The Global Multi-Resolution Topography (GMRT) Synthesis is maintained as a multi-resolution gridded global Digital Elevation Model (DEM) that includes cleaned processed ship-based multibeam sonar data at their full spatial resolution (~100m in the deep sea). Multibeam bathymetry data are unique among the marine geophysical data types in their relevance for a broad range of scientific investigations and non-academic uses, providing fundamental characterization of the physical environment and serving as primary base maps for multidisciplinary programs. While specialist expertise is needed to access, quality control and process multibeam bathymetry data files to generate high-quality bathymetric maps, the GMRT Synthesis provides free open access to bathymetric images and gridded bathymetric data for specialist and non-specialist users alike. Details about the tiling method and procedures used for creating and serving the GMRT synthesis is available in Ryan et al., 2009.
Source Data & Resolution
GMRT brings together a variety of elevation sources which are delivered as multi-resolutional images and grids of land and ocean elevations. A mask layer is available that highlights the high-resolution data. Source data include:
- Ship-based multibeam swath bathymetry data (100-m resolution) from research cruises assessed, cleaned, processed and curated by the MGDS. The current version (GMRT v3.3) was released in February 2017 and includes swath data from 931 cruises.
- Terrestrial elevation data (10-m resolution) for portions of the US from the USGS National Elevation Dataset (NED)
- Terrestrial elevation data (30-m resolution) from NASA’s Advanced Spaceborne Thermal Emission and Reflection Radiometer global DEM (ASTER)
- Gridded seafloor depth data (variety of scales) contributed by the international science community
- Gridded seafloor depth data (30 arc-second resolution) from the General Bathymetric Chart of the Oceans (GEBCO_2014)
- Gridded seafloor depth data (2-km resolution) from the International Bathymetric Chart of the Arctic Ocean (IBCAO) version 2.23
- Gridded seafloor depth data and ice surface data (500 m resolution) from the International Bathymetric Chart of the Southern Ocean (IBCSO)
MB Swath Data Preparation
Where do the source data come from?
We make use of multibeam data that are in the public domain at the National Geophysical Data Center (NGDC). Our efforts are focused primarily on the US Academic Research Fleet, but we also process and include data from non-US sources that are in the public domain. In addition to the data that have been included in GMRT, several cruises worth of data have been reviewed and not included - typically because they do not provide new coverage, and sometimes because of significant issues with data quality. Additional processed and/or gridded data are also contributed directly by scientists. Please contact us if you would like to contribute data.
How are data prioritized for inclusion in GMRT?
We prioritize data for inclusion in the GMRT Synthesis based on several criteria including: (1) extent of coverage and/or survey area, (2) the needs of NSF-funded research initiatives (e.g. GeoPRISMS), and (3) quality of sonar system. Having worked through much of the available archive of legacy data, our primary focus is now on data from recent research cruises. Please contact us to request that we prioritize data from a particular cruise or area.
How can users request new data in GMRT?
Please contact us if you would like to contribute data to GMRT or request that publicly available data be added to the compilation.
How are swath data processed?
We use a combination of software developed in-house and open source software to process swath data for GMRT. MB-System provides the backbone of the code that we use for our data cleaning and processing. Raw multibeam sonar ping files available for each cruise are first run through a series of crude filters (e.g. ocean depth), and are then used to generate a development set of 100-m resolution tiled images and grids. These images and grids are reviewed and quantitatively interrogated using a custom version of GeoMapApp. This allows our team to assess data coverage and data quality, and to identify problems in the data that can be addressed using multibeam processing tools and techniques available through MB-System.
Data are typically processed to 100-m resolution. While our overall goal is to include only high-quality data, we sometimes include lower quality data in areas where data coverage is sparse (e.g. Southern Ocean). The data are reviewed, edited as necessary and re-gridded iteratively until the quality is suitable for inclusion in the tile set. During this process, we also determine the maximum data resolution for the cruise, the data are either gridded at 100m (default), 50m, and 25m resolution depending on sonar system capabilities and survey area. The chosen resolution is applied to the entire cruise and is therefore limited by maximum water depth. During QA/QC we also assign a quality value to the cruise on the range of 1-5 with 10 being applied to the highest quality cruises. This value is applied to the data and affects the degree of blending that happens when the data are merged into the compilation.
Image Gallery of Common Problems Addressed During Multibeam Data Preparation:
Can the processing be automated?
Unfortunately, no. Successfully processing and cleaning sonar data collected at a variety of geologic settings throughout global oceans requires human intervention. While some automatic filtering techniques can be used for subsets of data, manual review and decision making is necessary to generate a consistent high-quality data product.
Accessing GMRT Grids & Images
- GMRT MapTool is an online tool that allows users to generate custom maps and grids directly from the GMRT compilation.
- GMRT is the default basemap in our visualization and analysis tool GeoMapApp, which can be used to create custom maps and grids, and to import and analyze other data sets within the context of GMRT.
- GMRT is the default basemap in our visualization and exploration tools Virtual Ocean and EarthObserver.
- GMRT images can be accessed as a Web Map Service (WMS)
- GMRT can be loaded into Google Earth
- The 100-m resolution swath layer of GMRT is integrated as part of the default ocean layer in Google Earth and Google Maps.
GMRT History & Evolution
- 1992 - Synthesis began as the Ridge Multibeam Synthesis Project (RMBS)
- 2003 - Compilation expanded to include Southern Ocean
- 2004 - GeoMapApp version 1.1 provides access to GMRT compilation
- 2005 - Expanded to include data from throughout global oceans
- 2005 - GMRT Web Services established
- 2008 - Virtual Ocean (3D Virtual Globe) released
- 2009 - GMRT Synthesis Paper published in G-cubed (Ryan et al., 2009)
- 2010 - GMRT version 2.0 – revised tiling scheme, new data prep tools, improved quality, improved land resolution, extended swath coverage
- 2010 - Earth Observer released for iPhone and iPad
- 2011 - GMRT 100-m swath bathymetry content added Google Earth & Google Maps
- 2011 - GMRT version 2.1 released in September with processed swath data from 77 additional cruises, representing 206,226 new trackline miles of data.
- 2012 - GMRT version 2.2 released in April with processed swath data from 41 additional cruises, representing 125,917 new trackline miles of data.
- 2012 - GMRT version 2.3 released in October with processed swath data from 37 additional cruises, representing 81,463 new trackline miles of data.
- 2013 - GMRT version 2.4 released in April with processed swath data from 40 additional cruises, representing 67,401 new trackline miles of data.
- 2013 - GMRT version 2.5 released in October with processed swath data from 41 additional cruises, representing 128,504 new trackline miles of data.
- 2014 - GMRT version 2.6 released in May with processed swath data from 48 additional cruises, representing 148,501 new trackline miles of data.
- 2014 - GMRT version 2.7 released in November with processed swath data from 29 additional cruises, representing 105,765 new trackline miles of data.
- 2015 - GMRT version 3.0 released in June with processed swath data from 38 additional cruises, representing 97,939 new trackline miles of data.
- 2015 - GMRT version 3.1 released in November with processed swath data from 31 additional cruises, representing 67,260 new trackline miles of data.
- 2016 - GMRT version 3.2 released in June with processed swath data from 28 additional cruises, representing 97,650 new trackline miles of data.
- 2017 - GMRT version 3.3 released in February with processed swath data from 28 additional cruises, representing 90,079 new trackline miles of data.
Attribution to Original Data Sources
Attribution to data providers has always been a priority with the GMRT Synthesis, and is accomplished in several ways.
Access to Original Data
Advanced users who seek access to multibeam sonar files used in the GMRT compilation can identify and download processed multibeam files using the Multibeam Swath Bathymetry Data Portal in GeoMapApp. After opening the Multibeam Swath Bathymetry Portal, clicking on a trackline of interest will illuminate the trackline for the entire cruise. The individual file of interest will turn red in map view, and the file name will be shown in the bottom of the GeoMapApp interface. By clicking on Download File, the individual data file will be downloaded from the MGDS
How to cite GMRT
Ryan, W.B.F., S.M. Carbotte, J.O. Coplan, S. O'Hara, A. Melkonian, R. Arko, R.A. Weissel, V. Ferrini, A. Goodwillie, F. Nitsche, J. Bonczkowski, and R. Zemsky (2009), Global Multi-Resolution Topography synthesis, Geochem. Geophys. Geosyst., 10, Q03014, doi: 10.1029/2008GC002332