About GMRT

GMRT Overview

The Global Multi-Resolution Topography (GMRT) Synthesis is a multi-resolution Digital Elevation Model (DEM) maintained in three projections and managed with a scalable global architecture that offers infrastructure for accessing the DEM as grids, images, points and profiles. A mask layer is available that highlights the location of high-resolution data. Most curatorial effort for GMRT is focused on cleaning and processing ship-based multibeam sonar data acquired by the US Academic Research Fleet (ARF) so they can be gridded at their full spatial resolution (~100m in the deep sea). These data are seamlessly overlain on lower resolution observed and predicted bathymetry data and are integrated with terrestrial elevation data to deliver to users the best resolution data that have been curated for a particular area area of interest.

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 multiple elevation sources that are managed as four discrete source types. Each of the discrete content types that are updated on different schedules including:


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 NOAA National Centers for Environmental Information (NCEI). 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. We are making our tools available so partner organizations and individuals can participate in the QA/QC of multibeam data in order to accelerate the pace of data integration. Our cruise-level metadata now includes information about the organization that processed the data, so the efforts of community members can be acknowledged and recognized. 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 1-10 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:

QA/QC Overview BadNav Too Shallow bad motion sensor Too Deep Nadir SVel map Smile toodeep overedit BadOuter

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 History & Evolution


Attribution to Original Data Sources

Attribution to data providers has always been a priority with the GMRT Synthesis, and is accomplished in several ways.

att1_th att2_th att3_th


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