Application Research Of The GPS On Underwater Topographic Survey

Underwater topographic survey is crucial in the development and utilization in rivers and lakes, and it is the basis of numerous water conservancy projects. Underwater topographic survey is usually conducted on board. The plane coordinates of target point is measured on the surface, while the elevation is measured by depth sounder. By combination of the two, the underwater terrain at the three-dimensional coordinates can be achieved, and thus the ultimate underwater topographic maps.The plane coordinates of a target point on surface are obtained in various ways. One method in concern is GPS. With its real-time, all-weather, high precision features, GPS has been widely used in navigation, survey, mapping, communications and other fields. In the underwater topographic survey, the application of GPS has also been very popular. Some important issues relating to the combination of GPS and sounder in the underwater topographic survey are investigated in this paper.The data from GPS is in the WGS-84 coordinate system, the data of underwater terrain point, however, is subjected to an engineering coordinate system. Thus, the method of data transfer has been investigated. As groundwork, the related theories, formulas are demonstrated. And, through reasonable analysis, some improvements on the formulas and methods are put forward. The convergence of some of the formulas are also verified, which enriches to the theory of GPS positioning calculation.Several common coordinate delivery problems are also studied in this paper. The coordinates of on board GPS antennas need to be delivered to the transducer of the sounder in order to obtain the coordinates of underwater terrain point, in which coordinate transformation is involved. Therefore, a ship hull oriented coordinate system was introduced in the paper, so that the coordinate delivery problem was translated to an issue of coordinate transformation. Several common issues of coordinate delivery were illustrated, and corresponding formula were deduced.Another problem is that the survey ship may be out of the measuring region before the survey. At this point, the navigation information should be provided, in order to guide the ships into the destination swiftly. Hence, several issues on calculation of navigation point and navigation route are also investigated, and calculating methods are presented.Time and data-processing problems are also examined in this article. GPS positioning and the depth sounding are not consistent in time. First, the positioning data and the elevation data should be synchronized, and then, the accuracy should be high enough. As in the data processing, the output of depth sounder is not stable, causing a high probability of error. In order to enhance the accuracy of underwater topographic survey, efficient and automatic error correction is very important.Based on the study above, a set of underwater topographic survey software was developed. The software proves to be effective in practice, and will serve to the real underwater topographic survey work.