Land and Marine Gravity Data - Dictionary of Terms

Dictionary of Terms

accuracy_code

ACCURACY OF THE DATA

The USGS accuracy code (see table below), provides a standardization of observed gravity precision, elevation control, and latitude and longitude control. In very general terms, observed gravity and elevation imprecision produce uncertainty in the Bouguer anomaly of roughly 1 mGal each within this entire data set. In addition, a 5 mGal total terrain correction has an uncertainty of perhaps 0.3 mGal while total terrain corrections of 10 mGal or above have ten percent uncertainties. It must be emphasized that this data tape was compiled from the work of many scientists using many individual reduction techniques. The use of uniform reduction criteria has been attempted in bringing the data together, but a careful accuracy analysis is impossible. The Bouguer and isostatic anomalies are thought accurate to about three or four mGal.

The single digit accuracy code used in southern California indicates the elevation control as follows: b = on or near bench mark, m = map elevation, t = 0.1 foot precision, u = "useful map elevation".

This accuracy code is unique to USGS California and Nevada data.

USGS FOUR DIGIT GRAVITY ACCURACY CODE TABLE

GENERAL LOCATION CODE (1st digit)
(vertical and horizontal)
CODE STATION LOCATION METHOD
A) SURVEY MARKS (VERTICAL) - TOPOGRAPHIC MAPS (HORIZONTAL): 1) Base plate directly on bench mark; B .............. a) USGS or USC and GS (NGS) level-line bench mark.; M .............. b) Level line bench marks other than a) such as USCE, BPR, CDH, private companies, etc.; V .............. c) VABM (Vertical angle bench marks).; 2) Base plate near bench mark.; N .............. a) USGS or USC and GS level-line bench mark.; E .............. b) Level-line bench marks other than a) such as USCE, BPR; H .............. c) VABM (Vertical angle bench marks).; P ..... 3) Base plate on or near other reference marks (such as stakes, paint, etc.) that have been surveyed by the group doing the gravity survey or other known people.; X ..... 4) On or near Section Corners, 1/4 section marks, 1/8 section markers, and other property boundary markers.; D ..... 5) Destroyed or not found bench or reference marks.; b ..... 6) Base plate on or near bench mark.; t ..... 7) Benchmark or surveyed location known to 0.1 foot.

B) TOPOGRAPHIC MAP LOCATIONS (VERTICAL AND HORIZONTAL): F .... 1) Black spot elevations - field checked.; G .... 2) Brown spot elevations and elevations taken off original manuscripts - not field checked.; W .... 3) Blue lake elevations.; ....... 4) Lake or reservoir elevations determined from leveling to bench marks, and water level is determined from gauging stations.; S .... 5) Sea level elevations.; C .... 6) Contour line interpolation.; Q .... 7) River gradient interpolation.; m .... 8) Map elevations.

C) AIR PHOTOGRAPHS (VERTICAL AND HORIZONTAL): T ...... 1) Elevations determined by U.S. Geological Survey National Mapping Division by Kelsh plotter or least squares computer system.; K ...... 2) Elevations determined by other groups by Kelsh plotter or least squares computer system.; L ...... 3) Elevations determined by laser methods.; J ...... 4) Elevations determined by other methods.

D) ALTIMETRY (VERTICAL) - TOPOGRAPHIC MAPS (HORIZONTAL): A ...... 1) Good control (Leap frog, double loop, two or more altimeters, etc.).; Y ...... 2) Poor control.

E) SPECIAL SOURCES: Z ....... 1) Elevations determined by methods such as mobile elevation recorders - horizontal control from topographic maps.; I ....... 2) Other special sources.

F) UNKNOWN ELEVATION SOURCES: U ..... 1) Elevation data sources unknown (this would include reference marks with unknown ties).

ELEVATION ACCURACY CODE (2ND DIGIT)
(Relative to 1929 USC and GS mean sea level datum)

Code	Elevation Accuracy(ft)	Typical types of Elevation Data	Approx.Gravity Effect (mGal)
1	0.2	On leveled bench marks	0.01
2	1/3	Beside bench marks	0.02
3	1	Transit and good alidade surveys	0.05
4	2	VABM's and most black map elevations	0.1
5	4	Black elevations on old maps, good photogrammetry	0.2
6	10	Brown elevations and normal photogrammetry on 20 ft contour interval maps	0.5
7	20	Brown elevations on 80 ft contour interval maps, good altimetry	1.0
8	40	Contour interpolation data from 50 ft contour-interval map	2.0
9	80	Poor altimetry; data from 200 ft contour-interval maps	5.0
0	80	Altimetry in very bad weather or equipment failures	5.0

LATITUDE ACCURACY CODE (3RD DIGIT)
(Lat. gravity effect based on mean value of 1.45 mGal/min at Latitude 37)

Code	Latitude Accuracy (min)	Equivalent Distance Accuracy (ft)	Typical Map Measurement Requirements in inches	Approx. Gravity Effect (mGal)
1	.0075	42	Triangulation or special survey data	0.01
2	.015	84	0.04 (1:24,000) map with special location care	0.02
3	.04	210	0.10 (1:24,000) normal survey; 0.04 (1:62,500) map	0.05
4	.07	420	0.21 (1:24,000) map; 0.08 (1:62,500) normal survey	0.1
5	.14	840	0.42 (1:24,000) map; 0.16 (1:62,500) map	0.2
6	.35	2,100	0.4 (1:62,500) map; 0.1 (1:250,000) map	0.5
7	.70	4,200	0.8 (1:62,500) map; 0.2 (1:250,000) map	1.0
8	1.4	8,400	1.6 (1:62,500) map; 0.4 (1:250,000) map	2.0
9	3.40			5.0
0	3.40			5.0

LATITUDE ACCURACY CODE (3RD DIGIT)
(Lat. gravity effect based on mean value of 1.45 mGal/min at Latitude 37)

Code	Latitude Accuracy (min)	Equivalent Distance Accuracy (ft)	Typical Map Measurement Requirements in inches	Approx. Gravity Effect (mGal)
1	.0075	42	Triangulation or special survey data	0.01
2	.015	84	0.04 (1:24,000) map with special location care	0.02
3	.04	210	0.10 (1:24,000) normal survey; 0.04 (1:62,500) map	0.05
4	.07	420	0.21 (1:24,000) map; 0.08 (1:62,500) normal survey	0.1
5	.14	840	0.42 (1:24,000) map; 0.16 (1:62,500) map	0.2
6	.35	2,100	0.4 (1:62,500) map; 0.1 (1:250,000) map	0.5
7	.70	4,200	0.8 (1:62,500) map; 0.2 (1:250,000) map	1.0
8	1.4	8,400	1.6 (1:62,500) map; 0.4 (1:250,000) map	2.0
9	3.40			5.0
0	3.40			5.0

OBSERVED GRAVITY ACCURACY (4TH DIGIT)
(relative to local base)

Code	Obs. Gravity Accuracy	Suggested Types of Gravity Measurements mGal
1	0.01	Local surveys with special meters
2	0.02	Multiple readings with LaCoste and Romberg meters
3	0.05	Average Lacoste and Romberg, and Multiple Worden gravity data
4	0.1	LaCoste and Romberg gravity data with small vibrations. Most USGS Worden gravity meter data
5	0.2	Gravity data from loops with closure errors this large
6	0.5	" " " " " " " " "
7	1.0	" " " " " " " " "
8	2.0	" " " " " " " " "
9	5.0	" " " " " " " " "
0	5.0	" " " " " " " "

agency_code
NGS AGENCY CODE TABLE

CODE	DEFINITION
1	Defense Mapping Agency
2	National Geophysical Data Center, Boulder
3	Geological Survey of Canada
4	Brown Geophysical Research Corporation
5	Georgia Institute of Technology
6	National Geodetic Survey
7	U.S. Geological Survey
8	Los Alamos Scientific Laboratory
9	Purdue University
10	Hawaii Institute of Geophysics
11	Tetra Technologies, Inc.
12	University of Texas at Dallas
13	Kansas Geological Survey

alternate_elev_ft
Usually, a map-derived elevation(spot elevation, contour-interpolated elevation, or a questionable altimetry elevation). When altimetry seems to provide a preferable station elevation the map-derived alternate elevation is usually used for the terrain correction and the altimeter is used for the slab portion of the complete Bouguer elevation. Unique to the Alaska ak1_iso and ak1_wpn data sets. Also see Notes on the Processing and Presentation of USGS Alaskan Gravity Data, by D. F. Barnes(#529).
alternate_elev_m
The alternate_elev_ft variable converted to meters.
alternate_elev_type
An elevation-type letter indicates the source of the elevation(see "source_accuracy_code"tables). The alternate station elevation type is unique to the ANWR data set and is described in Notes on the Processing and Presentation of USGS Alaskan Gravity Data by D. F. Barnes (#529).
alternate_station_name
Unique to ANWR data.
The alternate station name is unique to the ANWR data set and is described in Notes on the Processing and Presentation of USGS Alaskan Gravity Data by D. F. Barnes (#529). The following is an extract from this report.

The tabulation provides for two station numbers: a main number and an auxiliary number. The auxiliary number in some cases is an old number used for the station, and in other cases merely provides additional information about the station. Typical additional information includes identification of the station as a base, an indication of the type of field mark, or available reoccupation information. Thus typical phrases in the auxiliary number column may be "BASE"(for base stations), "MARK"(for usgs gravity marker), TEM1 (for tidal bench mark number 1), TB11 (for tidal bench mark number 11), FOTO (to indicate a photograph was taken), DESC (to indicate a description suitable for reoccupation is available), or a "/"followed by letters or numbers indicating the name of a vertical angle bench mark or triangulation station. A more complete lists of station naming and description conventions is given in "station_name"field description.
bath_corr_code
BATHYMETRIC CORRECTION CODE
As defined in Marine Geophysical Data Exchange Format - MGD77 by Hittelman,A., et al., this code details the procedure used for determining the sound velocity correction to depth:

01-55 Matthews' Zone - use only if Matthews' Zones were used to correct depth.

59 Matthews' corrections used but zones unspecified in data record.

60 S. Kuwahara Formula

61 Wilson Formula

62 Del Grosso Formula

63 Carter's Tables

88 Other (describe correction procedure in "Additional Documentation"portion of Header)

99 Unspecified
bath_corr_depth
BATHYMETRY,CORRECTED DEPTH

as defined in Marine Geophysical Data Exchange Format - MGD77 by Hittelman,A., et al., water depth measured by acoustic instruments and converted to units of meters.
bath_trav_time
BATHYMETRY, TRAVELTIME

as defined in Marine Geophysical Data Exchange Format - MGD77 by Hittelman,A., et al., 2-way travel-time in seconds corrected for transducer depth and other such corrections, especially in shallow water.
fbath_type_code
BATHYMETRIC TYPE CODE

as defined in Marine Geophysical Data Exchange Format - MGD77 by Hittelman,A., et al., indicates how the data record's bathymetric value was obtained:

1 = Observed

3 = Interpolated (explain scheme in Header Sequence No. 12, Columns 23-78)

9 = Unspecified
Bouguer_anom
The residual value obtained after latitude correction, elevation correction (including both free-air and Bouguer corrections) and terrain corrections have been applied to gravity data. In many situations the terrain correction is assumed to be zero. When this is explicitly documented, the term "Simple Bouguer Anomaly "is often used. When one explicitly documents that the terrain correction has been applied the term "Complete Bouguer Anomaly "is often used.
Bouguer_anom_243
A Bouguer anomaly calculated using a Bouguer slab density of 2.43g/cc.
Bouguer_anom_258
A Bouguer anomaly calculated using a Bouguer slab density of 2.58g/cc.
Bouguer_anom_267
A Bouguer anomaly calculated using a Bouguer slab density of 2.67g/cc.
Bouguer_anom_267_abs
Generally the absolute (unsigned) value of the Bouguer Anomaly that was calculated using a Bouguer slab density of 2.67g/cc.
Bouguer_anom_267_sign
The sign of the Bouguer Anomaly + or - .
Bouguer_anom_comp_dens1
A complete Bouguer anomaly, including terrain correction, using a Bouguer slab density specified in field labeled { density_1, density_2 }, unique to Alaskan gravity data sets Holitna1 and Holitna2.
Bouguer_anom_comp_dens2
A complete Bouguer anomaly, including terrain correction, using a Bouguer slab density specified in field labeled { density_1 , density_2 }, unique to Alaskan gravity data sets Holitna1 and Holitna2.
Bouguer_anom_sd
The standard deviation associated with the Bouguer Anomaly.
Bouguer_anom_simp
A simple Bouguer anomaly, i.e. Bouguer gravity anomaly that has not been terrain corrected.
Bouguer_anom_simp_dens1
A simple Bouguer anomaly, using a Bouguer slab density specified in field labeled { density_1, density_2 }, unique to Alaskan gravity data sets Holitna1 and Holitna2.
Bouguer_anom_simp_dens2
A simple Bouguer anomaly, using a Bouguer slab density specified in field labeled { density_1, density_2 }, unique to Alaskan gravity data sets Holitna1 and Holitna2.
Bouguer_anom_simp_elev1
A simple Bouguer anomaly, using a topographic map elevation (ft), unique to Alaskan gravity data sets Holitna1 and Holitna2.
Bouguer_anom_simp_elev2
A simple Bouguer anomaly, using an altimeter elevation (ft), unique to Alaskan gravity data sets Holitna1 and Holitna2.
comments_nev97
This is a mixed-use field that contains either a Simple Bouguer anomaly, in milligals to 0.01 mGal, or date that the station was occupied: month(a2), day(a2), and year(a2); or range in years(e.g., 72to76).
cross_ref_base_code
Cross-Reference to base station code numbers. Unique to DMANET data.
cruise_id
CRUISE IDENTIFIER

as defined in Marine Geophysical Data Exchange Format - MGD77 by Hittelman,A., et al., institution identification (for reference purposes). Unique to JODC data.
curvature_corr
The Bouguer gravity correction assumes that the mass between the measurement elevation and sea level can be approximated by an infinite slab (often using a slab density of 2.67 grams per cc). The curvature correction is subtracted from the Bouguer gravity correction and compensates for the mass of this slab which extends beyond the curvature of the Earth. The U.S. Geological Survey computes it as follows:
curvature_corr = elev*(1.4639108e-3+elev*(4.449648e-14*elev-3.532715e-7))
Where elev is the elevation of the gravity station in meters and the curvature correction is in units of milligals. This correction rarely exceeds a few tenths of a milligal and is often omitted.
data_record_type
Data records as defined in Marine Geophysical Data Exchange Format - MGD77 by Hittelman,A., et al., note that all data records are set to "3". Unique to JODC data.
data_set_code
Unique to the ANWR data set, this field is used to identify the data set from which the record originated. It can thus be used to determine the date of the gravity measurement, the meter used, the observers, the bases occupied, the densities used in reduction and a variety of supplementary information which may be desired if the data appear questionable.

NOTE: refer to original document for details.
data_set_id
A station identifier found in some data sets among the Minnesota gravity data sets.
day
Day of the month. Values range from 1 to 31.
deflec_of_vert_ew
The "East-West"component of the deflection of the vertical. Also known as deflection of the vertical in the prime vertical, represented by the Greek letter "Eta". Unique to the GEOID93 data set.
deflec_of_vert_ns
The "North-South"component of the deflection of the vertical. Also known as deflection of the vertical in the meridian, represented by the Greek character "Xi". Unique to the GEOID93 data set.
delta_n
The higher frequency geoid contribution from the Stokes' integration process including the correction for the indirect effect(in metres). This contribution represents the terrestrial part of the geoidal height above degree and order 20, i.e., above the GEM9(20,20) pure satellite gravity field solution. Unique to the Canada10 data set.
delta_n_indirect
The change in the geoid caused by the computational shifting of mass when reducing gravity data from the Earth's surface to the geoid, as required by the Stokes' integral. Unique to the Canada10 data set.
delta_n_sd
An estimate of the standard deviation of geoidal height in meters. Unique to the Canada10 data set.
density
Generally, the Bouguer slab density value used in the calculation of the Bouguer gravity anomaly.
density_1
Bouguer slab density value used to calculate the Bouguer correction, unique to Alaskan gravity data sets Holitna1 and Holitna2.
density_2
Bouguer slab density value used to calculate the Bouguer correction, unique to Alaskan gravity data sets Holitna1 and Holitna2.
density_count
The number of gravity observations in a given 3-minute cell. Unique to the GEOID93 data set.
description_avail_code
Unique to DMANET

Description Availability:

0 = Description, Sketch, and Photograph Not Available

1 = Description Available

2 = Sketch Available

3 = Description and Sketch Available

4 = Photograph Available

5 = Description and Photograph Available

6 = Sketch and Photograph Available

7 = Description, Sketch, and Photograph Available
dot
A period or decimal point.
edit_code
An NGS code used to summarize editing actions:
CODE            DEFINITION
0                            Not processed
1                            Preliminary rejection
2                            Finally rejected
3                            Accepted after quality check
4                            Corrected and accepted
elev_unit_code
A code used by DMA to specify units of elevation:

BLANK OR 0 - METERS

1-FEET

2-FATHOMS UNIQUE TO DMA FORMAT DATA
Eotvos_corr
EOTVOS CORRECTION

as defined in Marine Geophysical Data Exchange Format - MGD77 by Hittelman,A., et al., this is a correction for the movement of the gravity sensor relative to the Earth's surface rotation. The correction is in units of mgals and is calculated as E = 7.5 V cos(phi) sin(alpha) + 0.0042 V^^2 where V is the velocity in knots, phi is the latitude, and alpha is heading direction
file_maint_code
Unique to DMA - for internal use by DMA only:

0 - Add

1 - Replace

2 - Delete
file_maint_flag
l = Index Record
Blank = Supplementary Record
NOTE: A station may be described by one or more records. A record may contain all the information for a station (Index record); or certain additional information which more fully describes the station, such as a different station name or an additional cross-reference (Supplementary record).
Free_air_anom
The anomalous gravity difference between an observed value and theoretical value. The gravity data has been corrected for latitude and elevation (free-air correction), but not for the density of the rock between datum and the plane of measurement (Bouguer correction).
Free_air_anom_abs
Generally the absolute (unsigned), value of the Free-air Anomaly.
Free_air_anom_sd
The standard deviation of the Free-air Anomaly.
Free_air_anom_sign
The sign of the Free-air Anomaly + or - .
Free_air_anom_terr_corr
These are Helmert Anomalies, also known as Faye Anomalies, i.e. they are free-air anomalies which have had a terrain correction applied. Unique to the GEOID93 data set.
geog_quad_code
Unique to DMANET
Geographic Quad Code:

1 = Northeast

2 = Northwest

3 = Southeast

4 = Southwest
geog_unit_code
Unique to DMA data formats:

BLANK OR 0 - DEGREES AND MINUTES TO .01 MINUTE

1 - DEGREES, MINUTES AND SECONDS

2 - DEGREES TO .0001 DEGREES
geoid_height
Geoid heights referenced to the Geodetic Reference System 1980(GRS80) ellipsoid.
geoid_height_sd
The standard deviation associated with the geoid height.
geology_code
A three-unit digital geology model was created from mapped geology of Arizona, Utah, Nevada, and California. The units are:

1. Pre-Cenozoic basement rock

2. Cenozoic volcanic rocks

3. Cenozoic sedimentary cover, primarily Quaternary alluvium
Field is found in the USGS Basin and Range data set.
grav_anom
A general term for a gravity anomaly. In some data sets this may be a combination of more specific anomalies, such as Free_air_anom on ocean and Bouguer_anom on land.
grav_basement
A predicted Bouguer gravity anomaly(in milligals), which is due solely to the basement rock having had the contribution of the shallower Cenozoic basin rocks removed. See U.S.G.S. Open file report 90-404 for the details of it's derivation. Unique to the nvbasgrv data set for the State of Nevada.
grav_depth_to_basement
The predicted thickness(in meters), of Cenozoic deposits over basement for the State of Nevada. Unique to the nevdepth data set.
grav_gradient
The rate of change of the gravity in its steepest direction. Unique to the Absolute gravity data set.
grav_meter_id
Gravity meter name/number used in survey. Found only in the Holitna1 and Holitna2 Alaskan gravity data sets.
grav_uncertainty
In milligals, 0-uncertainty indicates less than 3 satellite orbits were available for average. Unique to the GEOSAT 44 data set.

Hammer_zone
Alpha character used to specify the annular zones employed when performing terrain corrections using terrain correction charts of the type designed by Sigmund Hammer.

Hammer Zone	Radii of zone(Meters)	# of compartments in zone
A	0-2	1
B	2-16.6	4
C	16.6-53	6
D	53-170	6
E	170-390	8
F	390-895	8
G	895-1529	12
H	1529-2615	12
I	2615-4469	12
J	4469-6653	16
K	6653-9903	16
L	9903-14742	16
M	14742-21944	16

Reference: Hammer, Sigmund, L.L. Nettleton, and W.K. Hastings: Gravimeter Prospecting for Chromite in Cuba; Geophysics, Vol 10 pp 34-39, 1945

header
Descriptive information about a data set.
IGSN71_conv_factor
A conversion factor for IGSN71/GRS67 to 0.01 mgals. Unique to Calif/Nevada data.
isostatic_anom
Isostatic residual anomaly to 0.01 mgal using average crustal thickness of 25 km, density of topography 2.67g/cc, and a lower crust-upper mantle density contrast of 0.40 g/cc (Jachens and Roberts, 1981). Unique to Calif/Nevada data.
isostatic_elev
Elevation data used for isostatic computations.
isostatic_grav
Isostatic residual gravity. Unique for the U.S. 2.5 minute grid.
isostatic_regional
Sea level isostatic regional gravity. Unique for the U.S. 8-km grid.
isostatic_residual
Isostatic residual gravity. Unique for the U.S. 8-km grid.
iso_tc_code
Indicates if isostatic anomaly or terrain correction is given in document:

0 - No isostatic anom. or t.c. in document

1 - Terrain correction given in document

2 - Isostatic anomaly given in document

3 - Both are given in document
iso_world_corr
A letter code, the "ISOW"indicates that data processing was completed through the world isostatic correction. Unique to the ak1_iso and ak1_wpn data sets.
latitude
This is the geographic latitude expressed as a decimal number. The units are degrees. The range is -90.0 to +90.0.
latitude_abs
The absolute (unsigned), latitude.
latitude_deg The degree portion of the latitude when the latitude is given in degrees, minutes, and seconds. The range is -90 to +90.
latitude_deg_abs
The absolute (unsigned), latitude.
latitude_min
The minute portion of the latitude when the latitude is given in degrees, minutes, and seconds. The range is 0 to less than 60.
latitude_sec
The seconds portion of the latitude when the latitude is given in degrees, minutes, and seconds. The range is 0 to less than 60.
latitude_sign
The sign of the latitude + or - . Plus signifies North and minus signifies South.
line_id
The alphanumeric survey line identification associated with the gravity measurement.
location_code
A 2 character alphanumeric country code(see table below), unique to DMANET.

01 ALABAMA

O2 ALASKA

03 ARIZONA

04 ARKANSAS

05 CALIFORNIA

06 COLORADO

07 CONNECTICUT

08 DELAWARE

09 DISTRICT OF COLUMBIA

10 FLORIDA

11 GEORGIA

12 HAWAII

13 IDAHO

14 ILLINOIS

15 INDIANA

16 IOWA

17 KANSAS

18 KENTUCKY

19 LOUISIANA

20 MAINE

21 MARYLAND

22 MASSACHUSETTS

23 MICHIGAN

24 MINNESOTA

25 MISSISSIPPI

26 MISSOURI

27 MONTANA

28 NEBRASKA

29 NEVADA

30 NEW HAMPSHIRE

31 NEW JERSEY

32 NEW MEXICO

33 NEW YORK

34 NORTH CAROLINA

35 NORTH DAKOTA

36 OHIO

37 OKLAHOMA

38 OREGON

39 PENNSYLVANIA

40 RHODE ISLAND

41 SOUTH CAROLINA

42 SOUTH DAKOTA

43 TENNESSEE

44 TEXAS

45 UTAH

46 VERMONT

47 VIRGINIA

48 WASHINGTON

49 WEST VIRGINIA

50 WISCONSIN

51 WYOMING

AD ADEN

AD AFGHANISTAN

AG ALGERIA

AL ALBANIA

AM ANDAMAN IS. AND NICOBAR IS.

AN ANDORRA

AO ANGOLA INC. CABINDA

AP AMSTERDAM IS. AND ST. PAUL IS.

AR ARGENTINA

AT AUSTRALIA

AU AUSTRIA

AY ANTARCTICA

AZ AZORES IS.

BA BAHRAIN

BB BARBADOS

BC BISMARCK

BD BERMUDA

BE BELGIUM

BF BAHAMAS

BG BRITISH GUIANA

BH BORHOLM IS.

BI BALEARIC IS.

BJ BONIN IS.

BK BECHUANALAND, BOTSWANA

BL BOLIVIA

BM BURMA

BN BRITISH HONDURAS

BO SABAH

BR BRAZIL

BS BASUTOLAND, LESCTHO

BT BHUTAN

BU BULGARIA

BV BOUVET IS.

BW WEST BERLIN

BX BRUNEI

BY BURUNDI

CA CANARY IS.

CB CAMBODIA

CD CENTRAL AFRICAN REPUBLIC

CE SRI LANKA (FORMERLY CEYLON)

CF CLIPPERION IS.

CG CHAD

CH CHINA NOT INC. TIBET

CI CHILE

CK COCOS IS. INC. CHRISTMAS IS.

CL CAROLINE IS.

CM CAMEROON

CN CANADA

CO COLUMBIA INC. PROVIDENCIA IS.

CP CONGO REPUBLIC

CR CRETE

CS COSTA RICA

CT CORSICA

CU CUBA INC. ISLE OF PINES

CV CAPE VERDE IS.

CX ZAIRE (FORMERLY REP. OF THE CONGO)

CY CYPRUS

CZ CZECHOSLOVAKIA

DA BENIN (FORMERLY DAHOMEY)

DE DENMARK

DO DODECANESE IS.

DR DOMINICAN REPUBLIC

EA EASTER IS. INC. SALA-Y-GOMEZ IS.

EC EQUADOR INC. GALAPAGOS IS.

EG UNITED ARAB REPUBLICS

El IRELAND

EP BANGLADESH (FORMERLY E. PAKISTAN)

ES EL SALVADOR

ET ETHIOPIA INC. ERITREA

FA FALKLAND IS.

FG FRENCH GUlANA

Fl FINLAND INC. ALAND IS.

FO FAROE IS.

FR FRANCE

FT FIJI AND TONGA

FW FRENCH WEST INDIES

GA GAMBIA

GC GABON

GE EAST GERMANY INC. EAST BERLIN

GH GHANA

GI GIBRALTAR (SEE SPAIN)

GL GREENLAND

GN GILBERT AND ELLICE IS.

GO GOTLAND IS.

GR GREECE

GT GUATEMALA

GU GUAM

GV GUINEA

GY GERMANY INC. SAAR NOT INC. WEST

HA HAITI

HK HONG KONG

HO HONDURAS

HU HUNGARY

ID INDONISIA

IF IFNI

IL ICELAND

IN INDIA INC. SIKKIM

IQ IRAQ

IR IRAN

IS ISRAEL

IT ITALY NOT INC.

IV IVORY COAST

JA JAPAN

JI JOHNSTON IS.

JK JAMU AND KASHMIR

JM JAMICA INC.

JN JAN MAYEN IS.

JO JORDAN

KE KENYA

KG KERGUELEN IS.

KM KURIA MURIA IS.

KN NORTH KOREA

KS SOUTH KOREA

KU KUWAIT

LA LACS

LC LACCADIVE IS.

LE LEBANON

LI LIBERIA

LN LINE ISLAND GROUP INC.

LS LIECHTENSTEIN

LW LEEWARD IS.

LX LUXEMBOURG

LY LIBYA

MA MALAGASY REPUBLIC INC.

MC MACAO

MD MADERIA IS.

ME MELILLA INC.

MF STATES OF MALAYA NOT INC.

MG MONGOLIA

MI MARSHALL IS.

MK WAKE IS.

ML MALTA INC.

MN MONACO

MO MOROCCO

MR MASCARENE IS.

MS MARIANAS IS.

MT MAURITANIA

MU MARCUS IS

MV MALDIVE IS.

MW MIDWAY IS.

MX MEXICO

MI MOZANBIGUE

NC NEW CALEDONIA

NE NETHERLANDS

NH NEW HEBRIOES

NI NIGERIA

NK NIGER

NN NETHERLANDS ANTILLES

NO NORWAY NOT INC.

NP NEPAL

NS SURINAM

NU NICARAGUA

NW NEW GUINEA

NY MALAWI

NZ NEW ZEALAND

OM MUSCAT AND OMAN

PA PARAGUAY

PB PRINCE EDWARD IS.

PE PERU

PH PHOENIX IS. INC.

PI PHILIPPINES

PK PAKISTAN (FORMERLY W. PAKISTAN)

PN PANAMA

PO POLAND

PR PUERTO RICO

PT PORTUGAL NOT INC.

PU PORTUGUESE GUINEA

PI PANAMA CANAL ZONE

QA GATAR

RA RHANDA

RI EQUATORIAL GUINEA INC.

RK RYUKYU

RM MALI

RK SOCCTRA IS.

RU RUMANIA

RY ZAMBIA

RI ZIMBABWE (FORMERLY RHODESIA)

SB SAN MARINO

SC SOLOMON IS.

SD SARDINIA

SE SEYCHELLES IS.

SF FRENCH SOMALILAND

SQ SARABAK

SH ST. HELENA INC.

SI SICILY INC.

SJ SOCIETY IS.

SK SENEGAL

SL SIERRA LEONE

SM SOMALI REPUBLIC

SN SINGAPORE

SO SAMOA

SP SPAIN NOT INC.

SQ ST. PIERRE AND MIQUELON

SR SAUDI ARABIA

SB SPANISH SAHARA INC.

ST SAO TOME E PRINCIPE

SU SUDAN

SV SVALBARD INC.

SW SWEDEN NOT INC.

KY SYRIA

SZ SWITZERLAND

TA TANGANYIKA, TANZANIA

TC TRUCIAL OMAN

TD TRINIDAD

TH THAILAND

TI TIBET

TK TURKEY

TM PORTUGUESE TIMOR

TO TOGO

TU TUNISIA

TW TAIWAN

UA SOUTH AFRICA INC. NAMIBIA

UG UGANDA

UK UNITED KINGDOM OF GREAT BRITAIN

UR UNION OF SOVIET SOCIALIST REPUBLICS

US UNITED STATES OF AMERICA

UV BOURKINA FASSO (FORMERLY UPPER VOLTA)

UY URUGUAY

VE VENEZUELA

VI VIRGIN IS. (U.K.)

VN NORTH VIETNAM

VS SOUTH VIETNAM

VT VATICAN CITY

VU VIRGIN IS. (U. K.)

WA SWAZILAND

WF WALLIS and FUTUNA IS.

WI WINDWARD IS.

WS WESTERN SAMOA

YE YEMEN

YU YUGOSLAVIA

ZA ZANZIBAR
location_type_code
A 1 letter code unique to DMANET:

Station Location Code:

0 = Refer to Base Station Descriptor (DMA Form 8342)

1 = Airport (Public Access)

2 = Airport (Restricted)

3 = Harbor (Dockside)

4 = Public (Sheltered)

5 = Public (Unsheltered)

6 = Restricted (Sheltered)

7 = Restricted (Unsheltered)

9 = Destroyed/Obsolete
longitude
This is the geographic longitude expressed as a decimal number. The units are degrees. The range is -180.0 to +180.0, where plus designates East longitude and minus designates West longitude.
longitude_abs
The absolute (unsigned) longitude.
longitude_deg
The degree portion of the longitude when the longitude is given in degrees, minutes, and seconds.
longitude_deg_abs
The absolute (unsigned) value of the degree portion of longitude.
longitude_east
A measurement of longitude starting at the Greenwich Meridian(zero degrees), and proceeding in a counter-clockwise direction(as viewed from above the North Pole), from 0 to 360 degrees.
longitude_min
The minute portion of the longitude when the longitude is given in degrees, minutes, and seconds. Values range from 0 to less than 60.
longitude_sec
The seconds portion of the longitude when the longitude is given in degrees, minutes, and seconds. Values range from 0 to less than 60.
longitude_sign
The sign of the longitude + or - . Plus designates East and minus designates West.
mag_diurnal_corr
MAGNETICS DIURNAL CORRECTION
as defined in Marine Geophysical Data Exchange Format - MGD77 by Hittelman,A., et al., (in nanotesias) if 9-filled (i.e., set to "+9999"), total and residual fields are assumed to be uncorrected; if used, total and residuals are assumed to have been already corrected.
mag_res_field
MAGNETICS RESIDUAL FIELD

as defined in Marine Geophysical Data Exchange Format - MGD77 by Hittelman,A., et al., residual field in nanoteslas. The reference field used is coded in Header record.
mag_sensor_depth
DEPTH OR ALTITUDE OF LEAD MAGNETIC SENSOR
as defined in Marine Geophysical Data Exchange Format - MGD77 by Hittelman,A., et al., + = Below sea level, - = Above sea level
mag_total_field_1
MAGNETICS TOTAL FIELD, 1ST SENSOR
as defined in Marine Geophysical Data Exchange Format - MGD77 by Hittelman,A., et al., total field in nanoteslas (gammas) for leading (closest to ship) sensor. This field is used if only one sensor is employed.
mag_total_field_2
MAGNETICS TOTAL FIELD, 2ND SENSOR
as defined in Marine Geophysical Data Exchange Format - MGD77 by Hittelman,A., et al., total field in nanoteslas for trailing sensor. Sensor separation defined in Header record.
mass_corr_offshore_164
Mass correction (offshore) (mgal), using density = 1.64g/cc.
NOTE: if you use the combined mapping (Bouguer anomaly onshore, free-air anomaly offshore) you have to subtract the offshore mass correction from the land data. Unique to ANDES data.
modeled_depth
Modeled Cenozoic basin depths(km).
month
Month of the year. Values range from 01 to 12.
monument_name
The name of the monumented gravity station. Unique to NGSNET data.
nav_quality_code
QUALITY CODE FOR NAVIGATION

as defined in Marine Geophysical Data Exchange Format - MGD77 by Hittelman,A., et al.,

5 - Suspected, by the originating institution

6 - Suspected, by the data center

9 - No identifiable problem found

NOTE: Institution will most frequently 9-fill this field; however, should they wish to code a "5", the data center will not contradict. The data center's quality control program, which performs (among other checks) a vectorial analysis of the navigation, is available in a printout form upon request.
network_code
Network or Special Group Code - Source Document. Unique to DMANET data. Values range from 001 to 186.
NOTE: this is an internal code for use by DMA only.
new_datum_code
This field is unique to ANWR data where the letter "N"is used to indicate a new datum.
num_obs_geoid_height
The number of points found in a one-eight degree bin, used for the computation of the geoid undulation. Unique to the Rapp92_deg and Rapp92_half data sets.
num_obs_grav_anom
The number of points found in a one-eight degree bin, used for the computation of the mean Free-air gravity anomaly. Unique to the Rapp_deg , Rapp_haf and fa8th_dg data sets.

obs_grav
The observed, or measured value of gravity in mGals at an occupied gravity station. In general, observed gravity varies from 978,000 to 984,200 mgals; therefore, many data contributors subtract a large constant from 900,000 to 982,000 from this field. Within this compact disc, the following data sets have had these offsets removed from them:

Antarc93	976,000
Anwr	900,000
ak1_iso	900,000
ak1_wpn	900,000
Bolivia	980,000
Calif	900,000
DMA93	976,000
Nevada	900,000
NGS93	978,000
NPRA	982,000
Ohio1	978,000
Wisc2a	980,000
90ngsnet	978,000

Data sets from the EROS data center:
Idaho	980,000
ID_Bath	980,000
ME_ofshr	980,000
ME_onshr	980,000
Ohio	980,000
Utah	980,000

obs_grav_sd
The standard deviation associated with the observed gravity value.
parent_ref_base
Parent Base of Base Station (DoD Code Number). Unique to DMANET data.
PGA_use_code
Point Gravity Anomaly File - Use Code
0 or Blank = Not found in PGA File
1 = Is found in PGA File
Unique to DMANET data.
position_type_code
POSITION TYPE CODE

as defined in Marine Geophysical Data Exchange Format - MGD77 by Hittelman,A., et al., indicates how the navigation was obtained:

1 = Observed fix

3 = Interpolated

9 = Unspecified
prin_investigators
Alphanumeric field describing the principle investigators involved in the collection of the data.
proprietary_code
A numeric code denoting whether a gravity data record is:

1)not proprietary data (unmonumented) code=1, or

2)not proprietary data (monumented) code=3

Unique to the NGS93 data set.
quad_code
Code for the 15-minute Quadrangle used. Unique to Wisc2b data. This code is the same as that used by the State of Wisc. Geological Survey on their maps.
ref_base
A gravity reference station that is used during a gravity survey to establish additional stations. Typically, a relocatable gravity station where the observed gravity is well established, perhaps through repeated occupations, which could be used to establish survey datum's and/or other reference bases. Unique to DMA format data.
ref_base_code
An alphanumeric code for the gravity base station site. Values are: blank; 0-9; A,B,M,N

NOTE: for internal use by DMA only.

Unique to DMA format data.
res_field_sensor
Sensor for residual magnetic field as defined in Marine Geophysical Data Exchange Format - MGD77 by Hittelman,A., et al. The magnetic field sensor used in the calculation of the residual magnetic field.

1 = 1st or leading sensor

2 = 2nd or trailing sensor

9 = Unspecified
sea_level_elev_ft
Height of the surface of observation above sea level, in feet
NOTE: most often from terrain surface but may be from lake surface.
sea_level_elev_km
Topography from North America Grid.
sea_level_elev_m
Height of the surface of observation above sea level, in meters (NOTE: most often from terrain surface but may be from lake surface).
sea_level_elev_m_abs
The absolute (unsigned) value in meters of the elevation (normally) with respect to mean sea level. The type of elevation is defined in "station_char_code"and the units of elevation are given by the "elev_unit_code ".
sea_level_elev_sign
The sign of the sea level + or - . On land, plus is above sea level and minus is below sea level; if the measurements are made in the ocean environment (i.e."station_char_code"is equal to 3, 4, or 5), depth is positive downward.
sea_surface_height
same as geoid_height
sea_surface_height_sd
The standard deviation associated with the sea surface height.
secondary_dist_ft
Generally, depth, in feet, of gravity instrument, land, lake or ice: Positive downward from surface. In some situations this may represent ice thickness or alternative sea-level distance measurement. See "station_char_code"for appropriate information. From DMA.
secondary_dist_m
The secondary_dist_ft variable converted to meters. From DMA.

security_class
Security codes used by DMA format.

U/2	UNCLASSIFIED	C/4	CONFlDENTlAL
F/3	FOR OFFICIAL USE ONLY	S/5	SECRET

security_control_code
Security control codes used by DMA format.

2-SPECIAL RELEASE - REQUIRED FROM DMAHQ

4-PROPRIETARY - (FULL TIME DOD EMPLOYEES ONLY)

Special Release From Originating Agency

Required For Dissemination to 3rd Party.
seismic_line_num
SEISMIC LINE NUMBER

as defined in Marine Geophysical Data Exchange Format - MGD77 by Hittelman,A., et al., used for cross referencing with analog seismic data.
seismic_shot_num
SEISMIC SHOT - POINT NUMBER

as defined in Marine Geophysical Data Exchange Format - MGD77 by Hittelman,A., et al., used for cross referencing with analog seismic data.
sequence
Gravity station number, sequence number of station in document, or page on which station is listed. Field is found in DMA, ANTARC, and VERNAL data sets.

source_accuracy_code
Unique to ANWR data

The source accuracy code consists of:

- a letter for location position type (see table 2)

- a letter for elevation type (see table 4)

- a letter for gravity type (see table 3)

- a number representing the estimated anomaly accuracy (see table 5)

The location-type letter indicates the kind of map used for the field work and for the measurements of latitude and longitudes (see table 2). The most common location type is a letter "A"indicating measurements from a modern inch-to-the-mile map, but users should he careful about letters "B"indicating doubtful locations on such maps. If such maps were used for latitude and longitude measurements of stations located in the field on smaller scale maps, the same letter "D"will be used if the transfer justifies the improved precision, but the letter "K"for 4-in-per-mile maps would be used if these were the original field map and if this scale map seemed to control the precision of the location. Letters early in the alphabet usually indicate the use of better maps than later letters in the alphabet.

Source or Type of Measurement
Codes for USGS Alaskan Gravity Data
Table 2
Position Codes

Map used for field work or for reading latitudes and longitudes or which controlled location precision	good location	poor location	transfer from photo	Transfer from smaller scale field map
Modern published maps, scale >= 1:63,360	A	B	C	D
Old or unpublished "">= 1:63,360	F	G	H	I
Modern 1:250,000 maps	K	L	M	N
Reconnaissance 1:250,000 maps	P	Q	R	S
Coast Survey or special maps	U	V	W	X

Data from other agency - Y

Position from special survey - Z

No location - ?

Estimated or assumed location - #

Near a bench mark - @

The gravity-type letter indicates the type of meter, the number of ties, and the duration of time between base station readings (see table 3 of ANWR doc)

Table 3
Gravity Codes

Type of meter	3 ties within 0.1 mgal	other multiple ties	ties or drift loops lasting,
			<6 hours	6-24 hours	1-4 days	>4 days
LaCoste Geodetic meters	A	B	C	E	D	F
Worden or (loop drift cntrl)	G	H	I	J	K	L
World-wide (other drift cntrl)	M	N	O	P	Q	R
meters (no drift control)	S	T	U
Old LaCoste or other thermostated meters	V	W	X
Data from other agencies	Y
Reasons to expect errors	Z

the gravity-accuracy type number indicates accuracy of the anomaly (same as calif; see table 5 and page 16-17 of ANWR doc)

The elevation-type letter indicates the source of the elevation (see table 4 of ANWR doc)

Table 4
Elevation Control Code

	Bench Marks	Highway and Railway Surveys	Sea Level	Special Surveys	USWB FAA Wisc
Surveys etc.	A	B	C	D	E
	black	brown	blue	unpublished maps
Topo Map Elevations	F	G	H	I

Contour intervals
	<-50 ft.	100 ft.	200 ft.
River gradien interpolation	J	K	L
Good contour interpolation	M	N	O
Poor contour interpolation	P		Q

Altimetry	Base Distance
	<15 miles	15-70 miles	70 miles
Good-repeated readings	R	S	-
Alticorder or other good base control	T	U	-
Poor control	V	W	X

Altimetry involving special adjustments - Y

No data - ?

Elevation from nearby bench marks - @

Table 5
Anomaly Accuracy Code
(similar to California USGS observed gravity code)

Code	Gravity Anomaly Accuracy milligals	Typical Gravity or Elevation Type
l	0.01	Local surveys with special meters and leveling
2	0.02	Multiple readings with LaCoste meters on hard, surveyed surfaces
3	0.05	Average LaCoste data at stable bench marks
4	0.10	Average LaCoste or Worden data at sea level or frost-affected bench marks
5	0.2	Worden or LaCoste data with poor drift or closure errors, or average data at vertical angle bench marks
6	0.5	Data from loops with closure errors this large, or good data using river gradients, good photogrammetric elevations or well controlled altimetry
7	1.0	Most surveys based on reasonable altimetry
8	2.0	Data using moderate-distance altimetry in variable weather or spot elevations on l00-foot contour interval maps
9	5.0	Data using long-range altimetry in bad weather or contour interpolation on 200-foot contour interval maps
10	5.0	Data from surveys using long-distance altimetry or altimetry with control failures or errors or some 500-foot-contour-interval reconnaissance maps

source_id
A numeric code for the contributing author or organizations of the data.
state_code
Code for state (G. Woollard Convention).
48 = Wisconsin
Unique to WISC2B data.

station_char_code
An alphanumeric code which indicates the type of elevation used in reducing the measured gravity to anomaly values. Generally, elevation of land gravity station, if ocean measurement, then ocean depth positive downward from surface. In some situations this may represent instrument or bottom depth, ice thickness or alternative sea-level distance measurement. From DMA.

CODE	DEFINITION
1	Land (surface observation)
2	Land (subsurface observation)
3	Ocean (surface observation)
4	Ocean (submerged observation)
5	Ocean (bottom observation)
6	Lake (surface observation, above sea level)
7	Lake (bottom observation, above sea level)
8	Lake (bottom observation, below sea level)
9	Lake (surface observation, above sea level with lake bottom below sea level)
A	Lake (surface observation, below sea level)
B	Lake (bottom observation with surface below sea level)
C	Ice cap (surface observation with bottom below sea level)
D	Ice cap (surface observation with bottom above sea level)
E	Transfer data given
X	Unspecified measurement on water (Canadian data)

station_id
Generally, an alphanumeric code uniquely identifying a gravity station within it's data set.
station_name
Unique to ANWR data.
The alternate station name is unique to the ANWR data set and is described in Notes on the Processing and Presentation of USGS Alaskan Gravity Data by D. F. Barnes (#529). The following is an extract from this report.

The tabulation provides for two station numbers: a main number and an auxiliary number The auxiliary number in some cases is an old number used for the station, and in other cases merely provides additional information about the station. Typical additional information includes identification of the station as a base, an indication of the type of field mark, or available reoccupation information. Thus typical phrases in the auxiliary number column be "BASE"(for base stations), "MARK"(for usgs gravity marker), TEM1 (for tidal bench mark number 1), TB11 (for tidal bench mark number 11), FOTO (to indicate a photograph was taken), DESC (to indicate a description suitable for reoccupation is available), or a "/"followed by letters or numbers indicating the name of a vertical angle bench mark or triangulation station. A more complete lists of station naming and description conventions is given in table 1.

Table 1
Preliminary List
Naming Conventions
Alaska Gravity Stations

Symbol etc.	Example
B---	BF31	At bench mark (such as F31)
@---	@F31	At or near a bench mark location which may or may not have been found (such as F31)
/---	/LOO /GEB	Vertical angle or triangulation station followed by first three letters of name or initials in name (such as stations "Loon"or "Glen East Base")
TB--	TB11
TBM-	TBM2	Tidal bench marks (such as numbers 11 or 2)
BASE		U.S.G.S. gravity base station (probably marked with hexagonal tablet)
MARK		Marked with USGS hexagonal gravity marker
LITE		On base of lighthouse or navigation light
FOTO		Station photographed
BLM-		At a Bureau of Land Management marker
DESC		Station described
T---	TB31	On railway track opposite (such as BM B3l)
MP--	MPl2	Milepost (such as milepost 12)
M---	M292	Milepost (such as at milepost 292)
TM--	TM69	On track or highway opposite a milepost (such as opposite milepost 69)
A---		Stations in Arctic Alaska or near Anchorage
F---	FM65	Stations obtained on float or ski-plane flights (such as Minto Flat ski-plane station 65)
H---	HTl5	Stations obtained on helicopter traverses (such as Takahula helicopter station 15)
---P	KETP	Station at Post Office (such as Ketchikan Post Office)
---A	WRNA	Station at airport (such as Wrangell airport)
---H	PETH	At harbor entrance (such as Petersburg harbor)
---D	HAND	At dock entrance (such as Haines small boat dock)
---F	MCGF	At Federal aviation building (such as McGrath FAA headquarters)
---R	ANCR	At railroad station (such as Anchorage)
---?	TB5?	At a questionably identified or hard-to-read mark (such as Tidal bench mark 5)
SA-- ,SB-- ,SC-- ,SO--,		Stations first read with Worden meter 226 in southeast Alaska in 1968
SZ-- ,SY-- ,SX-- ,SV-- ,ST--,SP-- ,SM--,		Stations first read with LaCoste meter G-08 in southeast Alaska in 1968
SW-- ,SU-- ,SQ-- ,SP-- ,SM--,SL-- ,SK--,		Stations first read with LaCoste meter G-17 in southeast Alaska in 1968
QA-- , QB--,		Stations first read with Worden meter 226 in southeast Alaska in 1969
QZ-- ,QY-- ,QX-- ,QW-- ,QV--,QU-- ,QT-- ,QR-- ,QP-- ,QM--,QS-- ,QQ-- ,QN-- ,QL--		Stations first read with LaCoste meter G-58 in southeast Alaska in 1969

terrace_reading
As a result of a terracing operation on isostatic residual gravity data, the smoothly varying gravity field is transformed into a stepped field of uniform domains separated by discontinuous boundaries. This results in an uncalibrated density value. Unique to the Idaho Batholith Study Area data set.
terr_corr
The terrain correction is an adjustment to the simple Bouguer correction that accounts for variations in terrain above and below the elevation of the gravity station at distances away from the point of measurement. Terrain below and above the level of the theoretical horizontal slab used to make the simple Bouguer correction results in a mass deficiency or excess that always yields an erroneously low gravity measurement at the point of observation. As a consequence, the terrain correction is always added to the simple Bouguer correction.
terr_corr_code
Denotes extent of inner-zone terrain correction and system, upper case denotes Hayford-Bowie system of zones, lower case denotes Hammer system of zones. Z, computer calculated from 0.0 - 0.59 km.
terr_corr_inner
Corrections for variations in terrain above and below the level of a gravity station may be carried out to an arbitrary radius from the station. In practice, terrain corrections are usually calculated in the field for distances relatively near to the gravity station, and in the office using digital computers and digital elevation models for more distant terrain. Historically, terrain corrections in the field were accomplished using charts developed by Sigmund Hammer and others to calculate the gravimetric contributions of variations in terrain in regions (compartments) of increasing radii called zones. These charts were typically divided into near, inner, and outer zones. For Hammer charts, inner zone corrections are carried out by estimating the average elevation or slope of the terrain out to distances between 2 meters and 170 - 390 meters (Zones B - D or E).
terr_corr_onshore_267
A terrain correction computed using a density of 2.67g/cc. Unique to ANDES data set.
terr_corr_outer
Corrections for variations in terrain above and below the level of a gravity station may be carried out to an arbitrary radius from the station. In practice, terrain corrections are usually calculated in the field for distances relatively near to the gravity station, and in the office using digital computers and digital elevation models for more distant terrain. Historically, terrain corrections in the field are were accomplished using charts developed by Sigmund Hammer and others to calculate the gravimetric contributions of variations in terrain in regions (compartments) of increasing radii called zones. These charts were typically divided into near, inner, and outer zones. For Hammer charts, Outer zone corrections to distances between 390 meters and approximately 22 kilometers (Zones F - M) are carried out manually using charts and topographic maps, or automatically using digital computers and digital elevation models.
terr_corr_sd
The standard deviation associated with the terrain correction.
terr_corr_total
The terrain correction is an adjustment to the simple Bouguer correction that accounts for variations in terrain above and below the elevation of the gravity station at distances away from the point of measurement. Terrain below and above the level of the theoretical horizontal slab used to make the simple Bouguer correction results in a mass deficiency or excess that always yields an erroneously low gravity measurement. As a consequence, the terrain correction is always added to the simple Bouguer correction. The total terrain correction is the sum of the inner and outer zone terrain corrections described above.
theor_grav
The value of gravity predicted by an International Gravity Formula for a given latitude, often referred to as the theoretical gravity. All theoretical gravity values on this CD_ROM are believed to be based on the Geodetic Reference System of 1967 (GRS67) and are obtained by the following formula:

g = 978031.85{1 + 0.005278895 sin(latitude)^^2 + 0.000023462 sin((latitude)^^4}mgals.
time_offset
In seconds, time 0 is an arbitrary reference point. Unique to the GEOSAT 44 data set.
time_zone_corr
TIME ZONE CORRECTION

as defined in Marine Geophysical Data Exchange Format - MGD77 by Hittelman,A., et al., Corrects survey time (in characters 15-27) to GMT when added: equals zero when time is GMT. Time zone normally falls between -13 and +12 inclusively.
utm_east
The linear distance from a central meridian computed using the ellipsoidal form of the Transverse Mercator projection and the parameters adopted by the U.S. Army in 1947 for designating rectangular coordinates on large scale military maps between 84 North and 80 South. Known as the Universal Trans-verse Mercator projection it is usually referred to as the UTM projection. The Army adopted the parameters of the Clarke ellipsoid of 1866 for use within the United States. Central meridians lie at 6 degree intervals and are numbered one ( 1 ) starting at 177 West longitude through sixty ( 60 ) lying at 177 East longitude. The utm_east value is taken to be 500,000 meters on the central meridian and increases to the East.
The Universal Transverse Mercator (UTM) projection is an ellipsoidal Transverse Mercator projection for which parameters such as the central meridian and scale factor are defined to ensure that maps within a specified zone may be mosaicked. The following parameters are for the Clarke 1866 reference ellipsoid:

Equatorial Radius : 6,378,206.4

Polar Radius : 6,356,583.8

Flattening : 1/294.98

Scale Factor : 0.9996

Base Latitude : 0.0
utm_north
The linear distance from a reference latitude computed using the ellipsoidal form of the Transverse Mercator projection and the parameters adopted by the U.S. Army in 1947 for designating rectangular coordinates on large scale military maps between 84 North and 80 South. Known as the Universal Trans- verse Mercator projection it is usually referred to as the UTM projection. The Army adopted the parameters of the Clarke ellipsoid of 1866 for use within the United States. Central meridians lie at 6 degree intervals and are numbered one ( 1 ) starting at 177 West longitude through sixty ( 60 ) lying at 177 East longitude. The utm_north value is measured positively northward from the equator in the Northern hemisphere. To decrease the magnitude of this coordinate when distant from the base latitude, a false northing is often removed from this coordinate.
The Universal Transverse Mercator (UTM) projection is an ellipsoidal Transverse Mercator projection for which parameters such as the central meridian and scale factor are defined to ensure that maps within a specified zone may be mosaicked. The following parameters are for the Clarke 1866 reference ellipsoid:

Equatorial Radius : 6,378,206.4

Polar Radius : 6,356,583.8

Flattening : 1/294.98

Scale Factor : 0.9996

Base Latitude : 0.0 ( the equator )
year
Generally, the year the gravity measurement was recorded.

Home | disclaimers | questions

Website of the US Dept of Commerce/NOAA/NESDIS/NCEI

Search
NOAA > NESDIS > NCEI (formerly NGDC) > Marine Geology and Geophysics > Geophysics	privacy policy