The Bathymetry Assessment System: efficient depth mapping in shallow seas using radar images

With the operation of the European Remote Sensing (ERS) satellites and RADARSAT, radar images are now readily available. One of the new applications of radar images is their use for bathymetric mapping in shallow seas. The Bathymetry Assessment System (BAS), described in detail in this paper, constructs accurate depth maps from radar images and a limited number of echo soundings. The BAS consists of a forward imaging model and an inversion part. The system needs a first guess depth map that may be derived from echo soundings or an old map of the area. The forward model calculates a simulated radar image. This is compared with the actual radar image by evaluating a penalty function. The penalty function also contains a term that compares model depths with measured depths and a term that contains a smoothness criterion, prohibiting speckle noise to be interpreted as depth variations. The inversion part of the system consists of optimization of the penalty function. This leads to an iterative procedure in which some model parameters are also estimated. When converged, the model depth is an estimate for the real depth. This model depth matches radar images and echo soundings as closely as possible. The system may be regarded as an intelligent interpolator: it interpolates depth between transects of echo soundings steered by the bathymetric information in radar images. The system has been applied many times and some examples are given in this paper. Its accuracy depends on the number of echo soundings fed into the system, the number and quality of the radar images, and the nature of the area under consideration. When a root mean squared error of 30 cm (compared to echo soundings) is acceptable, the distance between the tracks of echo soundings needed by the BAS varies between 600 in to 1 km or more. This should be compared to the usual track distance that is 200 m at most. Use of the BAS may therefore lead to a considerable improvement in efficiency. The accuracy of the system can be improved by using airborne radar images with higher resolution.