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Dictionary of Terms


A B C D E F G H I L M N O P Q R S T U Y

*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 & 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&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&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&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.

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*bath_corr_code
BATHYMETRIC CORRECTION CODE

As defined in “The 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 “The 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 “The 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 “The 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.

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*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 “The 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.

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*data_record_type
Data records as defined in “The 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.

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*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 “The 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

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*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.

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*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.

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*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.

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*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.

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*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 & 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.

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*mag_diurnal_corr
MAGNETICS DIURNAL CORRECTION

as defined in “The 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 “The 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 “The 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 “The 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 “The 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.

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*nav_quality_code
QUALITY CODE FOR NAVIGATION“

as defined in “The 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.

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*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.

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*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 “The 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.

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*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 “The 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

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*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 “The 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 “The 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.

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*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 & 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 &amp; 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 &amp; 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

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*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

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*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 “The 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.

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*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.

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