The projected coordinate system is defined in a two-dimensional plane. Unlike geographic coordinate systems, the length, angle, and area of a projected coordinate system are constant within a two-dimensional space. The projected coordinate system is always based on a geographic coordinate system, i.e.:

"Projected coordinate system = geographic coordinate system + projection algorithm function".

Our country's projection coordinate system mainly uses the Gauss-gram gauss–krüger projection, divides into 6 degrees and 3 degrees the projection, 1:25,000 -1:50 The scale topographic map uses the difference 6 degrees the belt, 1:10,000 scales the topographic map uses the difference 3 degrees to divide. The specific zoning method is: 6 degrees from the Prime meridian (Prime Meridian), according to the difference of 6 degrees for a projection belt from the west to the east, a total of 60 projection belts, China across 13-23 bands, 3-degree projection belt from the longitude 1 degrees 30 points longitude (1.5°), According to the deviation of 3 degrees for a projection belt from west to East division, the world is divided into 120 projection belts, China across 25-45 belt.

In the coordinatesystems\projected coordinate Systems\gauss kruger\beijing 1954 directory, we can see four different naming methods:

Beijing 1954 (Xian 1980) 3 degree GK CM 117E

Beijing 54 (XI ' an 1980) 3 degrees with no number

Beijing 1954 (Xian 1980) 3 degree GK Zone 25

Beijing 54 (XI ' an 1980) 3 degrees with band number

Beijing 1954 (Xian 1980) GK Zone 13

Beijing 54 (XI ' an 1980) 6 degrees with band number

Beijing 1954 GK Zone 13N

Xian 1980 GK CM 75E

Beijing 54 (XI ' an 1980) 6 degrees with no number

Note:GK is a Gaussian gram Gauss–krüger, CM is the Centralmeridian Central Meridian, Zone is a band number, n is a sign that does not show the band number.

3. coordinate system defined in 2.ArcGIS

All geographic datasets in ArcGIS require a coordinate system for displaying, measuring, and transforming geographic data, which is used in ArcGIS. If the coordinate system of a dataset is unknown or incorrect, you can use a tool that defines the coordinate systems to specify the correct system coordinates, and you must have learned the correct coordinate system for the dataset before using this tool.

The tool defines a coordinate system for a feature class or DataSet that contains an undefined or unknown coordinate system, located in Arctoolbox-data management tools-projections andtransfomations-define projections

Input DataSet: DataSet or feature class to define the projection

CoordinateSystem: The coordinate system defined for the data set

4. &NBSP;

3. ArcGIS-based projection transformation

In the operation of data, we often need to transform the data of different coordinate systems into a unified coordinate system. Convenient processing and analysis of data, software coordinate system conversion commonly used in the following two ways:

3.1 directly with the defined parameters to implement the projection transformation

In the ArcGIS software has defined the coordinate transformation parameters, you can directly call the coordinate system conversion tool, directly select the conversion parameters. Tool is located in Arctoolbox-data management tools-projections andtransfomations--feature-project (Raster data projection conversion tool   raster- Project raster), enter the following parameters in the tool interface:

Inputdataset: A feature class, feature layer, or feature dataset to be projected.

Outputdataset: A new feature dataset or feature class that has a coordinate system specified in the output coordinate system parameter.

Out_coor_system: The new coordinate system to which the feature class is known to be converted.

Geographictransformation: In the list is the conversion parameter, with gcs_beijing_1954 to gcs_wgs_1984 as an example, each conversion parameter has the following meanings:

Beijing_1954_to_ Wgs_1984_1 15918 Ordos basin

Beijing_1954_to_wgs_1984_2 15919 Yellow Sea sea Area

Beijing_1954_to_wgs_1984_3 15920 South China Sea-Zhujiang Estuary /p>

Beijing_1954_to_wgs_1984_4 15921 Tarim Basin

Beijing_1954_to_wgs_1984_5 15935 Beibu Gulf

Beijing_1954_to_wgs_ 1984_6 15936 Ordos basin

5. 3.2 Custom three-parameter or seven-parameter conversion

When the direct conversion between projections is not automatically implemented in ArcGIS software, a custom seven-parameter or three-parameter projection transformation is required, with the seven parameter as an example, the conversion method is as follows:

3.2.1 Custom seven-parameter geo-conversion

In Arctoolbox Select the Create Customgeographic Transformation tool, in the popup window, enter a name for the conversion, such as wgs84tobj54. Under define a geo-conversion method, select the appropriate conversion method, such as Coordinate_frame, in method, and then enter seven parameters, that is, the translation parameter, the rotation angle, and the scale factor.

3.2.2 Projection Transformation

Open the projections and Transformations>feature>project under the Toolbox, enter the data to be converted in the popup window, and the output coordinate System, Then enter the first step of the custom geographic coordinate system such as wgs84tobj54, start the projection transformation, complete the projection transformation:

coordinate system definition and projection transformation method in ArcGIS

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