Explanation of space delimited data with decimal points 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 Note**** These data have all been reprocessed using the same reduction equations. Past versions of this data set used the anomalies supplied by the source of the data. Slight differences in anomalies might be noticed from previous versions, including USGS data which had beeen proccesed by several different programs that had slightly different equations. 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 encripted GPS or with selected availability turned off.....................% Location from GPS with selected availability turned on...................................& 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 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 encripted GPS or with selected availability turned off.....................% Elevation from GPS with selected availability turned on...................................& 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