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.