Geometric Calibration Method For The Multibeam Bathymetric Data At Large Incidence Angles Based On The Laser Tie Points

The boat-based mobile mapping system (BoMMS), which is equipped with a laser scanner, a multibeam echo sounder, and an integrated navigation system, can simultaneously capture the topographic information both above and under water in the regions to be measured, e.g., islands, reefs, coasts, ports, and dams. Compared to the traditional method, which surveys the underwater topography with a single-beam sonar and surveys the land topography with handled RTK devices, the BoMMS has a higher surveying efficiency and higher density point cloud. To create a completely seamless topographical map both above and under water, not only the acquisition of complete terrain data in the regions near the borders of water and land is needed, but also it is necessary to unify the geographical frame for both above and underwater data. More importantly, the methods to calculate the positions of points both above and under water is also necessary, which are vital to improve the accuracy of points in the regions near the borders of water and land. To this end, this paper goes into more detail on several key technologies in the processing of multibeam data, the research results are as follows:(1 ) The method of calculating the geolocation of the bathymetries is studied.By analyzing the effect of sound ray bending on the geolocation of the bathymeties and the limitations of the convention method, which firstly reference the beam points to the coordinate system of transducer and then reference to the geographic coordinate system, this paper proposed an advanced method to calculate the coordinate of points underwater. In this new method, the orientations at time of transmit, the mounting angles of the transducer and the steering angle of transmit are considered at the same time to determine the geographic launch vector. And by implementing the process of ray tracing with the calculated geographic launch vector,the more accurate geolocation of the bathymetries can be acquired through the coordinate transform matrix. Consequently, the results showed that the proposed method can not only improve the accuracy of points underwater, but also can narrow the gap between the laser points and sounding points near the borders of water and land when working with the BoMMS.(2) The geometric calibration for multibeam bathymetries with large incidence angle is studied. When the incidence angle is beyond 60°, the geolocation accuracy of measured points are relatively low when using the conventional method of sound-ray tracing, which induces a mismatch between the abovewater laser scanner points and the underwater multibeam bathymetric points at the border between water and land. To solve this problems, a new geometric calibration method is proposed to address the accurate geolocation problem for the measured points of multibeam bathymetries at large incidence angles. First, based on the sound velocity profile(SVP) data in survey regions and the surface sound velocity (SSV) data along tracks,a refined sound velocity model (SVM) is established for the initial sound-ray tracing using the algorithms of empirical orthogonal function (EOF) analysis and the weighted linear interpolation. Then, several groups of tie points are extracted, which are located on the border between water and land and are acquired by the laser scanner and multibeam echo sounder. The laser tie points which have better geolocation accuracies, are used to calculate the rotation angle and scale coefficient to obtain the geometric calibration for every sound ray corresponding to an incident angle beyond 60°. The calibration results showed that the horizontal and elevation accuracies of the multibeam measured points corresponding to large incidence angles were approximately 2.5 cm (increased by 11.5 cm) and 1.3 cm (increased by 3.6 cm),respectively. The calibrated points are in accordance with the measurement accuracy demands of IHO. The conclusion is that this method has a significant effect on improving geolocation accuracy for measurement using sound ray with large incidence angles.