Appendix
The digital data and map of this open-file report are available from the
following U.S. Geological Survey URL:
http://wrgis.wr.usgs.gov/open-file/of03-269/
Data Contents
The digital dataset consists of one file (la-grav.dat) containing
the 3,914 point data that were gridded and contoured to produce gravity
contours on the map. The file contains the principal facts of the gravity
stations, with one measurement (or gravity station) point coded per line.
The format of the data is described below. Each gravity station has
a station name, location (latitude and longitude, NAD27 datum),
elevation, an observed gravity value, followed by free-air, Bouguer, and
isostatic anomalies and terrain correction values.
The data are on the IGSN71 gravity datum and the reference ellipsoid is the
Geodetic Reference System 1967 (GRS67). The free-air gravity anomalies were
calculated using standard formulas (Telford and others, 1976). The Bouguer,
curvature, and terrain corrections were applied to the free-air anomaly at each
station to determine the complete Bouguer gravity anomalies at a reduction
density of 2.67 g/cc (Plouff, 1977). An isostatic correction was then applied
to remove the long-wavelength effect of deep crustal and/or upper mantle masses
that isostatically support regional topography (Jachens and Roberts, 1981).
EXPLANATION OF PRINCIPAL FACT FORMAT
Item Explanation
STAT ..................Station Name--An alphanumeric combination of
up to 8 characters used for station
identification
LaD ...................Degree Latitude North
LaM ...................Decimal Minute
LoD ...................Degree Longitude West
LoM ...................Decimal Minute
ELEV ...................Elevation in feet
OG .........................Observed gravity, in mGal
FAA .......................Free-air anomaly, in mGal
SBA ........................Simple Bouguer anomaly, in mGal
ITC ........................Inner terrain correction for a density of
2.67 g/cc, in mGal. See Code column for
radius of correction.
TTC .........................Total terrain correction from the station to
166.7 km for a density of 2.67 g/cc, in
mGal.
Code .........................Letter denoting the zone (radius) of the
correction, "D"= 0.59 km; "G" = 3.52 km;
"M"= 58.8 km.
CBA ........................Complete Bouguer anomaly reduced for a
density of 2.67 g/cc, in mGal.
ISO ........................Isostatic residual anomaly,in mGal,assuming an
Airy model for isostatic compensation of
topographic loads. This model assumes a
a crustal thickness of 25 km, a topographic
load density of 2.67 g/cc, and a density
contrast across the base of the model crust
of 0.4 g/cc. Isostatic correction would be
difference between CBA and ISO values.
Example of format
98023 34 3.49 118 16.50 272.5 979573.64 -54.12 -63.42 0.03 0.68
D -62.85 -39.12
STAT: 98023
LaD: 34 degrees North
LaM: 3.49 minutes
LoD: 118 degrees West
LoM: 16.50 minutes
ELEV: 272.5 feet
OG: 979573.64 mGal
FAA: -54.12 mGal
SBA: -63.42 mGal
ITC: 0.03 mGal
TTC: 0.68 mGal
Code: D (0.59 km)
CBA: -62.85 mGal
ISO: -39.12 mGal
Jachens, R.C., and Roberts, C.W., 1981, Documentation of a Fortran program,
'ISOCOMP', for computing isostatic residual gravity: U.S. Geological Survey
Open-File Report 81-574, 26 p.
Plouff, Donald, 1977, Preliminary documentation for a FORTRAN program to
compute gravity terrain corrections based on topography digitized on a
geographic grid: U.S. Geological Survey Open-File Report 77-535, 45 p.
Telford, W.M., Geldart, L.O., Sheriff, R.E., and Keyes, D.A., 1976, Applied
Geophysics: New York, Cambridge University Press, 960 p.