channels.shp

Created: October 25, 2006

Author: Robert D. Conkwright
	Maryland Geological Survey
	Coastal & Estuarine Geology Program
	2300 St. Paul St.
	Baltimore, MD 21218
	(410)554-5545

Data Source:  NOAA/NOS 30 meter gridded DEM for Chesapeake Bay bathymetry
	See information below*
	DEM imported into ESRI ArcInfo 9.1 and converted to a elevation raster.
	Elevation raster contoured to 1 meter contour intervals.'
	Channels were digitized based on deepest contours, channel slope, channel width and bottom geomorphology.

Projection:
	Projected coordinate system name: WGS_1984_UTM_Zone_18N
	Geographic coordinate system name: GCS_WGS_1984

Bounding coordinates
	Horizontal
	  In decimal degrees
		West: -77.456151
		East: -75.610392
		North: 39.613362
		South: 36.786994
	  In projected or local coordinates
		Left: 289080.256617
		Right: 445553.830401
		Top: 4385037.801439
		Bottom: 4073849.629871

-----------------------------------------------------------------------------------------------------------------

* NOAA/NOS  Data Reference
Readme.txt for Estuarine Bathymetry Products
December 10, 1998

The products which have been zipped into this archive were generated by the Special Projects Office of the National Ocean Service, NOAA as part  
of a project to produce user friendly bathymetry for those estuaries that contained more than 80% spatial coverage of digital sounding data.   
Not every estuary contained sufficient source data in digital form, thus only 70+ of the 130+ estuaries are included in the full data set.   
These data sets can be retrieved from the NOS MapFinder service <http://mapfinder.nos.noaa.gov>.  

The included Metadata file should be consulted for details on the generation, quality, and format of these Bathymetric products.

The source data for these bathymetric products were soundings collected by The National Ocean Service over the last 150 years (retrieved from  
the national archives held by the National Geophysical Data Center in Boulder, CO) and Shorelines delineated by the National Ocean Service.  

Each Archive file contains the products applicable to one of four data sets possible for an estuary.  The four data sets include: 
1. Standard tiled DEMs for 30m gridded bathymetry,				[eda_D30.zip]
2. Standard tiled DEMs for 3 arc second [90m] gridded bathymetry,	[eda_D90.zip] 
3. a single BIG DEM for 30m gridded bathymetry, or				[eda_B30.zip]
4. a single BIG DEM for  3 arc second gridded [90m] bathymetry		[eda_B90.zip]
where "eda" represents a unique 4 character Code assigned by NOAA's Coastal Assessment Framework (CAF) to each estuary.  The first character of  
this code is a letter signifying N = North Atlantic,  M = Mid Atlantic, S = South Atlantic, G = Gulf of Mexico, L = Great Lakes, and P =  
Pacific.  The remaining 3 characters are digits which, in general, were assigned increasing clockwise around the coast.

A separate archive file for each estuary [eda_SC.zip] contains visual GIF images showing the locations of all soundings used to generate these  
bathymetric products superimposed on NOAA Nautical chart backgrounds.  These files can be accessed through the NOS MapFinder Service noted  
above. 

Standard DEMs are tiled so that their footprints correspond to standard USGS quadrangle boundaries; either 7.5 minute size for 30m grids or 1  
degree size for 3 arc second grids.
  
BIG DEMs cover a footprint which corresponds to the full extents (in Lat/Lon)of the estuary.  

The high resolution bathymetric data are gridded to 30m cells defined on a UTM projection where the locations of the value for each cell are at  
coordinates in UTM meter units evenly divisible by 30.  The origin of these units is 500,000 meters west of the central meridian for the UTM  
projection and the equator. It should be noted that these UTM grids do NOT align with geographic parallels and meridians which define the  
extents of these DEM data sets.  These 30m gridded data are equivalently referred to as UTM, 7.5 minute, or 30m data. In this context these  
terms are interchangeable. 

The lower resolution bathymetric data are gridded to 3 arc second cells defined on a geographic projection (latitude & longitudes) with units of  
arc seconds (1 degree = 3600 arc seconds).  The locations of the value for each cell are at geographic coordinates in arc second units evenly  
divisible by 3.  The origin of these units is the prime meridian and the equator.  The vertical extent of these grid cells are approximately 90m  
in length.  The horizontal extent of these grid cells vary with latitude and in the real world the cells are not square.  These grids are  
congruent with geographic parallels and meridians which define the extents of these DEM data sets.   These 3 arc second gridded data are  
equivalently referred to as 3 arc second, Degree Square (DSQ), or 90m data. In this context these terms are interchangeable.

The 3 arc second bathymetric data were derived from the higher resolution 30m gridded bathymetry and not generated independently.  

Null values (encoded -32767 within the DEMs) are used to represent all locations either above (outside) the local high water datum (the  
shoreline as defined by the National Ocean Service) or outside the extents of the estuary as delineated by the CAF.

Bathymetric elevations within these data sets are referenced to the local tidal datum which typically is Mean Lowest Low Water (MLLW) averaged  
over a 19 year tidal epoch.  Elevations above this datum (between the datum and the shoreline) have positive values (meters to centimeter  
resolution) while those below are negative.  Note that this datum is different from that used by USGS for land elevation data that it  
distributes in DEM form which are referenced to Mean Sea Level (MSL). 

INCLUDED PRODUCTS
Each of these data sets is supported by visual images and text files products.
The products include:
readme.txt		an overview +++ THIS FILE +++ 
metadata---.txt	A text file containing FGDC compliant metadata about the products.
				 The 30m and 90m grids have different metadata files.  
eda_desc.txt		A short descriptive text file for the estuary
eda_view.gif		A GIF image showing color coded bathymetry for the estuary
eda_GI.gif		A GIF image delineating the Standard DEM tile extents and names for this estuary
eda_idx.gif		A GIF image showing where the estuary is.	 
eda_ ----.dem		one or more DEMs containing the Bathymetry data.
