Readme for Open-File Report 2016-1027 High-Resolution Gravity and Seismic-Refraction Surveys of the Smoke Tree Wash Area, Joshua Tree National Park, California Data Contents The data file included in Open-File Report 2016-1027 are given as ascii data files on the web site URL http://dx.doi.org/10.3133/ofr20161027. The format of the data files are described below. GRAVITY DATA The gravity data are contained in one file (iso-all.txt) The horizontal and vertical locations for the stations are on the North American Datum 1927 (NAD27) and the North American Vertical Datum 1929 (NAVD29), respectively. The gravity stations were referenced to the International Gravity Standardization Net 1971 (IGSN71) datum and the reference ellipsoid is the Geodetic Reference System 1967 (GRS67). The free-air anomalies were calculated using a modified version of formulas from Swick (1942). The complete Bouguer anomalies were calculated from the free-air anomalies using the Bouguer correction, terrain corrections, a curvature correction, and a reduction density of 2,670 kg/m3. Isostatic corrections were calculated using an Airy-Heiskanen model of isostatic compensation. The depth of the crust-mantle boundary was controlled using the following parameters: a crustal thickness at sea level of 25 km, a density contrast of 400 kg/m3 between the crust and mantle, and a crustal density of 2,670 kg/m3. EXPLANATION OF PRINCIPAL FACT FORMAT FOR GRAVITY STATIONS (iso-all.txt) File iso-all.txt is tab-delimited format. Lines denoted by "\" are header lines that separate data of different sources. Item Explanation STATION_NAME: An alphanumeric combination of up to 8 characters used for station identification. LATD: Degree latitude. LATM: Minute latitude. LOND: Degree longitude. LONM: Minute latitude. ELEV: Elevation in feet. OG: Observed gravity in mGal (-999, unavailable) FAA: Free-air anomaly in mGal. SBA: Simple Bouguer anomaly in mGal (-999, unavailable) ITC: Inner zone terrain correction out to a radius of various distances (see TC CODE) from the station, for a density of 2,67 kg/m3. (-999, unavailable) TTC: Total terrain correction from the station to 166.7 km for a density of 2,670 kg/m3. (-999, unavailable) TC_CODE: Letter denoting the extent of the inner-zone correction, according to the Hayford-Bowie and Hammer templates (D=590 meters; G=1530 meters). See Spielman and Ponce (1984) for additional explanation. CBA: Complete Bouguer gravity anomaly reduced for a density of 2,670 kg/m3. ISO: Isostatic residual gravity anomaly values (reduced for a density of 2,670 kg/m3). Example of format for gravity file he008 33. 49.07 115. 51.56 4204.5 979293.97 56.09 -87.31 0.98 7.87 D -80.72 -20.42 STATION_NAME: he008 LATD: 33 degrees North LATM: 49.07 minutes LOND: 115 degrees West LONM: 51.56 minutes ELEV: 4204.5 feet OG: 979293.97 mGal FAA: 56.09 mGal SBA: -87.31 mGal ITC: 0.98 mGal TTC: 7.87 mGal TC_CODE: D (590 m) CBA: -80.72 mGal ISO: -20.42 mGal References Spielman, J.B., and Ponce, D.A., 1984, Handtc, a fortran program to calculate inner-zone terrain corrections: U.S. Geological Survey Open-File Report 84-777, 24 p. Swick, C.A., 1942, Pendulum gravity measurements and isostatic reductions: U.S. Coast and Geodetic Survey Special Publication 232, 82 p.