Projection is used to convert ellipsoidal coordinates into plane coordinates. The system comes with multiple commonly used projection methods, including Gaussian projection, horizontal Mercator projection, UTM projection, etc., and provides the ability to configure corresponding projection parameters.

In conventional applications, the parameter that users usually focus on is the central meridian. For custom coordinate systems, the central meridian can generally be determined based on the average longitude of the measurement area. To ensure projection accuracy, the deviation between the measurement area and the central meridian should not be too large, and it is usually recommended to control the longitude difference within 30 '.

• Central Meridian: Key parameters in projection definition. The system supports automatic acquisition of the central meridian corresponding to the current position, and also supports manual input. If the currently set central meridian does not match the actual position of the work area, the system will prompt the parameter deviation during the measurement process to remind the user to correct it to avoid abnormal coordinate results caused by projection parameter errors.
• Addition constant: The numerical range used to adjust the projected coordinate value. Usually, the east-facing constant is set to 500000 m by default to ensure that the coordinate value is positive. This parameter can be adjusted according to project specifications or local requirements.

The seven parameters are used to achieve coordinate conversion between different spatial Cartesian coordinate systems. The parameters usually include three translation quantities, three rotation quantities, and one scale factor. If the user has mastered the results of the seven parameters in the target area, they can directly input and apply them without recalculating through point correction.

Seven-parameter conversion usually requires at least three known common points for calculation. Known points can be control points in the national coordinate system or coordinate points with only minor system differences from WGS84. To improve model reliability, it is recommended to use more than three known points for checking point quality and parameter calculation results.

The seven-parameter method model is relatively rigorous and requires high accuracy for known points. It is suitable for medium and large-scale operations or scenarios that require high stringency for conversion models. When the accuracy of known points is insufficient, the distribution is unreasonable, or the local distortion is obvious, it is not recommended to use seven-parameter conversion.

Mainly used for plane coordinate conversion and local correction. After the system completes point correction and application, it will automatically generate corresponding plane conversion parameters and display them in the coordinate system parameter interface.

This type of parameter is usually suitable for plane coordinate fitting and transformation in a small area, suitable for local engineering measurement or application scenarios with limited control conditions. Compared with the seven-parameter method, its model is more suitable for local approximation processing, and the adaptation to point distribution and coordinate system differences is also different.

Elevation fitting is used to establish the correction relationship between GNSS elevation, engineering elevation, and leveling elevation. The system currently supports four elevation fitting algorithms: weighted average, plane fitting, surface fitting, and vertical adjustment, with plane fitting being the default.

• Single Benchmark: Weighted processing is performed based on the correction values of known elevation control points to generate unified or locally weighted elevation correction results. It is suitable for scenarios where the elevation changes slowly and the number of control points is limited.
• Plane Fitting: Establish the best fitting plane based on elevation anomalies at multiple elevation control points. When the fitting plane is parallel to the horizontal plane, it can be approximately understood as a fixed error correction. This method requires at least one starting point and is suitable for cases where the elevation correction change in the measurement area is relatively flat.3
• Surface Fitting: The best fitting surface is established based on elevation anomalies at multiple elevation control points, which is usually used to describe more complex elevation correction trends. This method requires at least 5 starting points and has high requirements for the number, distribution, and accuracy of control points. If the fitting quality is poor, it may cause unstable elevation correction results in the work area, and even divergence.
• TGO Vertical: The overall adjustment processing of elevation system differences is carried out through the vertical correction model, which is suitable for elevation unified scenarios with certain control foundations. In practical applications, the fitting method should be reasonably selected according to the measurement area range, terrain undulation, control point quality and distribution.

Used to describe the spatial distribution of plane coordinate correction information within a region. The model usually organizes data in the form of a two-dimensional regular grid, with each grid node corresponding to a plane correction value or transformation information. The grid position, direction, and resolution are all clearly defined in the coordinate system.

Through the planar mesh model, fine expression of local regional plane coordinate differences can be achieved, which is suitable for regional plane correction scenarios that are difficult to fully describe with conventional analytical parameters. Compared with a single parameter model, the mesh model can more accurately reflect spatial variation characteristics, thereby improving local conversion accuracy.

Used to describe the spatial distribution of elevation correction information within a region, it is a way to discretely express the elevation changes in three-dimensional space. The model usually divides the work area into continuous grid units and stores corresponding elevation correction values in each grid node or unit.

In practical applications, the elevation grid model can be used to express regional elevation anomalies, fit correction value changes, or local elevation transformation relationships. It is suitable for scenarios with large ranges, complex elevation changes, or where conventional fitting methods are difficult to meet accuracy requirements.

Used to establish the conversion relationship between ellipsoidal height and normal height or positive height, it is an important basic data for elevation calculation and elevation system unification.

The system supports importing various common formats of geoid model files, including * .tif, * .gtx, * .asc, * .grd, * .ggf, etc. Users can load corresponding geoid models according to the project area and achievement requirements to improve the accuracy and applicability of elevation conversion.