The Surface Under the Water – A Guide to Bathymetric Data in Global Mapper
Under the umbrella of hydrography, bathymetry refers to the measure of depth for oceans and other bodies of water. When collected, these depth measurements create a chart or 3D model describing the underwater topography. With the ocean floor represented as a gridded 3D surface, all the terrain analysis tools available in Global Mapper can be used to visualize, edit, and analyze the data.
Creating Ocean Floor Model
Underwater data collection presents a unique set of challenges, namely the presence of water covering the terrain. To collect 3D data for a terrestrial area, lidar data collection creates point returns based on light pulses reflected from the surface. With the ocean in the way, this method of measuring light pulse returns cannot be used to collect three-dimensional data for the seafloor. With over 70 percent of the Earth covered by oceans, this leaves an incredible amount of area that needs another method of data collection.
Bathymetric data can be collected in several ways, but most utilize sonar. Instead of using light, like with lidar data collection, acoustic beams are sent underwater, and the properties of their return are collected to form a 3D dataset.
As acoustic pulses are sent out from an echo sounder, the properties of these waves are recorded to determine the topography and other characteristics of the submerged terrain. Image Source
The device used to send and receive acoustic beams is called an echo sounder, and depending on the body of water being measured and the specific device being used, single beams or arrays of beams are transmitted. For smaller and shallower waterways, single beams may be used from an echo sounder attached to a boat, but to map the depths of the oceans, multiple beams are used, and depending on the level of data being collected, autonomous echo sounders may be deployed to detach the data collection from a manned boat.
With the acoustic pulses recorded, a data layer is generated consisting of points describing the submarine topography. This raw data loaded into Global Mapper can be treated in the same way as any other point cloud, and gridded to create a 3D terrain surface. After the bathymetric data is gridded to create a surface model of the seafloor, all of the terrain analysis tools in Global Mapper can be used for editing and analysis of the data. Often this process of generating the gridded layer is done by the organization that has collected the data. When the data is delivered it is already in a 3D gridded format and ready to be loaded and analyzed in Global Mapper.
Downloaded from the GEBCO data portal, this 3D data loads into Global Mapper as a terrain grid.
Raster data and Charts
Aside from 3D data describing the seafloor topography, many other maps and charts exist to provide information about the world’s oceans. Global Mapper provides access to various publicly available online sources, including bathymetric relief images. The General Bathymetric Chart of the Oceans (GEBCO) and National Oceanic and Atmospheric Administration (NOAA) base map sources are both shaded relief raster image layers useful for base maps or as a starting point for an initial exploration of a target area.
A shaded image showing the terrain can easily be streamed into Global Mapper for any project area.
Navigation charts contain a wealth of information about the seafloor, sea surface, and coastal characteristics that can be viewed in Global Mapper. Raster Navigational Charts (RNC) and Electronic Navigational Charts (ENC) in vector format contain much more detail than the shaded raster images streamed from the online sources. ENC layers contain bathymetric measurements, land boundaries, and additional information about the seafloor and water currents in vector format making this data ideal for use in a GIS program. The vector features contained in these charts can be searched, filtered, and edited as any other vector data in Global Mapper.
Vector navigational charts can be downloaded from NOAA and loaded into Global Mapper. Features can then be styled and edited as needed.
With 3D seafloor data and a RNC loaded, Global Mapper can texture the raster chart with the hill shade applied to the 3D data. This layer blending tool, enabled in the Display Options for the raster image, creates a layer that contains the labels and point information from the chart with the shaded relief shown in the GEBCO and NOAA base maps.
The Raster Navigational Chart (RNC) is textured using the downloaded GEBCO data.
Isolating the 3D surface data and using a custom shader in Global Mapper, the GEBCO shaded relief colors can be applied to create a similar visualization in the 3D data.
Instead of using a built-in shader for the bathymetric grid, a custom shader is used to show depth in varying shades of blue.
To more closely examine bathymetric features and seafloor elevation changes, the Path Profile tool can be employed to generate a cross-section of the terrain. Furthermore, the perpendicular path profile tool can create parallel cross-section views along the data. This capability allows the terrain to be examined and measured in steps down the length of a ridge.
A perpendicular path profile creates a series of cross section views along the drawn line.
These cross-sectional views can be exported as images, PDFs, or saved as 3D vector lines in the workspace. These lines describing the terrain can be edited in Global Mapper or exported to any supported vector format.
Each profile can be saved as a 3D line and shown in the 2D and 3D views.
A method of filtering the data by elevation, or depth value, can be applied from the Layer Options dialog. On the Alter Elevation Values tab, the valid elevation range can be set. Areas outside of this described range will be hidden from view in Global Mapper, effectively cropping the data by the elevation value.
The downloaded GEBCO grid contains data for the seafloor and landmass terrain. Filtering by elevation the area of interest can be narrowed down to areas with an elevation below sea level, 0 meters.
With the 3D submarine topography data in a terrain grid format, all of the available terrain analysis tools in Global Mapper can be used to further explore a project area. Contour lines along with peaks and depressions can be easily identified with the Generate Contours tool. This tool functions the same whether you are considering landmass terrain or 3D bathymetric data.
Contour lines generated from the GEBCO 3D data show local high spot elevations.
A common source of concern among bathymetric analysts is maintenance of shipping channels, within which the natural dynamics of seafloor material often results in sediment accumulation and blockage. Global Mapper includes an array of tools for simulating the dredging process by creating a modified surface and calculating the amount of material that will need to be removed. To edit the bathymetric terrain, any of the terrain editing tools in Global Mapper can be used. The Terrain Painting tool can be used to directly edit the depth values for the bathymetric grid by painting across it with the cursor. Using a 2D vector feature indicating the channel centerline as a guide, the Terrain Painting tool is used to manually edit the terrain in order to model the dredged channel approaching a port.
Using the option to Set Terrain Height, the channel elevations are lowered in the terrain grid. Along the drawn line, the channel will be set to the specified height and then blended into the surrounding area over the set feathering distance.
A line brush type is used to trace along the existing channel centerline feature in order to flatten the terrain along this line.
In the end the terrain is edited to show a depression in the surface for the channel. Loading, or creating an unedited version of this terrain grid allows a comparison to be seen in a Path Profile crossing the channel. The volume measurement tool can subsequently be used to calculate the volume of material by subtracting the per-pixel depth values in one layer from the other.
With multiple layers depicted in the cross-section view, it is clear how the terrain was edited to create this channel.
With freely available data and the tools in Global Mapper, bathymetric data can be explored and analyzed just as any other terrain layer. Considering all the tools in Global Mapper and the formats supported by the program a comprehensive analysis of submarine topography can be completed and tools such as terrain painting and volume calculation can take the workflow further into planning for coastal and marine research and construction projects.
The latest version of the Global Mapper and Lidar Module include several enhancements, many of which apply to bathymetric data analysis. If this blog piqued your interest and you’d like to find out if Global Mapper is the right application for you, download a 14-day free trial and request a demo today!
National Oceanic and Atmospheric Administration. “What Is Bathymetry?” NOAA’s National Ocean Service, 26 Feb. 2021, oceanservice.noaa.gov/facts/bathymetry.html.
Boldt, Justin. Bathymetric Surveys, USGS, www.usgs.gov/centers/oki-water/science/bathymetric-surveys?qt-science_center_objects=0#qt-science_center_objects.