July 2013

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Eelgrass Map (06-07)

Seagrass, a form of Submerged Aquatic Vegetation (SAV), serves as important fish and wildlife habitat and assists with storage and recycling of nutrients and the stabilization of sediments. The predominant seagrass found in Massachusetts coastal waters is eelgrass (Zostera marina). The Massachusetts Department of Environmental Protection (MassDEP) began surveying seagrass in the early 1990s and the MassDEP Eelgrass Mapping Project has produced multiple phases of seagrass mapping along the Massachusetts coastline.

This layer contains features mapped in the fourth phase of the MassDEP Eelgrass Mapping Project. Phase 4 mapping was conducted in 2010 (with follow-up in 2011) and focused on re-mapping a portion of the Cape Cod coastline running from Wood’s Hole to Chatham, including the Pleasant Bay area. Additionally, areas of the Wareham River, Weweantic river, Sippican Harbor, and Aucoot Cove as well as two embayments on Martha’s Vineyard (Lake Tashmoo and Lagoon Pond) were also included in this analysis.

The layer is named EELGRASS2010_POLY.


Digital imagery was acquired in late spring 2010 through a contract with GeoVantage Corporation (Peabody, MA). The GeoVantage sensor consists of a digital camera with four bands centered on blue (450nm), green (550nm), red (650nm) and near-infrared (850nm). The output of the camera system are GeoTIFF image products which were created from the true color imagery captured with a 0.5 meter ground sample distance resolution and 8 bit radiometric resolution. The images were orthorectified, terrain corrected (using 7.5m USGS DEM's), georegistered and mosaics were created for each flight mission with a spatial accuracy of +/- 3 meters (90% of pixels). The digital images were interpreted monoscopically on screen using the same photo-interpretative protocols as previous phases of the MassDEP Eelgrass Mapping Project. The digital imagery was analyzed and interpreted using a high resolution CRT 22 inch monitor.

Polygons of seagrass were hand digitized on-screen using ESRI ArcGIS Desktop software. Much of the Massachusetts coast has a sandy substrate providing a useful color contrast to map the darker seagrass photo-signatures. Nonetheless, there are still uncertainties with signature interpretation that required field verification. Extensive fieldwork was conducted to develop a quantitative understanding of the signatures and gather surface level data where the presence or extent of habitat was not apparent. The fieldwork methods and ground-truthing protocols were similar for this project as were utilized in the earlier two mapping efforts (1995 and 2001). Questionable points in the interpreted data were investigated in a small boat using underwater videography, high resolution acoustics and GPS (1 meter accuracy). These field points were logged into the on-board digital mapping database and used back in the laboratory for the final interpretation. Seagrass polygon vector layers were assessed for accuracy in September and October, 2011 (310 points). The assessments were based on random points generated within the polygon boundaries generated by the Hawth’s Tools extension within the ArcMap GIS environment. Navigation to each point in the field was accomplished using DGPS (1 meter accuracy. The underwater video camera was used to verify the presence of the mapped feature at the random point. The accuracy of the mapped seagrass was 89.9 % (278 out of 310 sites mapped correctly for the presence/absence of seagrass).


The attribute fields for this layer are:
YEAR Char Year of delineation; 2010
HABITAT Char Seagrass type (Eelgrass, Rupia)
AREA_ACRES Float Area of SAV feature in acres


This layer is maintained by the MassDEP Wetlands Conservancy Program. For additional information on the MassDEP Eelgrass Mapping Project, visit: http://www.mass.gov/eea/agencies/massdep/water/watersheds/eelgrass-mapping-project.html.

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Last Updated 7/12/2017