Gulf of Maine Bathymetry
go to: USFWS Gulf of Maine Watershed Habitat Analysis

Arnold Banner, USFWS-Gulf of Maine Program
June 10, 2002

RATIONALE:

The Gulf of Maine Program has need of bathymetric information to characterize inshore and wetland habitats for water birds, shorebirds, and fishes. The distribution of sounding data available for deeper areas is sufficient to generate a relatively coarse resolution bathymetry grid. We have need of finer spatial resolution around inshore features, but soundings of intertidal areas are not abundant. As an alternative, I elected to supplement bathymetric data with tidal exposure classifications described from aerial photography (e.g., "regularly flooded, irregularly exposed, irregularly flooded") in the USFWS National Wetland Inventory data sets, and also channels and tidal flats mapped from photography in Maine's Coastal Marine Geologic Environments (CMGE) and Massachusetts orthophoto wetlands maps. These classes were not translated into absolute elevations, but instead were coded as to position within the intertidal zone, and the general cover type (e.g., high marsh, lower intertidal flats, etc.), which may convey useful ecological information. The mapping area includes the U.S. portion of the Gulf of Maine: eastern coastal Massachusetts, coastal New Hampshire, and Maine.

DATA SOURCES:

NWI (for intertidal cover type, tidal position modifier; used for Maine, New Hampshire, Massachusetts).

Maine Geological Survey CMGE's (an alternative data set for intertidal flats and improved tidal channel characterization, to supplement NWI; used for Maine)

Massachusetts Office of GIS orthophoto wetlands, fairly general intertidal interpretations; partial coverage for Massachusetts.

USGS hydrologic features; polygons of these digital maps were coded with NWI attributes (based on NWI paper maps), to fill in areas without digital NWI or orthophoto wetlands (part of Massachusetts).

Surveys by the Jackson Estuarine Laboratory, original data obtained from Dr. Carl Friedrichs of Virginia Institute of Marine Science. Also, soundings obtained from the U.S. Army Corps of Engineers for the Piscataqua River and for Hampton Harbor; used forNew Hampshire.

USGS Gulf of Maine 500m (15 second) grid GOM15 (to supplement inshore data, used for areas deeper than -60' mlw). Obtained at http://oracle.er.usgs.gov/gomaine/bathy/data.htm; these were converted to UTM, depths in feet

NOAA NGDC bathymetry soundings (cdrom); used for Maine, New Hampshire, Massachusetts.

NOAA bathymetric grids (30 meter) of New England embayments at: http://mapfinder.nos.noaa.gov/; these high quality grids, were used to supplement or replace other subtidal bathymetry information; used for Massachusetts.

NOAA nautical charts were used as background for checking sources. Also, I digitized some line and point features for use in New Hampshire and Maine. I also used rasterized versions of charts to 'cut and paste' some channel and inlet depth zones where these were an improvement over the interpolation from limited point or line data. Arcs digitized from NOAA metric charts were provided by R. Kelley/OGIS, for use in Maine.

USGS depth contours were digitized for Cobscook Bay, Maine.

PROCESSING AND CODING:

Grids were produced with 15 m cells, Projection UTM zone 19, datum NAD27, depths in feet reference mean low water.

INTERTIDAL INFORMATION:

NWI

The following NWI coding modifiers were classified as upper intertidal or tidally influenced:

E2*' , 'M2*', 'R1US* = '*T', '*R', 'V*, '*P' , '*T', '*R', 'V*', '*P6', '*Pd'

Within this zone:

'E2EM*' , 'PEM*') were coded 1, high marsh

'E2US*', 'M2US*') were coded 2, high unconsolidated substrate

'E2RS*', 'M2RS*', 'E2AB*', 'M2AB*') were coded 3, high rocky/aquatic bed

'PFO*', 'PSS*' were coded 4, high shrub/forest

The following NWI coding modifiers were classified as middle intertidal:

'E2*' , 'M2*', 'R1US* <> '*P' and <> '*M', <> '*P6' or <> '*Pd'

Within this zone:

'E2EM*', 'PEM*' were coded 8, mid emergent

'E2US*', 'M2U*', 'R1US*' were coded 9, mid unconsolidated

'E2R*', 'M2R*', 'E2AB*', 'M2AB*' were coded 10, mid rocky/aquatic bed

The following NWI coding modifiers was classified lower intertidal:

'*M'

Within this zone::

'E2US*', 'M2US*' were coded 6, low unconsolidated

'E2RS*', 'M2RS*', 'E2AB*', 'M2AB*', 'E2/M2RF*' were coded 7, low rocky/aquatic bed

CMGES:

The following codings were used to supplement the NWI intertidal:

UN = "M*", "B*", "F", "F1" ,"F2" , "F3", "F4" , "F5" , "F6" )

Br, M were coded 10, mid rocky/aquatic bed

M1 were coded 1, high marsh

M2 & M3 were coded 8, mid emergent

M4 were coded 2, high unconsolidated substrate

rest were coded 9, mid unconsolidated

On checking against NOAA charts found serious discrepancies with ( [Un] = Me, Mf,

Mp, Mc (really were deep) selected and deleted all these

ADJUSTMENTS:

Used channels (through flats) to "erase" portions of NWI intertidal:

([Un] = "C*")

Combined Intertidal Grid:

1) Combined NWI with CMGEs, replacing only NWI subtidal, mid- and low intertidal with

CMGE values

2) Cut CMGE channels from above composite

SUBTIDAL BATHYMETRY

Data from all sources was converted to depth in feet reference mean low water, and projected as coverages to UTM.

TINs (triangulated irregular network; ESRI, ArcInfo) were created from the following data sets:

Subtidal bathymetry components:

Mean low water (0 depth line): The intertidal (0) line was the outer boundary of the intertidal grid, based on aerial photo-interpreted this was supplemented with arcs digitized from NOAA charts and USGS quad sheets.

Mean high water (elevation varies along coast): Used the USGS coast line coverage as mhw; made the depth value +19' at Calais, 16' at West Quoddy Head, 15 at Moose Cove, etc, based on tide table: 'Rise for substations of Portland'; set at + 6 to + 7 for Massachusetts and New Hampshire.

Soundings: NOAA points, from CDROM, < 30 meters only; used all Jackson Laboratory and Corps of Engineers New Hampshire soundings;

USGS and NOAA arcs.

COMBINING THE INTERMEDIATE PRODUCTS:

I first created TINs which were used to produce grids of nearshore bathymetry from the above sources, at 15 m and at 30 m resolution. The USGS 15 - second grid was used to fill in deeper areas; the resultant grid was overlain with the NOAA embayment data where this more closely corresponded with NOAA charts. Finally, the intertidal grids were superimposed.

Subtidal bathymetry of certain embayments and channels, mostly on the Maine coast, still was found a poor match with the published NOAA charts.  This was because the TINS were based on limited digital data sets for these areas, while the charts were based on far more complete soundings. Using scanned versions of the charts, I converted the images to grids. I then used the color rendition as the link to the depth range designated on the chart, and clipped out replacement 'patches' for each of the embayments to be corrected. The NOAA chart soundings were inspected, and a intermediate depth value was assigned each patch. Patches and base grid were combined by retaining the deeper value from either source.

Two final grids were produced: one retained all integer depth values, and a "descriptive" attribute to group intertidal features and depths into a few classes (see below). The other grid has only the classes, and therefore has a smaller file size.

ATTRIBUTES OF BATHYMETRIC GRIDS:

ITEM: DESCRIPTIVE CODE
upper intertidal; high marsh 1
upper intertidal; flats 2
upper intertidal; rock/algae 3
upper intertidal; shrub/wooded 4
lower intertidal; flats 6
lower intertidal; rocky/algae 7
mid-intertidal; low marsh 8
mid-intertidal; flats 9
mid-intertidal; rock/algae 10
subtidal; 0 - -15 ft 11
subtidal; -16 to -30 ft 14
subtidal; -31 to -60 ft 16
subtidal; -61 to -120 ft 18
subtidal; -121 to -240 ft 20
subtidal; -241 to -500 ft 22
subtidal; deeper than -500 ft 24
upland 99