What is point correction?

Keywords: GPS measuring instrument, intelligent RTKBeidouRTK Small RTK RTK Usage Instructions RTK Operation Steps GPS Measurement Experience

Point correction isTranslate into EnglishMeasured by RTKWGS-84Coordinate conversion toLocal Cartesian coordinate system.

The data collected by GPS satellite positioning system in engineering applications is WGS-84 coordinate system data, while currently our measurement results are generally based on the 1954 Beijing coordinate system or local (arbitrary | local) independent coordinate system. Therefore, it is necessary to convert the WGS-84 coordinates to the BJ-54 coordinate system or any local independent coordinate system.

The conversion between coordinate systems can utilize existing methodsSeven parameters（Three coordinatesoftranslation amountThe rotation angles of the three coordinate axes,The length of the same straight lineScale ratio in two coordinate systemsk）Three parametersA special case of seven parameters, with a scale ratio of1. In the case of the same direction）You can also use HuaceRTK comes withGeodetic software is used for point calibration, calculation of four parameters, and elevation fitting.

Single point correction: Using the WGS84 coordinates and local coordinates of a point, three translation parameters can be calculated, with zero rotation and a scaling factor of 1. Without knowing the rotation and scaling factors of the local coordinate system, the accuracy of single point correction cannot be guaranteed, and the control range cannot be determined. Therefore, it is recommended not to use this method as much as possible.

Two point correction: Three coordinate translation parameters, rotation, and scaling factors can be calculated, and each residual is zero. The scaling factor should be at least between 0.9999 * * * and 1.0000 * * * *. If it exceeds this value, accuracy may be compromised or there may be issues with known points; The rotation angle is usually relatively small, below minutes (0 degrees, 0 minutes, and 0.02 seconds). If the rotation is up degrees, attention should be paid to whether there is a problem with the known point or the * meridian.(Try not to use two-point calibration as much as possible)

Three point correction: Perform point correction on three points, with horizontal residuals and no vertical residuals.

Four point correction: Perform point correction on four points, including both horizontal and vertical residuals.

Precautions for point calibration:

1. Given that the points should be distributed along the edges of the entire work area, it is possible to control the entire area and avoid short edges controlling long edges. For example, if four points are used for point calibration, the measurement area is within the quadrilateral formed by these four points;

2. Be sure to avoid the linear distribution of known points. For example, if three known points are used for point correction, the triangle formed by these three points should be as close as possible to a regular triangle. If it is four points, it should be as close as possible to a square. It is important to avoid the distribution of all known points being close to a straight line, as this will seriously affect the accuracy of the measurement, especially the elevation accuracy;

3. If only horizontal coordinates are needed in the measurement task and elevation is not required, it is recommended that users use at least two points for calibration. However, if horizontal residuals of known points need to be checked, at least three points should be used; If both horizontal coordinates and elevation are required, it is recommended that users use at least three points for point correction. However, if they want to check the horizontal and vertical residuals of known points, they need at least four points for correction;

4. Pay attention to the coordinate system, meridians, projection surfaces (especially at high altitudes), and whether the control points and layout points are part of the same projection zone;

5. The degree of matching between known points is also important, such as GPSmeasurementIf both the known points of the triangle and the known points of the country are used simultaneously, the horizontal residuals may be significant during verification;

6. If there are 3 or more points for point correction, check the values of the horizontal and vertical residuals to see if they meet the user's measurement accuracy requirements. If the residuals are too large, do not exceed 2 centimeters. If they are too large, first check if there is an error in the input of the known points. If there is no error, it means there is a problem with the matching of the known points and they need to be replaced;

7. For elevation, special attention should be paid to the linear distribution of control points (several control points are distributed on a line), especially for line engineering. It is recommended that the number of elevation points involved in calibration should not exceed 2 points (that is, during calibration, vertical adjustment should not be selected for more than 2 points in the calibration method).

8. If an area is relatively large and has many control points, it is necessary to perform calibration in different zones. Do not have more than ten or more points in one area participate in calibration.

9. Note that only one point correction is required for a region, and subsequent measurements only need to reset the local coordinates.

The instrument in the picture is the Huace Intelligent RTKi80 GPS measuring instrument