Swath data collection creates non-uniform beam footprints resulting in a non-uniform sampling resolution. The across-track and along-track sampling resolutions are a function of distance from the boat or depth, angle of incidence, beam width, and slope of surface(International Hydrographic Bureau 2005, Diaz 2000). In general as beam angle or depth increase the size of the footprint increases resulting in a decreased sampling resolution. Slope of the surface being surveyed has a varying effect which is dependent upon the difference between the specific beam angle and the slope of the surface. The effect of beam angle at different depths for the RESON SeaBat 8101 is shown in Figure 2. For the idealized case of a flat bed, figure 2 reveals that a larger range of footprints exists as depth increases.
The concept of a beam footprint that varies in size is essential to understanding how uncertainty can be introduced into a surveyed point. When a beam footprint is large it has the potential to cover a large range of elevation values within the footprint, but will only return one representative value (see Quantifying Uncertainty for more info on this topic). By understanding the relationships between beam angle, depth, and slope of surface, surveys can be planned which reduce the footprint size and will reduce uncertainty introduced during data acquisition.
Given that the solution for beam footprint is a trigonometry function of depth, beam angle, beam width, and slope of surface this equation can be solved using GIS if the user provides a depth raster and slope raster (would need to have previously surveyed the site). These rasters would be used as input to the tool and the equation would be solved to minimize the beam angle (or a range as their generally is not one solution) on a cell by cell basis to create a 'best beam angle' survey map to guide the MBES operator's tracklines and survey strategy.
- International Hydrographic Bureau.(2005). "Manual on Hydrography:Publication M-13". 1st Edition May 2005.
- Diaz J.V.M. (2000) "Analysis of mulitbeam sonar data for the characterization of seafloor habitats". Master of Engineering, University of New Brunswick.