Gravity data files
Point gravity data is available with the following format:
Table 1.--Format of gravity data
[ft, feet; g/cc, grams/cubic-centimeter; mGal, milligal]
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Column Format Description
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1-8 A8 Station name
10-11 F2.0 Latitude, in degrees
12-15 F4.2 Latitude, in minutes to 0.01 minute
17-19 F3.0 Longitude, in degrees
20-23 F4.2 Longitude, in minutes to 0.01 minute
24-29 F6.1 Elevation, in ft to 0.1 ft
30-36 F7.2 Observed gravity, in mGal to 0.01 mGal, without leading 9
37-40 A4 Four character accuracy code
41-46 F6.2 Free-air anomaly, in mGal to 0.01 mGal
47-52 F6.2 Simple Bouguer anomaly, in mGal to 0.01 mGal
53-57 F5.2 Inner-zone terrain correction, in mGal to 0.01 mGal
58-62 F5.2 Total terrain correction, in mGal to 0.01 mGal
63 A1 Terrain correction code denoting 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
64-69 F6.2 Complete Bouguer anomaly reduced for a density
of 2.67 g/cc, in mGal to 0.01 mGal
70-75 F6.2 Isostatic gravity anomaly reduced for a density
of 2.67 g/cc, in mGal to 0.01 mGal
76-80 A4 Comment ______________________________________________________________________
Table 2.--Explanation of 4-digit accuracy code
[alt, altimetry; blk, black; brn, brown; elev., elevation;
NGS, National Geodetic Survey; NMD, National Mapping Division;
PG, photogrammetry; USGS, U. S. Geological Survey;
VABM, vertical angle bench mark]
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General elevation and location code--First digit
Code Explanation
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A Altimetry, good control
B On USGS or NGS level-line bench mark
C Contour line interpolation
D Destroyed or not found reference mark
E Near non-USGS level-line bench mark
F Map elev., blk or field checked
G Map elev., brn or not field checked
H Near vertical angle bench mark
I Other special source
K Photogrammetry by other than USGS NMD
M On non-USGS level-line bench mark
N Near USGS or NGS level-line bench mark
P On or near surveyed mark
Q River gradient interpolation
R Lake elev. by leveling
S Sea level elev.
T Photogrammetry by USGS NMD
U Unknown elev. source
V On vertical angle bench mark
W Map elev., blue
X On or near boundary marker
Y Altimetry, poor control
Z Special source
Approximate
Elevation gravity
Elevation code--Second digit accuracy effect
---------------------------------------------------- (ft) (mGal)
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1 On bench mark ------------------------------------ 0.2 0.01
2 Near bench mark ---------------------------------- 0.3 0.02
3 Transit or good alidade survey ------------------- 1.0 0.06
4 VABM or black map elev. -------------------------- 2.0 0.12
5 Blk elev. on old map or good PG ------------------ 4.0 0.24
6 Brn elev. or good PG on 20-ft contour map -------- 10.0 0.6
7 Brn elev. on 80-ft contour map or good alt ------- 20.0 1.2
8 Contour interpolation on 80-ft contour map ------- 40.0 2.4
9 Contour interp. on 200-ft contour map, poor alt -- 80.0 4.8
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Aprox.
Latitude code--Third digit Latitude Distance gravity
(based at lat 37) ------------------------- accuracy accuracy effect
---------------------------------------------- (min) -- (ft) (mGal)
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1 Triangulation or special survey data -------- 0.007 --- 42 0.01
2 Location known to 0.04 in on 1:24,000 map --- 0.014 --- 84 0.02
3 0.10 in, 1:24,000 or 0.04 in, 1:62,500 map -- 0.035 -- 210 0.05
4 0.21 in, 1:24,000 or 0.08 in, 1:62,500 map -- 0.07 --- 420 0.1
5 0.42 in, 1:24,000 or 0.16 in, 1:62,500 map -- 0.14 --- 840 0.2
6 0.40 in, 1:62,500 or 0.1 in, 1:250,000 map -- 0.35 - 2,100 0.5
7 0.80 in, 1:62,500 or 0.2 in, 1:250,000 map -- 0.7 -- 4,200 1.0
8 1.60 in, 1:62,500 or 0.4 in, 1:250,000 map -- 1.4 -- 8,400 2.0
9 4.00 in, 1:62,500 or 1.0 in, 1:250,000 map -- 3.5 - 21,000 5.0
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Approximate
Observed gravity code--Fourth digit gravity effect
---------------------------------------------- (mGal)
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1 Local survey with special gravity meter --------------- 0.01
2 Multiple observations with LaCoste and Romberg meter -- 0.02
3 Average LaCoste and Romberg or multiple Worden meter -- 0.05
4 LaCoste and Romberg observation with small vibrations
or average Worden meter --------------------------------- 0.1
5 Data from loop with closure error this large ---------- 0.2
6 Data from loop with closure error this large ---------- 0.5
7 Data from loop with closure error this large ---------- 1
8 Data from loop with closure error this large ---------- 2
9 Data from loop with closure error this large ---------- 4
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Click here for the same data in an Excel spreadsheet (224K).
