Number 2.-- CONSTRUCTION OF A 3 ARC SECOND LONG ISLAND SOUND REGION TOPOGRAPHIC GRID

Rich Signell  and Amy Farris

U.S. Geological Survey Woods Hole Field Center
(rsignell@usgs.gov, afarris@usgs.gov)

INTRODUCTION

This section describes the construction of the 3-second digital topographic grid of the Long Island sound area.  The grid was    constructed for use in circulation and wave modeling of the Sound.  Data from a variety of sources were combined and outliers were removed.  This grid has been used to create additional bathymetric products including contour lines and color shaded images.  

GRID PARAMETERS

Grid Extent:
Longitude: -74 to -71.2
Latitude: 40.5 to 41.5

Horizontal Datum:
NAD83

Vertical Datum:
Land = relative to Mean Sea Level
Bathymetry = relative to Mean Low Water

DATA SOURCES

The data we used for constucting the digital elevation/bathymetry grid came from four different sources:

• Data from NOS/NGDC CD-ROM set of NOS Hydrographic Surveys, version 3.3. This covered most of the Sound.  Data in NAD27 was converted to NAD83.
• Data digitized from NOS smooth sheets by Muriel Grim of the USGS in 1995. These were converted to NAD83 and used to fill most of the gaps in the NGDC data.
• Data digitized from NOAA fishing charts by Applied Geographics, Inc. under contract with the EPA was used to fill the remaining gaps in the Sound.  Isobaths were digitized in Arc/Info, converted from UTM to geographic coordinates, and the digitized points were then dumped out "pseudo soundings" for the pursose of constructing the digital grid.
• Data from USGS 3 arc second Digital Elevation Models (DEMs) were used to represent land-side topography.

Much of the above data used NAD27 as the horizontal datum, so we used a utility in the PROJ  Map Projection Software called  "nad2nad" (with the parameter "conus" specified) to convert the horizontal datum from NAD27 to NAD83. The vertical datum for bathymetry is Mean Low Water.

METHODS

We used Muriel Grim's digitized smooth sheet data only where there was no digital data from NGDC.  Then the last few gaps mostly near the shore were filled in with the Applied Geographics data dumped as points from Arc/Info.  Again, we only used the Applied Geographics data only where there was no other data. Before including the Grim and Applied Geographics we checked to see that all three data sets agreed where the boundaries abutted.

Visually scanning all the data together, we removed a few obvious outliers from the NGDC and Applied Geographics data.

We then combined the bathymetry data and land-based DEM data and constructed a 3 arc second digital grid using the freely-available GMT mapping package.  Since the land elevations and bathymetry soundings were relative to different vertical datums (water = MLW, land= MSL), we added one meter to the DEM data to approximate a datum shift of the land values from MSL to MLW (the average tidal range is about 0.5-1 m in this region). 

PROCEDURES

We used several GMT scripts to create the grids and images:

do_grd: Creates a grid from the scattered bathy soundings and the DEM land topography.   One meter is added to the DEM topography, a blockmedian filter is applied and then the blockmedianed data is gridded using GMT's "surface" routine.  The GMT grid produced is li.grd, and is dumped out as ASCII triplets (lon,lat, depth in meters) and gzip compressed into the file li.xyz.gz.

do_intens: Computes gradients of the bathymetry for use in color-shaded relief scripts.

do_image_page:  Creates a page sized color shaded relief map of Long Island Sound.  The GZIP PostScript file it creates is about 25 Mb when uncompressed.  It was also converted into several popular image formats:

do_image_poster: Creates a poster sized (26x12) color shaded relief map of the Sound. The GZIP compressed PostScript file is about 25 Mb when uncompressed.

do_contours: Creates bathymetric contour lines of the Sound at the following levels: [1:1:10  12:2:50 60:10:100].