Overview of Basic Principles
The GPS satellite ephemeris is based on the WGS-84 geodetic coordinate system, which means that most handheld GPS devices use this same system for positioning. However, many maps and geographic data available today are based on older local coordinate systems such as the 1954 Beijing Coordinate System or the 1980 Xi'an National Geodetic Coordinate System. This difference in coordinate systems can cause confusion and inaccuracies when using GPS data in conjunction with these maps.
To resolve this issue, it's necessary to convert GPS coordinates from WGS-84 to the local coordinate system used by the map. This process requires calculating transformation parameters between the two systems—typically five parameters: DX (X-axis shift), DY (Y-axis shift), DZ (Z-axis shift), DA (scale factor), and DF (rotation angle). Once these parameters are known and entered into the GPS device, it can automatically perform the conversion, allowing users to obtain coordinates in the desired system, such as Beijing 54 or Xi'an 80.
Below is a practical method for converting GPS coordinates to the Beijing 54 system, which can be adapted for other coordinate systems as well.
Step 1: Collect High-Accuracy Control Points
Begin by identifying at least three evenly distributed points within your area of interest. These should ideally be high-accuracy control points, such as those from a "B" level GPS network or survey benchmarks. The ideal location has no electromagnetic interference and offers a clear view of the sky for strong satellite signals. Visit your local surveying and mapping authority (such as a survey bureau or mapping institute) to obtain the corresponding coordinates. You'll need both the Gaussian plane rectangular coordinates (x, y) and the height (h), along with the geodetic latitude (B) and longitude (L) in the WGS-84 system for the Beijing 54 coordinate system.
Step 2: Determine Coordinate Conversion Parameters
Once you have the control point data, submit it to the technical support department of a relevant organization, such as Beijing Hezhong Zhuang Zhuang Corporation, to calculate the conversion parameters. Alternatively, you can use specialized software to determine these values. After obtaining the parameters, input them into your GPS device according to the instructions. Once set up, all GPS measurements taken in the area will automatically be displayed in the Beijing 54 coordinate system.
Step 3: Test the Conversion Parameters
After entering the five parameters (DX, DY, DZ, DA, DF), also input the central meridian longitude of your area. Set the projection scale to 1, the east-west offset to 500,000 meters, and the north-south offset to 0. To ensure accuracy, conduct field tests by measuring known feature points—such as intersections of linear features or independent landmarks—with good GPS signal reception. Ideally, use buried stone control points and compare their measured coordinates with their known theoretical values. If the difference exceeds the instrument’s specified accuracy, re-calculate the parameters or investigate the source of the error.
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