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All data have been reduced using the Geodetic Reference System 1967 (International Association of Geodesy, 1971), assuming a sea level datum and a Bouguer reduction density of 2.67 grams/cc. Corrections are made for earth curvature, but owing to the generally low topographic relief across most of Minnesota, no terrain corrections have been made. In their early compilation Chandler and Schaap (1991) estimated that reduced anomaly values typically had errors of 0.1—0.3 milligals, although some of the older stations could have errors of 0.3-0.5 milligals, possibly more. The above-described errors will be decreased somewhat in the pre-2011 data by the re-positioning and re-calculations made during the 2009-2011 upgrade (Chandler and Lively, 2011).
The Principal Facts Point Database of Gravity Stations is provided as an ESRI feature class (MINGRAV19_PF) within a File Geodatabase (gravity_sta_pf_2019.gdb) The parameters for each station are described as follows:
Table 1. Description of Principal Facts Gravity Data
UTME_N83 – UTM easting, NAD83 datum, Zone 15 N
UTME_N83 – UTM easting, NAD83 datum, Zone 15 N
LONG_DD – Longitude in decimal degrees
LONG_DD – Longitude in decimal degrees
FIELD_NO – Station number, assigned in field
ELEV_FT – Station elevation in feet above mean sea level
METER – Gravity meter that was used for this station
OBSERVER – Name of individuals reading the gravity meter
ACQ_DATE – Date of acquisition, as available. Format is yyyymmdd
MGS_QUAD – 7.5 minute quad station is located in, using MGS numbering system
ABS-G – Absolute gravity value in milliGals
FREE_AIR – Free air anomaly in milliGals
BOUGUER – Bouguer anomaly in milliGals
MAIN_BASE – Name of main base station, as available, usually part of a formal network.
FIELD_BASE – Name of field base, as available, usually tied to a major base and established as part of a local survey.