## Why Seabed 2030?
### At What Resolution Should We Map the Seafloor? As we plan for the Nippon Foundation--GEBCO Seabed 2030 Project, we must address the question \"at what resolution should we be mapping the seafloor?\" Ideally, one would want a resolution commensurate with that at which our land surfaces are mapped (sub-meter resolution). While this would be ideal, the limited propagation of electromagnetic waves in the ocean must be considered, and thus achieving sub-meter resolution with either acoustic or optical systems requires that the mapping system be within tens of meters of the seafloor. This is technologically achievable with surface vessels operating in shallow water environments. However, more than 90% of the ocean is deeper than 200 m water depth, and this would require the delivery of the measurement system close to the seafloor by either remotely operated vehicles (ROV) or autonomous underwater vehicles (AUV). Such systems are available and are capable of collecting very high-resolution acoustic or optical data near the bottom; however, at present, given their expense and slow speed of operation, both ROVs and AUVs are not realistic options for global mapping. This may change in the future (see new technology section below); however, if we truly aspire to global coverage, we will continue to rely largely on surface vessels, which will dictate achievable spatial resolution. Any discussion of seafloor mapping resolution must be presented in the context of changing depths, and as the effort to map the world's oceans is global, the resolution specified for a given sonar system must vary as a function of depth (Table 1). With the hope that much of the new data collected in support of the Nippon Foundation--GEBCO Seabed 2030 Project will be multibeam sonar data, we look at the achievable beam footprints for a range of water depths along with the percentage of seafloor area in each depth bin. Since few multibeam systems are \(1^{\circ}\times 1^{\circ}\) configuration, we have adopted a conservative approach and base the achievable mapping resolution on a \(2^{\circ}\times 2^{\circ}\) system deep water system (Table 1). Furthermore, as the beams are steered away from nadir, their footprints expand, decreasing the lateral resolution on the seafloor across the swath; at \(60^{\circ}\) from nadir (approximately 1.7 times the water depth) on each side, the beam footprint is double that found at nadir (Figure 6). Considering that high-quality multibeam sonar data typically can be retrieved to a maximum swath width of about four times the water depth we will adopt a feasible resolution that is constrained by the worst-case solution of the near outermost beams (at about \(60^{\circ}\) to either side of nadir). Thus, for the Nippon Foundation GEBCO Seabed 2030 Project, we will establish a depth-variable resolution  
Figure 6: Footprint of outer (\(60^{\circ}\) to one side) beam on seafloor (achievable lateral resolution) for \(2^{\circ}\times 2^{\circ}\) multibeam sonar, as a function of water depth. Swath width coverage (assuming \(4\times\) water depth) is shown in red. Left column is the percentage of the global seafloor in each depth bin. Note that in deepest water depths (\(>\)10 km) swath widths can be limited by attenuation. goal and consider the seafloor \"mapped\" if at least one sounding falls in a grid cell of the size described in Table 2. The depth regions corresponding to the calculated feasible grid resolutions are shown in Figure 7.  
The needs of the broad community of stakeholders who use deep ocean bathymetry vary with respect to required resolution. Mapping at the resolutions described above may not serve all needs; however, baseline bathymetry from the Nippon Foundation GEBCO Seabed 2030 Project will help identify those regions where higher-resolution data may be required. It will also serve as a baseline against which additional surveys can be compared to better understand processes that can change the seafloor in some regions.