Just like digital topography on land, digital bathymetry allows multiple views of the landscape. These are often displayed in shaded relief, and may superficially resemble side scan sonar imagery. They can be displayed with colors keyed to the elevation, but if other data will be displayed on the base map, then usually the bathymetry will be in grayscale.
Multibeam uses a number of sound pulses from a hull mounted system, that covers a fan on either side of the vessel, across track. The maximum scan angle is fixed, so the swath width depends on the water depth. Shallow water will have narrow swaths, and require many track lines for complete coverage. As the survey ship moves, each set of pings add a series of depth across track, and successive sound pulses build up the data along track.
|Large scale map of a small region. You can clearly see a single ship track, and the rectangular region in the center where they surveyed a small box. The black regions are where there is no good, high resolution data available. This is often referred to as "masked" data. This is collected using multibeam swath bathymetry, a sonar system.|
|Large scale map of the same small region. In this case, the best
available depth for each point is included in the data. You can
clearly see the high resolution data, the much poorer resolution data
beyond the swath covered by the ship tracks, and artefacts along the
edges where the two data sets were joined. This is often referred to as
"unmasked" data. While visually jarring, this does allow you to
see the regional context for the high resolution bathymetry.
The regional bathymetry will generally be from the radar altimetry, Smith & Sandwell Predicted topography, which is based on limited ship soundings and radar altimetry. Despite its shortcomings, this or the closely related ETOPO1 is the best global data set currently available for most of the world. The data comes from Radar Altimeters and the marine geoid.
|Medium scale map of a larger area. At this scale, some really low resolution data appears across the north edge of the map and in the NW corner, but it is very hard to see the difference in resolution for the data in the map above, which in the SE corner of this map.|
|Normally surveys proceed by "mowing the lawn", using adjacent,
overlapping tracks, whether for a survey ship or an aircraft. This
can be very slow. The example on the left leaves gaps between
profiles, and can cover about twice as much territory in the same time
as would be required for complete coverage. This is a difficult
tradeoff to make, but in many cases you can get a very good
understanding of the seafloor with tracks oriented perpendicular to the
trend of the topography
Note in this case that the ship ran tracks in two directions (E-W and N-S), suggested a change in the seafloor. The fact that detailed bathymetry exits also suggests a major feature, which justified the time and effort to do the surveys. This is likely to represent a ridge segment and an offsetting transform; most of the detailed bathymetry in the deep oceans has been done along these plate boundaries.
|A closer view of the map above shows the lack of detail in the regional bathymetry. The detailed, but separated tracks show the trend and size of the abyssal hills.|
|Swath bathymetry off Honolulu. Note that the swaths get
narrower approaching shore, in shallower water, because they record a
fixed angle below the survey ship and become wider as the depth
Image courtesy of NOAA, http://service.ncddc.noaa.gov/website/EXAtlas/viewer.htm
last revised 12/7/2016