Overview of the Basic Principles
The GPS satellite ephemeris is based on the WGS-84 geodetic coordinate system, which means that the handheld GPS devices use this same coordinate system. However, many map resources available today are based on older systems such as the 1954 Beijing Coordinate System or the 1980 Xi'an National Geodetic Coordinate System. This difference in coordinate systems can create challenges when trying to align GPS data with local maps or surveying projects. As a result, there is often a need to convert WGS-84 coordinates into either the Beijing 54 or Xi'an 80 coordinate systems.
Understanding the Need for Coordinate Conversion
Coordinate conversion between different systems involves both translation and rotation. To ensure accurate results, it's essential to calculate the transformation parameters between the WGS-84 system and the target coordinate system (such as Beijing 54 or Xi'an 80). These parameters typically include five values: DX, DY, DZ, DA, and DF. Once these are determined and entered into the GPS device, it can automatically perform the necessary conversions, allowing users to obtain data in their preferred coordinate system.
Example: Converting to the Beijing 54 Coordinate System
Below is a step-by-step guide on how to perform coordinate conversion using the Beijing 54 system as an example.
Step 1: Collect High-Precision Control Points
To begin the conversion process, you need to gather data from high-accuracy control points within your area of operation. Ideally, select three or more evenly distributed points, preferably in areas with clear satellite signals and minimal electromagnetic interference. These points could be B-level GPS network points or other high-precision benchmarks. It's best to choose locations with good visibility and strong signal reception. You can then visit your local surveying and mapping authority (e.g., a surveying bureau or institute) to obtain the corresponding Gaussian plane rectangular coordinates (x, y), height (h), and geodetic latitude and longitude (B, L) for the Beijing 54 system.
Step 2: Obtain Coordinate Transformation Parameters
Once you have the control point data, you can provide it to a technical support unit, such as Beijing Hezhong Zhuang Zhuang Corporation, to compute the necessary transformation parameters. Alternatively, you can use dedicated conversion software. After obtaining the parameters, input them into your GPS device as instructed. Once set up, all GPS readings taken in the area will automatically display in the Beijing 54 coordinate system.
Step 3: Test the Conversion Parameters
After entering the five parameters (DX, DY, DZ, DA, DF), make sure to also input the central meridian longitude of the measurement area. Set the projection scale factor to 1, the east-west offset to 500,000 meters, and the north-south offset to 0. After configuring these settings, conduct field tests to verify the accuracy of the conversion. Choose feature points with clear views and strong GPS signals, such as intersections of linear features or independent landmarks. If possible, measure existing benchmark stones and compare their theoretical coordinates with the GPS readings. If the results exceed the instrument’s specified accuracy, re-calculate the parameters or investigate potential issues.
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