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I. Basic concepts:

1. topographic map coordinate system: the topographic map in China adopts the Gaussian-gram plane Cartesian coordinate system. In this coordinate system, the horizontal axis: Equator, expressed by Y; the vertical axis: the central meridian, expressed by X; the coordinate origin: the intersection of the central meridian and the Equator, expressed by O. Negative to the south of the equator, positive to the north, positive to the east of the central meridian, negative to the west. China is located in the northern hemisphere, so the ordinate values are positive, but in order to avoid the negative value to the west of the central longitude line, the ordinate axis is moved 500 kilometers west.
2. Beijing 54 coordinate system: in 1954, China set up the earth coordinate origin in Beijing. The coordinates of the various Earth control points calculated based on the krausowski elliptical body are called the Beijing 54 coordinate system.
3. gs84 Coordinate System: The world's universal Coordinate System of longitude and latitude.
4, 6 degree belt, 3 degree belt, central longitude line.
China adopts 6-degree and 3-degree bands:
The topographic maps of and adopt a 6-degree band projection, that is, the longitude difference is 6 degrees, starting from the zero-degree meridian, and each longitude difference of 6 degrees from west to east is a projection band, the world is divided into 60 bands, with 1, 2, 3, 4, 5 ,...... Represents. That is, the eastern longitude 0 ~ The longitude of the center longitude is 3 degrees east longitude, And the longitude is 6 degrees east longitude ~ 12 degrees is the second band, and the longitude of the central longitude is 9 degrees. Our province is located in the east longitude between 113 degrees-east longitude between 120 degrees, across 19th belts and 20 belts, of which the east longitude is 114 degrees west (including the west of Xiazhuang township in Fuping County, the wentang in Pingshan, and the west of sujiazhuang, west of the mining area, west of Pulp Water Town in Xingtai County, west of live water township in wu'an, and whole of the county) is located in 19th, with the central longitude line of 111 degrees east longitude; 114 degrees east to Shanhaiguan are all in 20th belts, and its central longitude is 117 degrees.
The-10 thousand Topographic Map adopts a 3-degree band. It starts from the longitude line of the east longitude 1.5 degrees and ranges from 1 to 2, 3 ,...... It indicates that a total of 120 projection bands are divided around the world, that is, the longitude 1.5 ~ The longitude of the central longitude is 3 degrees, and the longitude of the longitude is 4.5 ~ The longitude of the central longitude is 6 degrees east longitude. our province is located in the east longitude 113 degrees-East Longitude 120 degrees, across 38th, 39, 40 a total of 3 belts, of which the east longitude 115.5 degrees to the west is 38th, its central longitude line is east longitude 114 degrees; eastern longitude 115.5 ~ 118.5 degrees for 39 belts, the central longitude line for the east longitude is 117 degrees; 118.5 degrees for the east longitude to the west of the mountain Customs is 40, and the central longitude line for the east longitude is 120 degrees.
The first two digits of the Cross-kilometer network coordinate on the topographic map are the belt number. For example, the horizontal coordinates of the 20345486-345486 Topographic Map in Our Province are, 20 of which are the belt number and are the abscissa value.

2. Calculation of the longitude of the local central longitude line
Calculate the longitude of the six-degree central longitude line: the longitude of the local central longitude line is 6 ° × the longitude of the local longitude line is-3 °. For example, the abscissa of the topographic map is 20345, the longitude of the central longitude of the six-degree belt is 6 ° × 20-3 ° = 117 ° (applicable to and topographic maps ).
Calculation of longitude of the Three-degree central longitude line: longitude of the central longitude line = 3 ° × Local longitude line number (applicable to topographic map ).
The calculation method of the central longitude line mentioned in gps320/315 is incorrect. avoid misleading during use.

Iii. GPS coordinate system and coordinate System Transformation
The GPS receiver is based on the WGS84 coordinate system (longitude and latitude coordinate system. The topographic map currently used in China belongs to the Beijing coordinate system (bj54) in 50 thousand, which is usually called the KM network coordinate. However, the GPS receiver has preset the coordinate conversion formula between the WGS84 and the KM network coordinates. Therefore, we only need to input the necessary parameters into the GPS receiver to automatically convert them. The parameters are as follows:
Longitudeorigin: (Central longitude): based on the preceding content, the scale is determined based on the map of different scales and the location of the local location; Scale (projection ratio): 1.0000000; false 'E' (East-West deviation): 500000.0; false 'n' (North-South deviation): 0.0.
Because the WGS84 coordinate system and the KM network coordinate system usually have 80 ~ For the difference of 120 meters, Accurate Correction is also required to obtain precise km network coordinates. Parameters in different regions are slightly different. Beijing parameters can be used in counties near Beijing, other regions in Hebei Province can use the parameters of Hebei province. The regions to the west of e114 ° (including the west of Xiazhuang township in Fuping County, wentang in Pingshan, west of sujiazhuang, west of Jinjing mining area, and west of pulp and Water Town in Xingtai county, west of the Active Water township in wu'an, covering the whole county). You can try parameters in Shanxi Province, using. take E111 ° for the central meridian of a topographic map of 5 or, and other parameters remain unchanged.

4. GPS error correction using coordinate points
1. Use GPS to measure known coordinate points to obtain the coordinates xgps and ygps;
2. Calculate the difference between the two: △x = XGPS-X known △y = YGPS-Y known
3. Calculate the false 'E (East-West deviation) and false 'n' (North-South deviation)
East-West deviation = 500000-△x North-South deviation = 0-△y
4. Change the values of false 'E' (East-West deviation) and false 'n' (North-South deviation) in the GPS parameter)
To obtain very precise coordinate points, we can only obtain them from the Surveying and Mapping Department. However, we can also obtain accurate coordinate points on KM network through the registered topographic map in the geographic information system. Under normal circumstances, you can also take the coordinates of multiple easy-to-determine special points from the topographic map, compare them with the coordinates measured by GPS, and obtain the mean deviation value.

5. Notes
Currently, most GPS functions to calculate the area, but if the area is small, you need to perform repeated measurement multiple times to obtain its average value.
Shutdown or battery replacement will not cause loss of system parameters. Therefore, it is recommended that you do not modify the parameters easily after setting the system parameters to avoid unnecessary errors due to misoperations.
In addition, due to the satellite signal wavelength, GPS is difficult to receive signals in concrete buildings, and positioning and navigation operations are not allowed.

How to calculate the local central Meridian?
The local central meridian is determined by the local Cartesian coordinate system. First, determine whether your Cartesian coordinate system is a 3-degree or 6-degree projection, and then calculate it based on the following formula:

Calculation Formula for 6-degree central Meridian: Local longitude/6 = N; Central meridian L = 6 x n

When N has remainder, the central meridian L = 6 x n-3

Calculation Formula for a 3-degree band central Meridian:

Local longitude/3 = N; Central meridian L = 3 x n

 

Specifically registered to correct the 6-degree calculation formula with central longitude in the post.
"6 degree with central meridian L = 6 x n-3" is incorrect, it should be L = 6 x (n + 1)-3.
When there is no remainder in the local longitude, N should be the first carry number, and the local carry number is n + 1.
I have seen many posts on the Internet that reference the wrong 6-degree calculation method with central Meridian. I would like to point it out so as not to mislead you.