Readme.txt Explanation of data format Fortran Column Format Contents ------ ------- -------- 1-4 a4 Station name 5-8 a4 Auxiliary station name, descriptor, or addition to station name 9-11 f3.0 Latitude of station in degrees 12-15 f4.2 Minutes of latitude to 0.01 minute 16-19 f4.0 Longitude in degrees 20-23 f4.2 Minutes of longitude to 0.01 minute 24-29 f6.1 Elevation in feet to 0.1 feet 30-36 f7.2 Observed gravity to 0.01 milligal minus 900,000.00 37 a1 Location source code 38 a1 Gravity source code 39 a1 Elevation source code 40 a1 Simple Bouguer anomaly accuracy code 41-46 f6.2 Free air anomaly to 0.01 milligal 47-52 f6.2 Simple bouguer anomaly to 0.01 milligal 53-57 f5.2 Inner-zone terrain correction from station to 0.39 km to 0.01 milligal 58-62 f5.2 Terrain correction from 0.39 km to 166.7 km to 0.01 milligal 63 1x Blank 64-69 f6.2 Complete bouguer anomaly to 0.01 milligal 70-75 f6.2 Isostatic anomaly to 0.01 milligal 76-79 a4 Data set code 80 a1 Datum code 81-85 f5.0 Second elevation in whole feet 86 1x Blank 87 a1 Second elevation source code 88-92 f5.1 Simple Bouguer anomaly based on second elevation to 0.1 milligal 93-96 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 Verticle Datum of 1929 (NGVD29). Theoretical gravity at sea level is based on the Geodetic Referenc 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 37 [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 38 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 39 [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 40 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 76-79 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 80 N or blank.........1967 ellipsoid and conversion of Barnes Alaska datum to IGSN-71 datum Isostatic anomaly code, columns 93-96 ISO Indicates isostatic correction made from station to 166.7 km. ISOW Indicates isostatic correction made for the whole earth