**Table 3.** Explanation of data

Fortran

Column Format Contents

1-4 a4 Station name

5-8 a4 Auxiliary station name, descriptor, or addition to station name

9 1x Blank

10-11 f2.0 Latitude of station in degrees

12 1x Blank

13-17 f5.2 Minutes of latitude to 0.01 minute

18 1x Blank

19-21 f3.0 Longitude in degrees

22 1x Blank

23-27 f5.2 Minutes of longitude to 0.01 minute

28 1x Blank

29-35 f7.1 Elevation in feet to 0.1 feet

36 1x Blank

37-45 f9.2 Observed gravity to 0.01 milligal

46 1x Blank

47 a1 Location source code

48 a1 Gravity source code

49 a1 Elevation source code

50 a1 Simple Bouguer anomaly accuracy code

51 1x Blank

52-58 f7.2 Free air anomaly to 0.01 milligal

59 1x Blank

60-66 f7.2 Simple Bouguer anomaly to 0.01 milligal

67 1x Blank

68-73 f6.2 Inner-zone terrain correction from station to 0.39 km to 0.01
milligal

74 1x Blank

75-80 f6.2 Terrain correction from 0.39 km to 166.7 km to 0.01 milligal

81-83 3x Blank

84-90 f7.2 Complete Bouguer anomaly to 0.01 milligal

91 1x Blank

92-98 f7.2 Isostatic anomaly to 0.01 milligal

99 1x Blank

100-103 a4 Data set code

104 1x Blank

105 a1 Datum code

106 1x Blank

107-112 f6.0 Second elevation in whole feet

113 1x Blank

114 a1 Second elevation source code

115 1x Blank

116-121 f6.1 Simple Bouguer anomaly based on second elevation to 0.1
milligal

122 1x Blank

123-126 a4 Isostatic anomaly code

**Table 3.** ­Continued.

Latitudes and longitudes are on the North American Datum of 1927
(NAD27).

Elevations are on the National Geodetic Vertical Datum of 1929 (NGVD29).

Theoretical gravity at sea level is based on the Geodetic Reference
System 1967 (GRS67).

Observed gravity is on the IGSN71 datum.

Terrain corrections are calculated from 0.39 km to 166.7 km by computer
program.

> Inaccuracies in station elevations can generate large inner zone
> corrections between the station and 0.39 km.

Values shown in the inner zone column are usually field corrections,
which are field estimates from the station to a radial distance of 223
ft.

The isostatic reduction assumes an Airy-Heiskanen model with the
following parameters from the station to 166.7 km:

> density of topography above sea level, 2.67 g/cc
>
> crustal thickness at sea level, 25 km
>
> density contrast across the base of the model crust, 0.4 g/cc
>
> From 166.7 km to a point on the opposite side of the Earth, isostatic
> and terrain corrections were taken off maps by Karki.

Location source code, column 47

\[NGS, National Geodetic Survey\]

Map used for field work transfer from

or for reading latitudes and longitudes good poor transfer smaller scale

or which controlled location position location location from photo field
map

Modern published maps, scale 1:63,360 A B C D

Old or unpublished maps, scale 1:63,360 F G H I

Modern 1:250,000 scale maps K L M N

Reconnaissance 1:250,000 scale maps P Q R S

NGS or special maps U V W X

Data from other agencies\...\.....Y

Location from special survey\....Z

No location\...\...\...\...\...\...\...?

Estimated or assumed location\...#

Near a bench mark\...\...\...\...\...@

Location from differential GPS..\$

Location from GPS\...\...\...\...\...%

Gravity source code, column 48

3 ties other ties or drift loops lasting

within multiple

Type of meter 0.1 mGal ties \<6 hours 6-24 hours 1-4 days \>4 days

LaCoste & Romberg Geodetic A B C D E F

meters

Worden or (loop drift control) G H I J K L

World-Wide (other drift control) M N O P Q R

meters (no drift control) S T U

Old LaCoste & Romberg or other

thermostated meters V W X

Data from other agencies\...Y

Reasons to expect errors\...Z

**Table 3.** Continued.

Elevation source code, column 49

\[USWB, U.S. Weather Bureau; FAA, Federal Aviation Administration;

Wisc, University of Wisconsin\]

highway & USWB

railway special FAA

bench marks surveys sea Level surveys Wisc

Surveys, etc. A B C D E

black brown blue unpublished maps

Map elevations F G H I

Contour interval

0-50 ft 100 ft 200 ft

River gradient interpolation J K L

Good contour interpolation M N O

Poor contour interpolation P 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\...\...\...\...\...@

Elevation from differential GPS\...\...\...\...\...\...\$

Elevation from GPS\...\...\...\...\...\...\...\...\...\....%

**Table 3.** Continued.

Simple Bouguer anomaly accuracy code, column 50

Gravity

Anomaly

Accuracy,

Code milligals Typical gravity or elevation types

1 0.01 Local surveys with special gravity meters and leveling

2 0.02 Multiple readings with LaCoste & Romberg meters on hard, surveyed

surfaces

3 0.05 Average LaCoste & Romberg data at stable bench marks

4 0.10 Average LaCoste & Romberg or Worden data at sea level or frost-

affected bench marks

5 0.2 Worden or LaCoste & Romberg 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 100-ft contour interval maps

9 5.0 Data using long-range altimetry in bad weather or contour

interpolation on 200-ft contour interval maps

0 \>5.0 Data from surveys using long-distance altimetry or altimetry
with

control failures or errors or some 500-ft contour interval

reconnaissance maps

Data set code, columns 100-103

This is a unique code for each data set of USGS Alaska gravity data.
This code refers

to the original field data which include the project name, traverse,
date collected,

identity of gravity meter used, gravity and altimeter bases, observed
gravity of gravity

base, time of each reading, meter reading, altimeter reading, reference
elevation, base

altimeter, and wet and dry bulb temperatures

Datum code, column 105

N or blank\...\...\...1967 ellipsoid and conversion of Barnes Alaska
datum to IGSN-71

datum

Isostatic anomaly code, columns 123-126

ISO Indicates isostatic correction made from station to 166.7 km.

ISOW Indicates isostatic correction made for the whole earth