CONTACTS
Paul Johnson, Rachel Morrison, and Larry Ward
DISCLAIMER
All information provided here is for displaying seafloor properties and cannot be used for navigation purposes.
Full Report
Ward, L.G., Morrison, R.C., Bogonko, M., and Johnson, P., 2026, Gulf of Maine Bathymetry, Backscatter, and Coastal Topography. University of New Hampshire Center for Coastal and Ocean Mapping, Technical Report, 25 Colovos Road, Durham, NH 03824, 41 pp. DOI: ????????
Overview
The Gulf of Maine (GOM) was shaped by the interactions of marine processes, riverine sediment input, glaciations, and sea level fluctuations. As a result, the seafloor is extremely complex and can vary significantly over short distances (tens of meters). To study the surficial geology of the GOM seafloor, a comprehensive review of available bathymetry and backscatter data was undertaken, comprised mainly of multibeam echo sounder (MBES) surveys. The high-resolution MBES bathymetry data, supplemented by nearshore lidar surveys, was successfully brought together and combined into a high-resolution bathymetry map gridded at 4m. To expand the bathymetry coverage over the GOM, a lower resolution bathymetry map (16m) was developed primarily utilizing the MBES and Global Multi-Resolution Topography (GMRT). Where available, backscatter co-registered with the MBES bathymetry is largely presented as individual surveys with detailed metadata. The exception is two previously published surveys off northern Massachusetts and New Hampshire. The expanded shelf bathymetry is complemented by a high-resolution topographic lidar compilation (4m grid) of the adjacent coastal upland extending from Maine to Rhode Island.
Collectively, the compilations provide a unique land-sea perspective of the physiography of the region revealing extensive bedrock outcrops, glacial deposits on land and offshore, bathymetric highs, deep shelf basins, shoals, and fluvial deposits. This knowledge facilitates a better understanding of the regional seafloor, sea level history, and potential marine resources of the GOM continental shelf. More details of the development of the Gulf of Maine Bathymetry, Backscatter, and Coastal Topography maps can be found in Ward et al. (2026).
The GOM high-resolution bathymetry and topography maps, as well regional bathymetry can be downloaded from the UNH/CCOM-JHC GIS Portal.
Map Products
Gulf of Maine High-Resolution Bathymetry
The high-resolution bathymetry compilations presented in this report are comprised predominantly of MBES surveys (207), supplemented by bathymetric lidar surveys (14) in the shallow coastal region, all gridded at 4 m. The new bathymetry compilations provide a significant expansion of coverage of the inner continental shelf between Cape Cod and mid-coast Maine (Figure 1) than the earlier maps published by the University of New Hampshire (UNH) Center for Coastal and Ocean Mapping (CCOM) (Ward et al., 2016; 2021). The GOM high-resolution bathymetry now covers an area of approximately 25,420 km2.
Bathymetric data for the GOM (typically archived as Bathymetric Attributed Grid “BAG” files), as well as backscatter data where available, were primarily obtained from the National Centers for Environmental Information (NCEI) repository. Additional MBES surveys by UNH CCOM between 2021–2025 as part of the “Summer Hydro” field course and recent MBES surveys (2023–2025) off the coast of Maine by the Maine Coastal Mapping Initiative (MCMI) were provided directly to CCOM. The CCOM and MCMI surveys are not presently available through NCEI. Footprints (outlines) of all 207 surveys included in the newest bathymetry compilation are shown in Figure 2.
All of the acquired bathymetric datasets were first ordered according to spatial resolution, where highest-resolution grids were placed at the top and coarsest-resolution datasets were positioned at the bottom using Global Mapper Pro (version 24), ensuring that the most detailed datasets would be visible at the surface in areas where more than one survey (of varying resolutions) had been completed or different surveys overlapped. A unified bathymetric surface was then compiled in Global Mapper and exported as a GeoTIFF with 4m resolution (Figure 1). More detailed discussion of the methodology is given in Ward et al. (2026).
Figure 1. GOM high-resolution bathymetry and topography compilation maps (gridding 4m; scale 1:2,200,000). See Figure 2 for survey footprints, and see Ward et al. (2026) for survey metadata. Figures 4 and 5 show enlargements of the high-resolution maps.
Figure 2. Footprints (outlines) of all surveys included in the current bathymetry compilation. Note that many outlines overlap; the highest-resolution surveys were chosen to display at the surface. See Ward et al. (2026) for a list of all surveys and their associated metadata. Metadata can also be viewed on the UNH/CCOM-JHC GIS Portal. Map scale is 1:1,000,000.
Gulf of Maine Regional Bathymetry
To provide broader coverage, the MBES and lidar were combined with the most recent GMRT grid (Global Multi-Resolution Topography v4.4.0) to develop a regional map gridded at 16 m (Figure 3). GMRT is gridded at varying resolutions as a function of its source datasets (Ryan et al., 2009). Within Global Mapper, the high-resolution bathymetry surface was layered on top, and then the GMRT bathymetry underneath to fill in spatial gaps in the high-resolution coverage and to expand the entire map. The unified regional bathymetric surface was exported as a GeoTIFF with 16m resolution and completes the coverage of the study region, expanding the areal coverage to ~102,870 km2 and extending ~300 km offshore.
Gulf of Maine High-Resolution Coastal Topography
Like the GOM, the adjacent upland has been shaped by terrestrial, riverine, and glacial processes and share many of the same features. Fortunately, high-resolution terrestrial lidar surveys are now available which cover the entire coastal states adjacent to the GOM. These lidar surveys were brought together in a 4m gridded compilation that provides high-resolution topographic coverage of the coastal regions extending ~45,940 km2 (Figures 1 and 3). The bathymetry and topography compilation maps are independent of each other with different color ramps, and when shown together allow for visualization of onshore-offshore geologic features (Figures 4 and 5).
The high-resolution Digital Elevation Model (DEM) was compiled from lidar data covering the coastal regions of Maine (ME), New Hampshire (NH), Massachusetts (MA), and Rhode Island (RI) to assist in surficial geology mapping and tracking historical sea level changes. The surveys used in the compilation were flown between 2010 and 2023 (see Ward et al., 2026 for a complete list of data sources). Lidar data for NH was downloaded as a single file from NH GRANIT (https://granit.unh.edu/), while data from ME, MA, and RI were collected by the USGS and are available through NOAA.
Lidar data was processed using the GDAL open-source library (Geospatial Data Abstraction Library) for OSGeo4W software package (https://trac.osgeo.org/osgeo4w) and ArcGIS Pro (version 3.6). All files were first individually re-gridded to 4m resolution, merged into a single tiff file for each state, and then merged into one file. This map was then clipped to refine both the landward and seaward boundaries. The landward portion was defined using a manually-drawn polygon in ArcGIS Pro. For coastal boundaries, the OpenStreetMap (OSM) worldwide “land polygons” shapefile layer was used (https://osmdata.openstreetmap.de/data/land-polygons.html) in combination with a CCOM-developed shoreline for additional detail. The final lidar compilation GeoTIFF is gridded at 4m resolution to align with the WGOM bathymetry compilation.
Figure 4. Enlargement of the high-resolution coastal topography map shown in Figure 1 south of Portsmouth, NH (gridding 4m; scale 1:120,000). Note the extensive glacial features on land including drumlins and eskers (blue arrows), and a small glacial delta (red arrow). These same features are found offshore, although modified by marine processes (see Figure 5).
Figure 5. Enlargement of the high-resolution bathymetry map shown in Figure 1 west of the Isles of Shoals (gridding 4m; scale 1:30,000). Note the extensive marine-modified glacial features including drumlins and eskers (blue arrows) and a marine submerged shoal (red arrow).
A major effort was made to identify the backscatter surveys that are now available in the GOM. The backscatter surveys that are co-registered with the MBES bathymetry are shown as individual surveys with different grid sizes and color ramps (Figure 6). Since the previous compilation by CCOM (Ward et al., 2021), fifty new MBES backscatter surveys were added to the sixty-two existing surveys. Survey sources are presented in the metadata and in Ward et al. (2026). All individual backscatter surveys were brought into ArcGIS Pro 3.6, and standard coloring/symbology settings used to visualize and display the surveys. Note that many older surveys, such as those collected using side-scan sonar, were not included in our database due to frequent issues with earlier backscatter survey quality.
Two previously published backscatter mosaics covering the northern Massachusetts and New Hampshire inner shelf from Cape Ann to Portsmouth Harbor (Figure 7) are also included in the interactive webpage. The first covers Cape Cod to the Merrimack Embayment and was previously produced by the United States Geological Survey (USGS) (Pendleton et al., 2013). The second covers the New Hampshire inner shelf and was developed by CCOM (Ward et al., 2016). More information on the backscatter mosaics can be found in the cited references.
Figure 6. Individual MBES backscatter surveys in the GOM shown on top of the regional bathymetry compilation. The surveys were identified during a thorough review of MBES surveys. As is apparent, not all past MBES surveys used for the high-resolution bathymetry compilation had usable backscatter. See Ward et al. (2026) for footprints and metadata for the backscatter surveys shown here. Map scale is 1:1,600,000.
Figure 7. Backscatter mosaics developed by CCOM (Ward et al., 2016, outlined in red) and the USGS (Pendleton et al., 2013, outlined in yellow). Map scale is 1:620,000.
Previous Versions of the Bathymetry and Backscatter Compilations
Two earlier versions of the GOM bathymetry and backscatter compilations were completed by UNH CCOM including the original maps in 2015 which focused on the Western Gulf of Maine (Ward et al., 2016), followed by an update in 2020 to include new surveys and expand the area of coverage to Long Island Sound (Ward et al., 2021). The newest compilation presented here and in the associated report updates and expands upon earlier bathymetry and backscatter coverage (Figure 8) and also adds topography maps of the adjacent coastal upland.
Summary
The “Gulf of Maine Bathymetry, Backscatter, and Coastal Topography” brings together multibeam echo sounder (MBES) surveys, selected backscatter data, and onshore topography (lidar) surveys for the Gulf of Maine area. The bathymetry and topography are presented as compilations. The backscatter surveys are shown individually, except for earlier syntheses by UNH CCOM/JHC and by the United States Geological Survey. This report presents a significant expansion of the bathymetry and backscatter maps for the region published in Ward et al. (2016; 2021) by the addition of sixty-five newly available MBES bathymetry and fifty backscatter datasets. In addition, terrestrial lidar surveys conducted over the last fifteen years are brought together in a compilation that provides high-resolution topographic coverage of the onshore coastal regions.
The overarching goal of this report is to present a synthesis of available high-resolution bathymetry, backscatter, and topography for the GOM seafloor and adjacent upland. This knowledge facilities a better understanding of the regional seafloor, sea level history, and potential resources of the continental shelf. The surfaces shown here reveal complex physiographic features in the area such as extensive bedrock outcrops, glacial and fluvial deposits, bathymetric highs, and deep ocean basins. Collectively, the high-resolution topographic and bathymetric coverages provide a valuable asset for studying the GOM.
Reference List
Pendleton, E.A., Baldwin, W.E., Barnhardt, W.A., Ackerman, S.D., Foster, D.S., Andrews, B.D., and Schwab, W.C., 2013, Shallow geology, seafloor texture, and physiographic zones of the Inner Continental Shelf from Nahant to northern Cape Cod Bay, Massachusetts: U.S. Geological Survey Open-File Report 2012–1157, 53 p. http://pubs.usgs.gov/of/2012/1157
Ryan, W. B. F., et al. (2009), Global Multi-Resolution Topography synthesis, Geochem. Geophys. Geosyst., 10, Q03014. https://doi.org/10.1029/2008GC002332
Ward, L.G., Johnson, P., Nagel, E., McAvoy, Z.S., and Vallee-Anziani, M., 2016, Western Gulf of Maine Bathymetry and Backscatter Synthesis: BOEM/New Hampshire Cooperative Agreement (Contract M14ACOOO10) Technical Report, Department of Interior, Bureau of Ocean Energy Management, Marine Minerals Division, 45600 Woodland Road, Sterling, VA, 20166, 19 pp. https://dx.doi.org/10.34051/p/2021.27
Ward, L.G., Johnson, P., Bogonko, M., McAvoy, Z.S., and Morrison, R.C., 2021, Northeast Bathymetry and Backscatter Compilation: Western Gulf of Maine, Southern New England, and Long Island Sound: BOEM/New Hampshire Cooperative Agreement (Contract M14ACOOO10) Technical Report, Department of Interior, Bureau of Ocean Energy Management, Marine Minerals Division, 45600 Woodland Road, Sterling, VA, 20166., 23 pp. https://dx.doi.org/10.34051/p/2021.28
Ward, L.G., Morrison, R.C., Bogonko, M., and Johnson, P., 2026, Gulf of Maine Bathymetry, Backscatter, and Coastal Topography. University of New Hampshire Center for Coastal and Ocean Mapping, Technical Report, 24 Colovos Road, Durham, NH 03824, 41 pp. ??????????????????????????
Use of Compilations
The information provided here is freely available for use. However, the University of New Hampshire (UNH) Joint Hydrographic Center/Center for Coastal and Ocean Mapping (JHC/CCOM) should be acknowledged for the bathymetry and lidar compilations. The agencies that conducted individual surveys are given in the metadata displayed in the footprints.
Recommended Citation
Ward, L.G., Morrison, R.C., Bogonko, M., and Johnson, P., 2026, Gulf of Maine Bathymetry, Backscatter, and Coastal Topography. University of New Hampshire Center for Coastal and Ocean Mapping, Durham. (https://www.ccom.unh.edu/gulf-maine-bathymetry-backscatter-coastal-topography)
Use the “CCOM Bathymetry Downloader Tool” available on the UNH/CCOM-JHC GIS Portal to download data layers (https://gis.ccom.unh.edu/portal/home/).
Map Coordinate System, Projection and Datum
Coordinate System: WGS 1984 UTM Zone 19N
Projection: Transverse Mercator
Vertical Datum: MLLW (bathymetry); NAVD88 (topography)
Acknowledgements
The development of the “Gulf of Maine Bathymetry, Backscatter, and Coastal Topography” report was supported by the University of New Hampshire/National Oceanic and Atmospheric Administration Joint Hydrographic Center Award Number NA20NOS4000196. We gratefully acknowledge Jesse Minor at Maine Coastal Mapping Initiative for providing hydrographic and backscatter survey data. Juliet Kinney and Colleen Mitchell provided valuable technical advice and support.