Mankinen, Edward A., Sweetkind, Donald S., McKee, Edwin H., and Morin, Robert L., 2005, Gravity and ground magnetic data from selected traverses in the Amargosa desert and vicinity, Nevada and California: Open-File Report 2005-1444, Geological Survey (U.S.).
What coordinate system is used to represent geographic features?
Horizontal positions are specified in geographic coordinates, that is, latitude and longitude.
Latitudes are given to the nearest 0.0001667.
Longitudes are given to the nearest 0.0001667.
Latitude and longitude values are specified in decimal degrees.
The horizontal datum used is World Geodetic System 1984.
The ellipsoid used is WGS 84.
The semi-major axis of the ellipsoid used is 6378137.
The flattening of the ellipsoid used is 1/298.257.
Textual identifiers without distinct scientific significance
Range of values
Range of values
Free air gravity anomaly
Range of values
Terrain inner zone correction
Inner-zone terrain corrections (zones A, B, C, and D of Hayford and
Bowie, 1912), which are necessary to account for variations in
topography near a gravity station, were obtained using a program
developed by Spielman and Ponce (1984).
Gravity data were obtained using a LaCoste and Romberg meter (USGS
meter G8) and observed gravity values were referenced to the
International Gravity Standardization Net 1971 (ISGN 71) gravity
datum (Morelli, 1974). Gravity observations were tied to two ISGN
71 base stations (Harris and others, 1989); the original Beatty
post office (BPO), now the Death Valley visitor station, and DUNE,
west of the junction of U.S. Highway 95 and Nevada State Highway
29 in Amargosa Valley. The observed gravity value given for BPO in
Harris and others (1989) has been revised to 979,566.45 mGal (D.A.
Ponce, USGS, written communication, 1997).
Observed gravity at each station was adjusted by assuming a
time-dependent linear drift between readings of a base station at
the start and finish of each daily survey. This adjustment
compensates for drift in the instruments spring. All gravity
data and their associated parameters are given in Table 1.
Observed gravity values are considered accurate to about 0.05 mGal
based on repeat measurements over several mountain calibration
loops (Barnes and others, 1969; Ponce and Oliver, 1981). Gravity
data were reduced using standard gravity corrections (Harris and
others, 1989) and a reduction density of 2670 kg/m3. Inner-zone
terrain corrections (zones A, B, C, and D of Hayford and Bowie,
1912), which are necessary to account for variations in topography
near a gravity station, were obtained using a program developed by
Spielman and Ponce (1984). The calculations use digitized
topography in a digital elevation model (DEM) and a procedure by
Plouff (1977). A regional isostatic field was calculated using an
Airy-Heiskanen (Heiskanen and Vening Meinesz, 1958) model for
local compensation of topographic loads (Simpson and others, 1983,
1986). This model assumes a crustal thickness of 25 km, a crustal
density of 2670 kg/m3, and a 400 kg/m3 density contrast between
the crust and mantle. This regional isostatic field was subtracted
from the complete Bouguer anomaly, thus removing longwavelength
variations in the gravity field that are inversely related to
topography. The resulting isostatic residual gravity anomaly,
therefore, is a reflection of local density distributions within
middle to upper crustal levels. The isostatic residual gravity
anomaly for the Amargosa Valley study area, incorporating the 152
new gravity observations from Table 1, is shown in figure 4. The
new gravity stations were established along the same traverses as
the ground magnetic profiles described in the following section
and were intended to complement those data.
Date: 05-Jun-2008 (process 2 of 2)
Creation of original metadata record
Person who carried out this activity:
How accurate are the geographic locations?
Locations of gravity stations were determined using differential GPS, with surveyed benchmarks
used for control and are accurate to within about 0.1 m, both horizontally and vertically
This report is preliminary and has not been reviewed for
conformity with U.S. Geological Survey editorial standards (or
with the North American Stratigraphic Code). Any use of trade,
product, or firm names is for descriptive purposes only and does
not imply endorsement by the U.S. Government.