Principal aquifers of Puerto Rico and the U.S. Virgin Islands

For publications, at a scale of 1:500,000 or smaller. Because of the small
scale, the primary use is for regional and national data display rather than
specific local data analysis.

This is polygon data of shallowest principal aquifers of Puerto Rico and the
United States Virgin Islands, developed as part of the effort to produce a map
published at 1:500,000 in Chapter HA 730-N of the "Ground Water Atlas of the
United States" series of printed publications. See Other_Citation_Details for
publication information. The published map contains base and cultural features
not included in this data. The Ground Water Atlas publication also includes
information that may be relevant to the use of this data, such as figures
showing where an aquifer may extend beyond the area delineated in this data,
but be overlain by one or more other aquifers and/or low-permeability
material.

The areal extent of the aquifers, as shown in this data, represent the area in
which a named aquifer is the shallowest of the principal aquifers. These
aquifer areas are not necessarily the only area in which ground water can be
withdrawn because the aquifers shown may have a larger areal extent than is
represented here. The boundaries in this data generally represent an
interpretation of the surface location (outcrop), or near surface location
(shallow subcrop) of the uppermost principal aquifer for the area. An aquifer
may extend beyond the area shown in this data, but be overlain by one or more
other aquifers, and/or low-permeability material.

These aquifer outcrop and shallow subcrop boundaries represent broad, regional
categories and should not be interpreted as site specific. Comments regarding
the names of aquifers, or the hydrogeologic interpretation of the aquifers can
be can be directed to the USGS Water Resources Division, Office of Ground
Water, Roy Sonenshein, [e-mail], [phone].

Introduction This data was created as part of the effort to produce an aquifer

map published at 1:500,000 in Chapter HA 730-N of the "Ground Water Atlas"
series of printed publications. See reference for more publication details.
Since this data was created to produce a printed map, the map can be
considered the source for the data. The information below is provided for more
detail regarding the map's compilation procedures. The coverages for Puerto
Rico, and the Virgin Islands were processed separately, and later appended
into one coverage. The processing steps are discussed separately for Puerto
Rico, and the Virgin Islands

PUERTO RICO

*Aquifer contact lines

The aquifer contact information was originally compiled by Robert A. Renken at
a scale of 1:250,000 on scale-stable material. The contacts were scribed at a
lineweight of 0.004". Several lat-long intersections were also scribed for
later use in georeferencing. A film positive made from the scribecoat was
scanned at a resolution of either 300 or 400 pixels per inch (exact resolution
was not recorded). The raster image was converted to an Arc/Info grid and
vectorized using the Arcscan module with these parameters:
ArrowLength 32
ArrowWidth 3
BackTrack YES
BRANCH STRAIGHT
DASH 0.033
EndOfLine NOMANUAL
EndOfSession EDIT
FAN 15.000,30.000
ForeGround 1
GAP 0.025
GeneralizeTOLerance 0.005
HOLE 0.002
JunctionSensitivity MEDIUM
LineSymbol 8
LineVariation 2.000
LineWidth 14.000
MarkerSymbol 0
MODE CENTER
RegionOfInterest 9999.000,9999.000,-9999.000,-9999.000
RETRACE NO
SearchRadius 10.000
SKIP 0
STraightenAngle 0.000

STraightenCorner 0.000
STraightenDistance 0.000
STraightenRange 0.000

The resulting lines were edited in Arcedit for completeness and vectorizing
errors. Paper plots were made to compare the resulting linework to the
original compilation. Discrepancies were noted and corrected. The low
generalize tolerance used in the vectorizing process resulted in lines that
closely matched the original compilation, but had a small vertex spacing. This
small vertex spacing would have in turn resulted in large file sizes and an
unnecessary level of detail for the intended publication scale. The arcs were
generalized, splined, generalized, and unsplit in Arcedit, tolerances were not
recorded. The appearance and accuracy of the resulting linework was judged
against publication standards for the intended publication scale (1:500,000).
The aquifer contact lines were transformed into their proper projection
through the use of tic features located at several latitude- longitude
intersections included in the scribed linework. The resulting RMS error was
not recorded. The coverage was plotted and checked against the source
compilation.

Coastline

The coastline was derived from an Arc/info coverage received from the USGS
Water Resources Division Caribbean District in January, 1996. This coverage
had been originally created from 1:20,000 USGS maps, and subsequently modified
by the Caribbean Disctrict for use at a smaller scale. This coverage, as
received contained more detail than was needed for the intended publication
scale for the aquifer map (1:500,000). The coastline coverage was generalized
for publication purposes, and the generalized version is now part of this
data. The data linework was generalized by converting the line coverage into a
grid , with a cell spacing of 25 meters. This grid was vectorized in Arcedit,
using the Arcscan module. The parameters for vectorizing were:

ArrowLength 32
ArrowWidth 3
BackTrack YES
BRANCH STRAIGHT
DASH 500.000
EndOfLine NOMANUAL
EndOfSession EDIT
FAN 45.000,90.000
ForeGround 1
GAP 0.025
GeneralizeTOLerance 25.000
HOLE 25.000
JunctionSensitivity MEDIUM
LineSymbol 3
LineVariation 2.000
LineWidth 400.000
MarkerSymbol 0
MODE CENTER
RegionOfInterest 9999.000,9999.000,-9999.000,-9999.000
RETRACE NO
SearchRadius 375.000
SKIP 0
STraightenAngle 15.000
STraightenCorner 30.000
STraightenDistance 250.000
STraightenRange 750.000

This procedure resulted in a less detailed version of the original coastline.
The resulting coastline was compared onscreen to the original. The aquifer
contact lines were snapped to, and appended with the final coastline. Polygons
were attributed in Arcedit.

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

VIRGIN ISLANDS

*Aquifer contact lines

The aquifer contact information was originally compiled by Robert A. Renken at
a scale of 1:100,000 on scale-stable material. The contacts were scribed at a
lineweight of 0.006". Several lat-long intersections were also scribed for
later use in georeferencing. A film positive made from the scribecoat was
scanned at a resolution of of either 300 or 400 pixels per inch (exact
resolution was not recorded). The raster image was converted to an Arc/Info
grid and vectorized using the Arcscan module with these parameters:

ArrowLength 32
ArrowWidth 3
BackTrack YES
BRANCH STRAIGHT
DASH 0.033
EndOfLine NOMANUAL
EndOfSession EDIT
FAN 45.000,90.000
ForeGround 1
GAP 0.025
GeneralizeTOLerance 0.005
HOLE 0.002
JunctionSensitivity MEDIUM
LineSymbol 8
LineVariation 2.000
LineWidth 5.000
MarkerSymbol 0
MODE CENTER
RegionOfInterest 9999.000,9999.000,-9999.000,-9999.000
RETRACE YES
SearchRadius 1.000
SKIP 0
STraightenAngle 0.000
STraightenCorner 30.000
STraightenDistance 0.000
STraightenRange 0.000

The resulting lines were edited in Arcedit for completeness and vectorizing
errors. Paper plots were made to compare the resulting linework to the
original compilation. Discrepancies were noted and corrected. The low
generalize tolerance used in the vectorizing process resulted in lines that
closely matched the original compilation, but had a small vertex spacing. This
small vertex spacing that would have in turn resulted in large file sizes and
an unnecessary level of detail for the intended publication scale. The arcs
were generalized, splined, generalized, and unsplit in Arcedit, tolerances
were not recorded. The appearance and accuracy of the resulting linework was
judged against publication standards for the intended publication scale
(1:500,000). The aquifer contact lines were transformed into their proper
projection through the use of tic features located at several latitude-
longitude intersections included in the scribed linework. The resulting RMS
error was not recorded. The coverage was plotted and checked against the
source compilation.

Coastline

The coastline was derived from coverages originally digitized from 1:24,000
USGS maps, as received from the USGS Water Resources Division Caribbean
District in early 1992. These coverages contained more detail than was needed
for the intended publication scale for the aquifer map (1:500,000). The
coverages were appended, and edited to contain only coastline information. The
coastline coverage was generalized for publication purposes, and the
generalized version is now part of this data. The data linework was first
generalized to a level of detail subjectively considered appropriate for
display at a scale of 1:250,000. This was done by converting the line coverage
into an Arc/Info grid, cell spacing unrecorded. This grid was vectorized in
Arcedit, using the Arcscan module. The parameters for vectorizing were:

ArrowLength 32
ArrowWidth 3
BackTrack YES
BRANCH STRAIGHT
DASH 1500.000
EndOfLine NOMANUAL
EndOfSession EDIT
FAN 45.000,90.000
ForeGround 1
GAP 25.000
GeneralizeTOLerance 0.500
HOLE 75.000
JunctionSensitivity MEDIUM
LineSymbol 3
LineVariation 2.000
LineWidth 1200.000
MarkerSymbol 0
MODE CENTER
RegionOfInterest 9999.000,9999.000,-9999.000,-9999.000
RETRACE YES
SearchRadius 1125.000
SKIP 0
STraightenAngle 15.000
STraightenCorner 30.000
STraightenDistance 750.000
STraightenRange 2250.000

This procedure resulted in a less detailed version of the original coastline.
This coastline was used in an intermediate compilation step, and was further
generalized for publication at 1:500,000. The further generalization
presumably included the use of the Arcedit generalize command, as well as
on-screen editing, and use of cartographic judgement, however, records were
not kept. The resulting coastline was compared onscreen to the original. The
aquifer contact lines were snapped to, and appended with the final coastline.
Polygons were attributed in Arcedit.

The Puerto Rico, and Virgin Islands coverages were appended to create one
coverage. The coverage was used to generate an output file that was imported
by Adobe Illustrator for map publication. The completed map, at a scale of
1:500,000, was reviewed for content and accuracy by the Ground Water Atlas HA
730-N lead cartographer, Water Resources Division Geohydrologic Map Editor,
and chapter author.

Related Spatial and Tabular Data Sets_

A data set of principal aquifer outcrops of the 48 contiguous United States is
available at: "http://water.usgs.gov/lookup/getspatial?/aquifers_us" A data
set showing the extent of glacial deposits in the contiguous U.S. is in
progress, as of 8/25/98. A data set of aquifers of Alaska will be available
Nov 1 1998 at: "http://water.usgs.gov/lookup/getspatial?/aquifers_ak" A data
set of aquifers of Hawaii is planned.