# 3-degree band and 6-degree band

Source: Internet
Author: User

1. 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.

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 a topographic map are belt numbers. For example, the cross-coordinate on a-50 thousand topographic map is 20345486, where 20 is the belt number, and 345486 is the X-coordinate 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 ).

3. 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 based on a 3-degree or 6-degree projection formula:
Formula for Calculating the 6-degree belt central Meridian: Local longitude/6 = N; Central meridian L = 6 * n (belt number)
When N has remainder, the central meridian L = 6 * n-3
Calculation Formula for a 3-degree central Meridian: Local longitude/3 = N; Central meridian L = 3 x n
China's longitude ranges from 73 ° to 135 ° to the West, which can be divided
Eleven in the Six-Degree belt (belt 13-belt 23). The central longitude lines of each belt are (75 °, 81 ° ,...... 123 °, 129 °, 135 ° );

Carry 22 of the three degrees (24-45 ). The central longitude of each belt is (72 °, 75 ° ,...... 132 °, 135 ° );

The six-degree belt can be used for small and medium scale (such as) ing, and the three-degree belt can be used for large-scale (such as) ing. The urban construction coordinates usually use the Gaussian projection of the Three-degree belt.

4. How to determine whether the projection coordinate is 3-degree coordinate or 6-degree Coordinate
For example, (4231898,21655933) where 21 is the belt number, the same defined East pseudo offset value also needs to be added with a number, for example, the East pseudo offset value of 21 is 21500000 meters. If the longitude of your work zone is in the range of 120 degrees to 126 degrees, the coordinate system is a 6-degree coordinate system with the central longitude of 123 degrees.
For example, (2949320,36353822) 36 indicates the belt number. It is known that the location is near Guiyang City. On the map, we can see that the longitude of Guiyang is about 108 degrees east longitude, therefore, it can be 36*3 = 108. Therefore, the coordinate system is a 3-degree coordinate system with a central longitude of 108 degrees. But it cannot be 6 degree band: 36*6 = 216.

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