RATIONALE AND OPERATIONAL PLAN TO UPGRADE 
THE U.S. GRAVITY DATABASE

INTRODUCTION
A concerted effort is underway to prepare a substantially upgraded digital gravity 
anomaly database for the United States and to make this data set and associated 
usage tools available on the internet. This joint effort, spearheaded by the geophysics 
groups at the National Imagery and Mapping Agency (NIMA), University of Texas 
at El Paso (UTEP), U.S. Geological Survey (USGS), and National Oceanic and 
Atmospheric Administration (NOAA), is an outgrowth of the new geoscientific 
community initiative called Geoinformatics (www.geoinformaticsnetwork.org).  This 
dominantly geospatial initiative reflects the realization by Earth scientists that 
existing information systems and techniques are inadequate to address the many 
complex scientific and societal issues.  Currently, inadequate standardization and 
chaotic distribution of geoscience data, inadequate accompanying documentation, 
and the lack of easy-to-use access tools and computer codes for analysis are major 
obstacles for scientists, government agencies, and educators.  An example of the type 
of activities envisioned, within the context of Geoinformatics, is the construction, 
maintenance, and growth of a public domain gravity database and development of 
the software tools needed to access, implement, and expand it.  This product is far 
more than a high quality database; it is a complete data system for a specific type of 
geophysical measurement that includes, for example, tools to manipulate the data 
and tutorials to understand and properly utilize the data.  On August 9, 2002, 
twenty-one scientists from the federal, private and academic sectors met at a 
workshop to discuss the rationale for upgrading both the United States and North 
American gravity databases  (including offshore regions) and, more importantly, to 
begin developing an operational plan to effectively create a new gravity data system.   
We encourage anyone interested in contributing data or participating in this effort to 
contact G.R. Keller (keller@geo.utep.edu) or T.G. Hildenbrand (tom@usgs.gov).
This workshop was the first step in building a web-based data system for sharing 
quality gravity data and methodology, and it builds on existing collaborative efforts.  
This compilation effort will result in significant additions to and major refinement of 
the U.S. database that is currently released publicly by NOAAÕs National 
Geophysical Data Center and will also include an additional objective to 
substantially upgrade the North American database, released over 15 years ago 
(Committee for the Gravity Anomaly Map of North America, 1987).  
The seventeen workshop attendees spanned a variety of organizations:
Allen Briesacher, National Imagergy Mapping Agency, DoD 
Ron Buhmann, National Geophysical Data Center, NOAA
Dave Dater, National Geophysical Data Center, NOAA
Guy Flannigan, Society of Exploration GeophysicistsÑPhillips Petroleum
Alan Herring, Society of Exploration GeophysicistsÑEDCON
Tom Hildenbrand, U.S. Geological Survey
Bill Hinze, Purdue University
Allen Hittelman, National Geophysical Data Center, NOAA
G.R. Keller, University of Texas, El Paso
Robert Kucks, U.S. Geological Survey
Don Plouff, U.S. Geological Survey
Tiku Ravat, Southern Illinois University
Walter Roest, Geological Survey of Canada
John Seeley, University of Texas, El Paso 
Dru Smith, National Geodetic Survey, NOAA
Mike Webring, U.S. Geological Survey
Dan Winnester, , National Geodetic Survey, NOAA
The workshop agenda (see Appendix) provided time to discuss the rationale for 
assembling a gravity data system but concentrated on developing a realistic 
operational plan for the upgrade.  In preparation for the workshop, participants were 
selected to resolve some of the anticipated issues in assembling a national gravity 
database by interviewing colleagues or by forming focus groups.  Topics and 
participants were:
Web site Appearance and Options:  Robert Kucks and John Seeley 
Initial U.S. Nonproprietary Database To Be Upgraded and the Effective Use of 
Proprietary Data: Allen Briesacher (chairperson), Tom Hildenbrand, Allen 
Hittelman, LeRoy Schmieder (NIMA), and Dru Smith
Data and Metadata Formats:  Guy Flanagan (Chairperson), Allen Briesacher, Tom 
Hildenbrand, and Allen Hittelman 
Horizontal, Vertical, and Observed Gravity Datum:  Dru Smith (Chairperson), Don 
Plouff, and Mike Webring
Terrain and Bathymetry Corrections:  Mike Webring and Don Plouff
Theoretical Gravity, Free-Air Anomaly, Bouguer Correction, Spherical Cap, Isostatic 
Anomaly, and Miscellaneous:  William Hinze
The resulting interviews and group meetings helped to focus workshop discussions 
on the remaining critical issues.  The rationale for the mission and a summary of the 
evolving operational plan are provided below.

RATIONALE TO UPGRADE THE U.S. DATABASE
Workshop presentations stressed that the primary goals of upgrading the gravity 
database and ultimately creating a robust data system are to provide more reliable 
data that support societal and scientific investigations of national and international 
importance.  Although most of the workshop activity focused on the U.S. gravity 
database, the broader goal of improving the North American gravity database is 
critical in understanding the tectonic evolution of the continent and elucidating 
regional geologic features, particularly those crossing national boundaries.
Applications of a modern gravity database included studies of the EarthÕs gravity 
field in geodesy, geophysics, and geology.  At regional scales, for example, gravity 
data are useful in determining the shape of the Earth, in accounting for the orbits of 
satellites, determining the Earth's mass and moment of inertia, and conducting 
geophysical mapping and interpretation of lithospheric structure and geodynamic 
processes.  In local studies of the upper crust, gravity data such as those shown in 
Figure 1 can effectively address a broad range of basic geologic questions, delineate 
geologic features related to natural hazards (faults, volcanoes, landslides), and aid in 
the search for natural resources (groundwater, oil, gas, minerals, geothermal energy).  

OPERATIONAL PLAN TO UPGRADE THE U.S. DATABASE
Workshop discussions focused on a viable operational plan to develop a modern 
gravity data system from existing personnel, equipment, and technology.  Upgraded 
principal facts (observed gravity, geographic coordinates, and elevation) are 
recognized as the single greatest legacy of the effort to develop a national gravity 
data system.  Secondary products are proposed and include an easily accessible 
website and associated tools to generate anomaly data, grids and maps.
It was determined that the NIMA comprehensive database will be used as a starting 
point in the upgrade process, supplemented by a significant quantity of new data. 
An extensive search is planned to identify and acquire existing data that have not 
been sent to NIMA (about 200,000 new gravity stations in the United States already 
have been identified).  Contributed data will be (1) specifically recognized with the 
contributor's name as appropriate, at the website discussed below, (2) processed in a 
consistent manner with the other data, and (3) like all the acquired data, freely 
distributed upon request. 
The scientific community working with gravity data has been traditionally small and 
well networked with a history of cooperation and sharing of data.  However, 
technical developments such as the use of GPS and the rising recognition of the 
utility of gravity data have led to a demand for gravity data by non-specialists, as 
well as an expanding ability to collect new gravity data.  Thus, the discussions 
centered on preparing a gravity database and system that will serve the purposes of 
a diverse user community while being simple to access and flexible enough to meet 
the range and changing needs of users.  Moreover, the proposed reduction 
procedures must follow internationally accepted standards and methodologies.
Web-Based Gravity System:  Of primary importance is that the data system be 
accessible from a website.  The database will have terrain and bathymetry corrections 
and anomaly calculations based on a reduction density of 2,670 kg/m3.  The web site 
will permit users to output data using a different reduction density.  If the user 
prefers data for different datums related to geographic coordinates or observed 
gravity (see below), the user will be supplied software from the website to make the 
conversions. 
The ultimate website must be a fully integrated data system "populated" with 
evaluated or quality data, as well as a robust set of software for data reduction and

 

analyses.  For example, a web-based toolkit will permit access to gravity data and 
manipulation of the data using tools that support processing, such as modeling, 
mapping, filtering, and construction of profiles.  Another goal is to produce a web-
based data system that will facilitate the efforts of researchers and students who 
want to learn about the gravity method and, perhaps, wish to collect data in areas 
currently not represented adequately in the database.  To that end, the website will 
contain tutorial information on topics such as:
_ Utility of gravity surveying and mapping
_ Guide for teachers
_ Location and proper use of base stations
_ Gravity standards (including ties to the standardization network)
_ Guide to internet resources
_ Online data reductions and plotting
_ Software for reduction, gridding mapping, and modeling that can be 
downloaded. 
Workshop discussions also addressed optimum approaches to the reduction and 
computation of gravity corrections and anomalies used in preparing a modern 
gravity database, which led to tentative decisions on what quantities should be 
calculated and what reduction formulas should be used.  Standardization and quality 
control will be critical in this effort.
Data and Metadata Formats:  A recommendation was made to develop a new ASCII 
format for the gravity data to meet the objective of the database.   This new format 
must include accuracies of values expected in a modern database, for example: 
geographical coordinates in decimal degrees to 7 places, elevation in meters to 3 
decimal places, observed gravity in mGal to 4 places.  Most data sets will not require 
this high accuracy format, but allowing higher-resolution data increases the usability 
of the database.  The data contributors should include an estimated accuracy for their 
data.  A decision was made to convene a focus group to recommend a 
comprehensive and lasting gravity data format. 
For the near-term, metadata residing at the NIMA will be incorporated in the web 
site to convey important information on the various data sources and associated 
data.  Any information provided by data contributors, such as a descriptive report of 
the survey, method of measuring or estimating inner terrain corrections, and detailed 
description of station locations, should eventually be incorporated as metadata.  
Contributors of new data will be asked to fill out a form that at least describes the 
survey procedures (e.g., GPS locations, inner terrain estimations), datums used, and 
accuracy of data collected.  
Data Evaluation:  An important issue arose in the development of an upgraded U.S. 
database, namely estimating reliability of the data.  It was decided that both NIMA's 
data evaluation process and UTEP's new advanced techniques of identifying and 
flagging erroneous data points and duplicates in a gravity database will be applied to 
the U.S. database.  In addition, proprietary data held by NIMA will be used to assist 
in quality assuring the nonproprietary data but only the resulting evaluated 
nonproprietary data will be made available.
Erroneous data will be removed or corrected using three different approaches.  First, 
using knowledge of the database, old clearly erroneous data will be removed or 
flagged.  Second, using an elevation filter algorithm to be developed, bad points due 
to elevation or location errors will be removed.  Third, using gravity anomaly values 
of nearby stations to determine doubtful station values will lead to the identification 
of data to be removed or flagged.
Duplicate data will be removed using station identification (ID).  Because the station 
ID may have changed, duplicate stations also need to be identified by latitude, 
longitude, elevation and observed gravity. 
Redundant data will be flagged, and the most accurate redundant station at or near a 
given location will be retained in the national database.  Nearby stations or previous 
knowledge of superior data sets will be used to determine which station is to be 
retained in the national database.
Horizontal, Vertical, and Observed Gravity Datums:  The default horizontal datum 
will be WGS84 (NAD83) but software will be provided for optional conversion to 
other datums, such as NAD27.  The default vertical datum will be the classical 
NGVD29 height above mean sea level on topographical maps but consideration will 
be given to a future change to a more modern datum.  Observed gravity will be tied 
to the International Standardization Net 1971 (IGSN71) (Morelli and others, 1974). 
The IGSN71 values will have the Honkasalo (1964) correction for tidal deformation 
removed (Moritz, 1980).  Morelli and others (1974) provide the Honkasalo term as 
follows:
          ) mGal,

where   is the geocentric latitude.  The correction varies from +0.04 at the equator to 
Ð0.07 mGal at the poles (Uotila, 1980).  
Gravity Anomalies:  The workshop offered the opportunity to discuss the reduction 
and computation of gravity corrections and anomalies that should be used in 
preparing the updated U.S. gravity database and led to tentative decisions on what 
values should be calculated and how the calculations should be done.  The gravity 
method depends on the removal of predicted gravity effects to enhance the 
expression of geologic targets.  For example, to derive the Bouguer gravity anomaly, 
corrections are made that relate to the total mass, rotation, and ellipsoidal shape of 
the Earth, to the elevation of the gravity station, and to the attraction of nearby 
topographic or bathymetric relief.  The following discussions outline the preferred 
anomaly equations and approaches to derive the Bouguer anomaly and other 
anomaly calculations and corrections. The reduction procedures follow 
internationally accepted standards and methodologies.
Theoretical gravity (on ellipsoid):  Both the rotation of the Earth and its elliptical 
shape affect gravity as a function of latitude.  To account for the mass, shape and spin 
of the Earth, a theoretical gravity value is subtracted from observed gravity.  Here we 
propose using the 1980 International Gravity Formula (IGF) in the Somigliana closed 
form (Moritz, 1980):

                   Gal.

An option to output data in the older Reference System 1967 (Morelli and others, 
1974) will be included at the website.
Atmospheric correction: The atmosphere is included in the total mass of the Earth 
and its effect is a function of altitude and reduces the predicted gravity value.  The 
correction varies from 0.87 mGal at the ellipsoid to zero at about 34 km above the 
ellipsoid.  We plan to use linear interpolation of the table given in Moritz (1980).
Height or free-air correction: The decrease in gravity above the ellipsoid is computed 
with a second order height correction equation provided by Heiskanen and Moritz 
(1969, page 79) and reprinted in Li and Gotze (2001).  Following Heiskanen and 
Moritz (1969), the height correction is calculated by:

               mGal,    
     
where the GRS80 ellipsoid has the following parameter values:
		h = elevation above the elipsoid, m
a = semimajor axis = 6378.137 km
		b = semiminor axis = 6356.7523141 km
		f  =  flattening = 0.003352810681
		ga = 978.03267715 Gal
		m =  = 0.00344978600308
		 = angular velocity = 7292115 x 10-11 radians s-1
     		GM = geocentric gravitational constant = 3986005 x 108 m3 s-2.

As mentioned previously, the data will be presumed to be on the NGVD29 datum 
unless documented as being on another datum.  We will supply anomaly values 
based on this datum or transform the elevations as needed to heights above the 
GRS80 ellipsoid (see details in next section).
Indirect reduction or correction:  The indirect correction takes into account the 
gravitational effect of the difference in height between the ellipsoid and the geoid 
(ranging from Ð107 to 85 m).  The resulting gravity effect ranges from about Ð22 to 
+18 mGal.  The indirect gravity effect has been generally neglected because geoidal 
heights change slowly with distance and, thus the effect is small over limited 
distances.  Because the difference is small in local areas, the geoid variation was 
previously interpreted as part of the regional gravity field.  This approximation is 
now unnecessary with the availability of reliable geoid models and it is now possible 
to supply the user with height corrections computed from the ellipsoid rather than 
using heights above mean sea level.  However, since many users will want to merge 
their own data and may not be able to apply the geoid, height corrections will also be 
made available using NGVD29 heights.  When doing ellipsoid height calculations, 
our plans include using (1) bilinear interpolation of the VERTCON grids (see 
http://www.ngs.noaa.gov/PC_PROD/pc_prod.shtml), (2) biquadratic interpolation 
of the GEOID99 grids (see http://www.ngs.noaa.gov/GEOID/GEOID99) and (3) 
application of a 7 parameter transformation to obtain International Terrestrial 
Reference Frame heights (e.g., http://www.ngs.noaa.gov/PUBS_LIB/gislis96.html). 
Bouguer correction: The Bouguer correction takes into account the gravitational 
attraction of the mass between the station elevation and the elevation datum.  We 
propose employing a spherical cap (LaFehr, 1991) computed to a radial distance of 
166.7 km instead of a horizontal infinite slab corrected to a spherical cap.  A spherical 
Earth radius of 6371 km, a crustal density of 2,670 kg/m3, and the Newtonian 
gravitational constant 6.673 +/- 0.01 x 10-11 m3kg-1s-2 (Mohr and Taylor, 2001) will be 
applied.  
Terrain corrections: A 3-part process generally will be employed: (1) utilize near-
station topographic information collected in the field to a distance of about 50 to 100 
m; (2) use high-resolution data in selected areas to compute an inner terrain 
correction to 895 m; and (3) compute an outer terrain correction from 0.895 to 167 km, 
based on digital elevation model (DEM) data.  The data collector provides the near-
station and inner terrain corrections, if available. The selection of an arbitrary 
intermediate radius of 895 m (Hammer, 1939, zone F) is consistent with the 
resolution of 15-second terrain grid (~ 450 m).  The outer terrain correction will 
initially be computed using Plouff's (1966; see also Godson and Plouff, 1988) 
computer algorithm and 15-sec, 1-min and 3-min topographic grids.  Spherical Earth 
curvature is used beyond 14 kilometers.  A terrain correction to 895 m can be roughly 
estimated with Plouff's program, but it would be stored separately from the outer 
(0.895 to 167 km) correction and users advised of its limitation.
Future possibilities include adding bathymetry corrections and extending gravity 
terrain corrections to 500 or 1000 kilometers to meet some user's survey size and 
accuracy needs.  The extended terrain correction can be computed using 2-minute 
bathymetry and an elliptical Earth.  Extended corrections would probably be 
complete terrain models, eliminating the spherical cap and traditional terrain 
correction approach.  Since the gravity terrain corrections will be computed offline 
and stored, the best algorithms available can be used without regard to excessive 
computer time.
Moreover, terrain corrections will be brought closer to the station with higher 
resolution topographic data by obtaining a complete set of 30-meter data.  Although 
a large number of the 30-meter quads have corrugation patterns, they have at least 
three times better resolution than 15-second (~ 450 m) data, allowing a decrease in 
the inner zone radius.  Other data sets like 10-meter, 1-second and high-altitude 
radar data will be co-located and utilized when they become available.  Further 
radius reductions in the mid-term are problematic since many of the station locations 
in the present database are mis-located by many 10's of meters. 
Bathymetry correction: The gravity terrain correction can be supplemented by an 
additional correction to account for the presence of seawater rather than rock below 
sea level in offshore areas.  Therefore, gridded bathymetric data needs to be 
assembled in the next phase to implement a bathymetric correction.  The technique 
being considered is to apply a numerical integration of sloping cylindrical surfaces 
(Olivier and Simard, 1981) to model water columns over sloping ocean bottoms.  
Isostatic anomaly:  Bouguer gravity anomaly maps traditionally have been used to 
provide a geologic picture of the subsurface over land areas (Simpson and others, 
1986).  At regional scales the Bouguer gravity map displays broad anomalies 
inversely correlated with regional elevations.  This inverse correlation commonly is 
interpreted to reflect the gravimetric response to a reduction of average crustal 
density or a thickened crust to topographic loads above the geoid.  To remove the 
effects of these topographic roots, an isostatic regional field is computed in a manner 
similar to the terrain model using the Airy-Heiskanen model.  A modified version of 
Jachens and Roberts (1981) method that assumes local compensation will be used to 
calculate the isostatic correction of varying depths to a hypothetical crust-mantle 
boundary caused by varying loads of topography above the geoid.  For each gravity 
station in the national database, topography will be modeled assuming a density of 
2,670 kg/m3 and using 3-minute elements of topography to 166.7 km plus an 
interpolated value to 180¡ from Karki and others (1961). 
Additional Products:  Bouguer and isostatic anomaly maps and associated grids of 
the conterminous United States including Alaska are additional proposed products.  
The grid spacing will be based on the data resolution of the final database (a spacing 
of probably between 2 and 4 km).  The selected map scale of 1:2,500,000 matches that 
of the previous Bouguer anomaly map of the conterminous U.S. (Society of 
Exploration Geophysicists, 1982).  
Although the need to compile an improved gravity database for North America was 
discussed, an associated operational plan was not formulated.  It was decided to 
schedule a meeting with colleagues of similar interests in North America gravity to 
launch this concerted effort. 
 
SCHEDULED ACTIVITIES AND SUMMARY
The near-term goals in chronological order will be to (1) gather existing gravity data 
presently not in the NIMA's database, (2) conduct quality control analysis including 
filtering out erroneous data, (3) compute terrain corrections and gravity anomalies 
consistent with internationally agreed upon procedures and with full documentation, 
(4) have available the upgraded U.S. database on the Web by May 2003, (5) produce a 
4-km grid of the upgraded U.S. data on the Web by June 2003, and (6) publish new 
U.S. Bouguer and isotatic gravity maps by October 2003.  The mid-term goals to be 
completed within the next three years will be to (1) develop new methods to improve 
data quality, (2) add bathymetry corrections, (3) extend outer terrain correction to 500 
or 1000 km and compute inner terrain corrections using 10-m DEMs, (3) improve on 
the contents and appearance of the established website by input and contributions 
from the scientific community, and (4) complete development of a gravity data 
system for North America.  The overall long-range goal of our concerted effort is to 
facilitate the creation of an open and flexible data system populated and maintained 
by user-members of the earth science community.  Community involvement will be 
central to the vitality and usability of both the U.S. and the North American data 
systems. 
The envisioned new gravity data system will offer significantly improved U.S. and 
North American gravity databases, which represent a resource fundamental to 
geoscience investigations.  The resulting data system will be freely accessible to all on 
the Internet.  

REFERENCES
Committee for the Gravity Anomaly Map of North America, 1987, Gravity Anomaly 
Map of North America:  Geological Society of America, Continental-Scale Map-
002, scale 1:5,000,000.
Godson, Richard H., Plouff, Donald, 1988, BOUGUER version 1.0ÑA microcomputer 
gravity-terrain-correction program: U.S. Geological Survey Open-File Report 88-
644-A, Documentation 22 p.; 88-644-B, Tables, 61 p., 88-644-C, 5 1/4-inch diskette.
Hammer, S., 1939, Terrain corrections for gravimeter stations, Geophysics, v. 4, p. 
184-194.
Heiskanen, W.A. and Moritz, H., 1969, Physical Geodesy: W.H. Freeman Co.
Honkasalo, T., 1964, On the tidal gravity correction: Boll.Geof. Terror. Appl., VI, p. 
34-36.
Jachens, R.C., and Carter, Roberts, 1981, Documentation of program, ISOCOMP, for 
computing isostatic residual gravity: U.S. Geological Survey, Open-File Report 
81-0574, p. 26.
Karki, Pentti, Kivioja, Lassi, and Heiskanen, W.A., 1961, Topographic isostatic 
reduction maps for the world to the Hayford Zones 18-1, Airy-Heiskanen System, 
T=30 km: Publication of the Isostatic Institute of the International Association of 
Geodesy, no. 35, 5 p., 20 pl.
LaFehr, T.R., 1991, An exact solution for the gravity curvature (Bullard B) correction: 
Geophysics, v. 56, p. 1179-1184.
Li, Xiong, and Gotze, H-J., 2001, Ellipsoid, geoid, gravity, geodesy, and geophysics, 
Geophysics, v. 66, p. 1660-1668.
Morelli, C., and others 1974, The International Gravity Standardization Net 1971: 
International Association of Geodesy, Special Publication No. 4, 194 p.
Moritz, H., 1980, Geodetic Reference System 1980: Bulletin Geodesique, v. 54, p. 395-
405.
Mohr, P.J., Taylor, B.N., 2001, The fundamental physical constant: Physics Today, v. 
54, no. 8, part 2, p. 6-16 [http://physics.nist.gov/constants]. 
Olivier, R.J., Simard, R.G., 1981, Improvement of the conic prism mode for terrain 
correction in rugged topography: Geophysics, v. 46, no. 7, p. 1054-1056.
Plouff, Donald, 1966, Digital terrain corrections based on geographic coordinates 
(abs.): Geophysics, v. 31, no. 6, p.1208.
Simpson, R. W.,  Jachens, R. C., Blakely, Richard J., and Saltus, R. W., 1986 A new 
isostatic residual gravity map of the conterminous United States with a discussion 
on the significance of isostatic residual anomalies: Journal of Geophysical 
Research, v. 91, p. 8348-8372.
Society of Exploration Geophysicists, 1982, Bouguer gravity anomaly map of the 
United States (excluding Alaska and Hawaii): Society of Exploration 
Geophysicists, scale 1:2,500,000 (2 sheets).
Uotila, U.A., 1980, Note to users of international gravity standardization net 1971: 
Bulletin Geodesique, v. 54, p. 407.

APPENDIX A
WORKSHOP AGENDA:
UPGRADING THE U.S. AND NORTH AMERICAN
GRAVITY DATABASES
August 9, 2002

8:30Ð8:40 a.m: Welcoming RemarksÑTom Hildenbrand, USGS, and
	 Randy Keller, UTEP
8:40Ð9:30 a.m.: Rationale for Upgrading Gravity Databases of North America
Geoinformatics and EarthScopeÑRandy Keller (20 min.)
Rationale for the Upgrade of the U.S. DatabaseÑTom Hildenbrand (15 min.)
Rationale for the Upgrade of the Canadian and Mexican DatabasesÑWalter Roest, GSC, 
and Randy Keller (15 min.)
9:30 a.m.Ð10:00:  Operational Plan
Web site Appearance and OptionsÑJohn Seeley, UTEP, and Bob Kucks, USGS (30 min.)
10:00Ð10:15 a.m.:  Coffee Break
10:15Ð11:30p.m.:  Operational Plan (Cont.)
Initial U.S. Nonproprietary Database To Be Upgraded and the
Effective Use of Proprietary DataÑAllen Briesacher, NIMA (15 min.)
Data and Metadata FormatsÑGuy Flanagan, SEG (15 min)
Handling of Erroneous, Redundant and Duplicate DataÑRandy Keller (30 min.)
Horizontal, Vertical, and OG DatumsÑDru Smith, NGS (15 min.)
11:30Ð1:00 p.m.:  Lunch
1:00Ð2:10 p.m.:  Operational Plan (Cont.)
Terrain and Bathymetry CorrectionsÑMike Webring, USGS (20 min.)
Theoretical Gravity, Free-Air Anomaly, Bouguer Correction, Spherical Cap, 
Isostatic Anomaly, Etc.ÑBill Hinze, Purdue U. (30 min.)
Final Products (maps, grids, databases, web sites, and software)ÑGroup discussion 
led by Tom Hildenbrand and Randy Keller (20 min.)
2:10Ð3:00 p.m.:  Closing Discussions/Action Items
Ð12Ð