readme-of99-398.txt The digital data and map of this open-file report are available from the following U.S. Geological Survey URL: http://pubs.usgs.gov/of/1999/of99-398/ Data Contents The digital dataset consists of one file (lv100k.iso) containing the 2,506 point data used to create the map. The file contains the principal facts of the gravity stations, with one point coded per line. The format of the data is described below. Each gravity station has a station name, location (latitude and longitude, NAD27 projection), elevation, and an observed gravity reading. The data are on the IGSN71 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. 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. EXPLANATION OF PRINCIPAL FACT FORMAT Item Explanation STATION NAME (a8) ................An alphanumeric combination of up to 8 characters used for station identification LAT (f3.0,f4.2)...................Latitude in degrees and minutes, to 0.01 minute LON (f4.0,f4.2)...................Longitude in degrees and minutes, to 0.01 minute ELEV (f6.1).......................Elevation, to 0.1 ft OG (f7.2).........................Observed gravity, to 0.01 mGal FAA (f6.2) .......................Free-air anomaly to 0.01 mGal SBA (f6.2)........................Simple Bouguer anomaly to 0.01 mGal ITC (f5.2)........................Inner terrain correction for a density of 2.67 g/cc, to 0.01 mGal, followed by a letter denoting the extent of the correction. "Z" indicates computer terrain correction from the station out to 166.7 km with inner terrain correction out to D zone. TC (f5.2).........................Total terrain correction from the station to 166.7 km for a density of 2.67 g/cc, to 0.01 mGal. TC CODE (a1)......................Letter denoting the extent of the correction, according to the Hayford-Bowie template (e.g. 'D' means 590 meters). CBA (f6.2)........................Complete Bouguer anomaly reduced for a density of 2.67 g/cc, to 0.01 mGal. ISO (f6.2)........................Isostatic residual anomaly values 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. Example of format 98NYE074 36 12.94 115 57.44 2931.1 979537.98 -23.26 -123.23 0.02 1.71 D -122.53 -4.74 STATION NAME: 5962D800 LAT: 36 degrees 12.94 minutes North LON: 115 degrees 57.44 minutes West ELEV: 2931.1 feet OG: 979537.98 mGal FAA: -23.26 mGal SBA: -123.23 mGal ITC: 0.02 mGal TTC: 1.71 mGal TC CODE: D (590 m) CBA: -122.53 mGal ISO: -4.74 mGal Telford, W.M., Geldart, L.O., Sheriff, R.E., and Keyes, D.A., 1976, Applied Geophysics: New York, Cambridge University Press, 960 p.