The base station is erected at an unknown point, and the mobile station collects the known point to calculate four parameters, which are used for lofting. The lofting accuracy is mainly affected by the accuracy of solving four parameters, that is, the accuracy of solving the known control points of four parameters. Usually, when using the mobile station to collect known control points to solve four parameters, the observation time is relatively long, and the observation value with the largest number of visible satellites and the smallest DOP value is selected.

At the same time, the influence of receiver system error is inevitable. The plane accuracy of general RTK is1cm+1ppm; RTK elevation accuracy: 2cm+1ppm; Ordinary site lofting can meet the precision requirements of lofting. If the construction lofting requires high precision, it is best to use total station.

Question 2:

You mentioned here several conversion methods between WGS84 coordinate system and the coordinate system adopted by China, namely

1. three-parameter method

2. Four-parameter method

3. Seven-parameter method-bursa model method

First of all, the seven-parameter method is the most rigorous mathematical model for spatial coordinate transformation among the three methods, but they all have their own advantages and disadvantages.

1. The three-parameter coordinate transformation model is only a simple translation of the three-dimensional coordinate system, without considering the difference between WGS84 reference ellipsoid and China's reference ellipsoid. This method is simple and practical for small survey area. But among these three methods, the accuracy of coordinate transformation is relatively lowest. For a large survey area, it is not competent.

2. The four-parameter coordinate transformation model does not consider the difference between the two reference ellipsoids, and performs rotation, translation and scale transformation on the two-dimensional plane rectangular coordinate system, that is, one rotation angle, two translations and one scale. It does not involve elevation transformation, but a simple transformation of two dimensions. According to the conclusions of many papers published in China, for the survey area with the longest side within 10km, the plane coordinate transformation accuracy of the four-parameter method is higher than that of the strict seven-parameter method. This is the most reliable, simple and practical coordinate transformation method for small-scale survey areas within 10km.

3. The seven-parameter coordinate transformation model considers the difference between two reference ellipsoids, which are the rotation, translation and scale transformation of three-dimensional rectangular coordinates, that is, the rotation, three translations and one scale of three coordinate axes. It is the most rigorous spatial coordinate transformation model, and the calculation is more complicated. For the survey area with the longest side above 15km, the seven-parameter method must be used for coordinate transformation, which has high precision. However, for a small survey area, due to the influence of the conversion between geodetic height and normal height, the accuracy of the converted plane coordinates is not as high as that of the four parameters.

The third question:

The reference station is erected at different unknown points, and its signal acceptance conditions are different, and the distance from the lofting point is also different, which is related to the deviation of the lofting point, but the most important thing is the accuracy of the coordinate conversion parameters adopted.