Readme for Open-File Report 2013-1024–H Gravity Survey and Interpretation of Fort Irwin and Vicinity, Mojave Desert, California Data Contents The three data files included in Open-File Report 2013-1024–H are given as ascii data files on the web site URL http://pubs.usgs.gov/of/2013/1024/h/. The format of the data files are described below. 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 (iso.txt) 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 2.67 g/cc. 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 0.40 g/cc between the crust and mantle, and a crustal density of 2.67 g/cc. The other two files contain gridded values of the isostatic residual gravity (iso_grid.txt) and of the depth to basement (depth2basement.txt). The data in both grids were were interpolated to a rectangular grid with nodes spaced 250 m apart using a minimum curvature algorithm (Briggs, 1974) using a Transverse Mercator projection (central meridian -117.0, base latitude 0.0; scale factor 0.9996). There are three fields in each file, with longitude and latitude (NAD27 datum) being the first two. In iso_grid.txt, the third field is isostatic residual gravity value in mGal. In depth2basement.txt, the third field is depth to basement in kilometers, with negative values being z in the down direction. EXPLANATION OF PRINCIPAL FACT FORMAT FOR GRAVITY STATIONS (ISO.TXT) File iso.txt is fixed format. Format is as follows: Item Explanation STATION NAME: An alphanumeric combination of up to 8 characters used for station identification. Columns 0-8 LATD: Degree latitude. Columns 9-11 LATM: Minute latitude. Columns 12-17 LOND: Degree longitude. Columns 18-21 LONM: Minute latitude. Columns 22-27 ELEV: Elevation. Columns 28-35 OG: Observed gravity. Columns 36-45 FAA: Free-air anomaly. Columns 46-58 SBA: Simple Bouguer anomaly. Columns 59-66 ITC: Inner terrain correction out to a radius of various distances (see TC CODE) from the station, for a density of 2.67 g/cc. Columns 67-73 TC: Total terrain correction from the station to 166.7 km for a density of 2.67 g/cc. Columns 74-80 TC CODE: Letter denoting the extent of the inner-zone correction, according to the Hayford-Bowie and Hammer templates (D=590 meters; f=895 meters; M=2000 meters; F=2291 meters; Z=590 meters calculated from 15-sec data; no letter means unknown, but likely D). See Spielman and Ponce (1984) for additional explanation. Column 82 CBA: Complete Bouguer anomaly reduced for a density of 2.67 g/cc. Columns 83-90 ISO: Isostatic residual anomaly values. Columns 91-98. Example of format for gravity file 51636676 36 12.16 114 29.15 2480.0 979579.35 -23.18 -107.77 3.13 5.96 D -102.71 -3.14 STATION NAME: 51636676 LATD: 36 degrees North LATM: 12.16 minutes LOND: 114 degrees West LONM: 29.15 minutes ELEV: 2480.0 feet OG: 979579.35 mGal FAA: -23.18 mGal SBA: -107.77 mGal ITC: 3.13 mGal TC: 5.96 mGal TC_CODE: D (590 m) CBA: -102.71 mGal ISO: -3.14 mGal 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.