## Why Seabed 2030? ### How Much of the Seafloor Is Actually Mapped? Answering the question of \"how much of the seafloor is actually mapped\" is far from simple. Aside from the issues of knowing how much data exists from hundreds of potential sources around the world (an issue that will be addressed later), there is the fundamental question of what does \"mapped\" mean. When a lead-line makes a measurement on the seafloor, the actual spot sampled is, in reality just a few tens of centimeters wide, as represented by the diameter of the weight (Figure 2). When a single-beam echo-sounder makes a depth measurement, the shallowest point in the area ensonified (with a diameter on the order of one half the water depth for a sounder with a beam width of 30\({}^{\circ}\); Table 1) is reported, but with no angular information and thus the location of the sounding is reported as the downward projection of the position of the vessel on the seafloor no matter where the shallow feature really is within the ensonified area (Figure 2). For example, in 4000 m of water depth, a single-beam sounder with a 30\({}^{\circ}\) beam width will ensonify a diameter of approximately 2100 m on the seafloor. The shoalest feature within that circle will be the reported depth at a position that is at the center of the ensonified diameter (Figure 1). We can thus be confident that the depth of shoalest feature within the ensonified area has been measured, but we do not know where in that area the feature is and there is no other information about what else is in the 2100 m diameter ensonified area. Should we consider the entire ensonified area mapped for each single-beam sounding? The question of what is mapped or not is somewhat simpler for modern multibeam sonars that use beam forming and orthogonal transmitter and receiver arrays (Mills Cross) to form many narrow beams across a wide (athwartship) swath. The fundamental lateral resolution on the seafloor is controlled by the length of the arrays and the intersection (cross-product) of the transmit and receive beams (Figure 2). Most modern deep-water multibeam sonars form beams that are 1\({}^{\circ}\)\(\times\) 1\({}^{\circ}\), 1\({}^{\circ}\)\(\times\) 2\({}^{\circ}\) or 2\({}^{\circ}\)\(\times\) 2\({}^{\circ}\) producing ensonified areas on the seafloor (and thus fundamental feature resolutions) that vary with depth (Table 1). Figure 2: The mapped area of the seafloor (ensonified area) using different methods. (**a**) Lead line. (**b**) Single-beam echo-sounder (SB). (**c**) Multibeam echo-sounder (MB). GEBCO's latest product (GEBCO_2014 [1]) is a mixture of both single-beam, multibeam, and satellite altimetry-derived bathymetry assembled into a model gridded at a regular interval of 30 arc-seconds (Figure 3). The Arctic Ocean in GEBCO_2014 is comprised of a separate grid provided by the International Bathymetric Chart of the Arctic Ocean (IBCAO; [21]) and the Southern Ocean consists of a similar grid created by the International Bathymetric Chart of the Southern Ocean (IBCSO; [22]), two Regional Mapping Projects working within GEBCO. GEBCO_2014 also includes the GMRT compilation of multibeam bathymetry from research expeditions throughout the world ([[http://gmrt.marine-geo.org](http://gmrt.marine-geo.org)]([http://gmrt.marine-geo.org](http://gmrt.marine-geo.org)); [23]), which covers \(\sim\)8% of the world ocean and is the largest source of multibeam-derived soundings contributing to the GEBCO_2014 grid. In addition, some regions are covered by other regional projects such as EMODnet, covering European waters (www.emodnet-bathymetry.eu), and the Baltic Sea Bathymetry Database ([[http://data.bshc.pro](http://data.bshc.pro)]([http://data.bshc.pro](http://data.bshc.pro)); [24]). The GEBCO_2014 grid was based on all publically available bathymetric data at the time of compilation (Figure 3). Track-line plots such as Figure 3 are particularly deceptive in terms of depicting coverage.