## Why Seabed 2030?
### How Much of the Seafloor Is Actually Mapped?
Given the scale of these figures, each of the track-lines shown covers a width of 100s of kilometers while in reality, a multibeam swath is typically on the order of four times the water depth (\(\sim\)16 km in 4000 m water depth) and a single beam would ensoirly a diameter of approximately 2 km in 4000 m water depth. Lines this thin would be impossible to display on global (or even regional-scale) maps and thus one gets the impression of much greater coverage than actually exists. As previously mentioned, the vast majority of the world ocean has not been sampled by echo sounders even at a resolution of about 30 arc seconds. Considering that many of the approximately 1 km grid cells only have a single sounding in them, the percentage of the seafloor that has actually been measured by echo-sounders is considerably less than 18%. Between track-lines, large areas of the GEBCO_2014 grid are based on interpolation guided by satellite altimetry-derived bathymetry data except in most of the two polar regions where sea ice precludes the use of satellite altimetry-derived data. GEBCO_2014 started with the base grid from the previous GEBCO_08 version, which included the altimetry-derived bathymetry model SRTM30_PLUS [25]. Satellite altimetry-derived bathymetry has represented a remarkable advancement in our mapping of oceans, providing global coverage of general estimates of depths and coarsely filling gaps between sparse ship soundings [2]. Satellite altimetry-derived bathymetry is, however, far less precise than echo sounder-derived data and has far less resolution than modern multibeam sonars, but the method is objective and, in most areas without sea ice, superior to interpolation between sparse ship tracks by mathematical algorithms or hand-contouring. Altimetry-derived data are particularly  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **SB Beam Width** & \multicolumn{3}{c}{**MB Beam Width (Transmit \(\times\) Receive)**} \\  & **Foot Print Diameter** & \multicolumn{2}{c}{**Footprint Diameter in Meters. (For the 1\({}^{\circ}\times\) 2\({}^{\circ}\) Configuration,** **in Meters**} \\  & **in Meters** & **the Semi-Minor and Semi-Major Axes of the Footprint Ellipse Is Given)** \\ \hline Depth & 30\({}^{\circ}\) & 1\({}^{\circ}\times 1^{\circ}\) & 1\({}^{\circ}\times 2^{\circ}\) & 2\({}^{\circ}\times 2^{\circ}\) \\
0 & 0 & 0 & 0 & 0 \\
500 & 268 & 9 & 9 \(\times\) 19 & 17 \\
1000 & 536 & 17 &  
good for mapping tectonic-scale features such as spreading ridge segments and the fracture zones that offset them, but much finer-scale features and accurate depths are often difficult to derive (Figure 4). Figure 4: Satellite-derived bathymetry (including single-beam sonar data) from SRTM 30_Plus off Hawaii (**left**). Single swath of 12 kHz multibeam sonar over the same area—swath width = 20 km for scale (**right**). Figure 3: GEBCO_2014 bathymetric data coverage. At this scale, the World Ocean appears much better covered with ship soundings than it is. The fact is that the available bathymetric data used to compile GEBCO_2014 provided depth control points to only 18% of all the 30 arc-second (926 m at the equator) grid cells. Figure is from [1]. The achievable resolution of a bathymetric model (grid) is a function of the underlying data density--in the case of the GEBCO_2014 grid, the coverage of satellite tracks for depth estimates and ship soundings. Ship-borne multibeam surveys collect depth measurements at very high density and may be designed with overlapping swaths to provide full map coverage. When multibeam data are incorporated into a bathymetry model, their very high-lateral resolution (70 m \(\times\) 140 m for a 1\({}^{\circ}\)\(\times\) 2\({}^{\circ}\) multibeam system in 4000 m water depth) and high data density (up to 800 soundings across a 20 km wide swath in 4000 m water depth) improves the resolution of seafloor details (Figure 4).