Polygon shapefile of the interpretation of the seabed geologic substrates in quadrangle 6 of the Stellwagen Bank National Marine Sanctuary region offshore of Boston, Massachusetts based on data collected by the U.S. Geological Survey from 1993-2004 (Geographic, NAD 83)

A polygon shapefile was created in ArcCatalog (version 9.0) with a custom Mercator projection using the following parameters: central longitude -70 degrees 19 minutes; latitude of true scale 41 degrees 39 minutes; false easting and false northing, both 0; datum, NAD 83.

Interpretations were drawn on paper maps, which displayed the sun-illuminated topography with the locations of sediment grain size, with textural analysis, and video/photo stations. These paper maps were printed at a scale of 1:25000 on a large format HP plotter. The interpretation was based on seabed samples to determine sediment grain size distribution, photo and video imagery to identify small seabed features, multibeam sonar data to identify large seabed features and relative reflectivity (hardness and softness) of seabed sediments. The paper interpretations were transferred to ArcGIS visually based on background data layers in the GIS. The process was an iterative one, with the interpretation then printed on the paper maps with underlying bathymetry or backscatter, and interpretation lines adjusted based on the other data printed on the maps.

The following information was used as guidelines to derive individual units.

Individual sediment samples were aggregated into substrate types by comparing their textural composition, with special emphasis placed on:

(1) mean grain size in phi units;
(2) standard deviation from the mean of grain size distribution (degree of sorting) in phi units;
(3) significant grain size(s) in phi units;
(4) mud, sand and gravel contents in weight percents of the sample;
(5) sand fraction subdivided into fine grained sand (3 and 4 phi) and coarse grained sand (0, 1, and 2 phi) in weight percents of the sample;
(6) gravel fraction subdivided into G1 (-1 and -2 phi) and G2 (-3, -4, and -5 phi) in weight percents of the sample.

Gravel pebbles larger than -5 phi (6.4 cm) were not collected in this study, although larger pebbles, cobbles, and boulders were observed in seabed imagery.

Mobility of sediments was determined by the presence or absence of sand ripples, as viewed in video and photographic imagery.

Substrate layering was determined by the presence of a layer of finer sediment overlying (but not completely covering) coarser sediment, as viewed in video and photographic imagery.

This process took place over a number of years, 2005-2010.

In ArcGIS 9.3.1, slivers between polygons were identified and removed. Much of this work was accomplished using ArcToolbox - Data Management Tools - Feature Class - Integrate. Overlap between polygons was also taken care of often using the clip option from the editor menu discarding the overlapping portion. Further tweaking of the polygons was necessary. To maintain integrity between adjacent polygons, the remaining edits were done using map topology. Shapefiles don't have topology, but by using the map topology capabilities of ArcGIS, editing of shared nodes was possible.

The final piece of the geology interpretation was the already published boulder ridge data. The previously published ridge polygons (SIM 2840) were published as shapefiles in two separate data layers. One data layer consisted of boulder ridges < 1 meter in height, with the other data layer having boulder ridges greater than or equal to 1 meter in height. In edit mode in ArcGIS 9.3.1, the ridge shapefiles were copied and added (pasted) to the geology interpretation shapefile. With the ridges selected, the clip option was chosen from the edit menu to clip the underlying polygons and discard the overlap. Additionally, any ridges falling outside quadrangle 6 were deleted.

The geology interpretation polygons serve multiple functions. With this in mind, the attribute table was constructed such that any of the geologic interpretations associated with quadrangle 6 could be symbolized from the single shapefile. For instance, the geologic substrate unit A1 is equivalent to the mobile sediment unit of the substrate mobility map. Additionally, geologic substrate unit A1, combined with several other geologic substrate units, comprises the single coarse-grained substrate. In this manner, the geologic substrate units can be used in various combinations of units to derive all subsequent geologic maps in quadrangle 6.

The interpretation shapefile was projected from the custom Mercator projection to a geographic coordinate system using ArcToolbox - Data Management Tools - Projections and Transformations - Feature - Project. The output coordinate system was geographic, NAD83. No datum transformation was necessary.

Added keywords section with USGS persistent identifier as theme keyword.