Readme: UTIG Thwaites Glacier Airborne Free-Air Gravity Data Please cite the Diehl et al. (2008a) paper in G-cubed for survey and processing information and the NGDC data release website (Diehl et al., 2008b) as the source of the data files. (See References below) General Information: This airborne free-air gravity dataset over Thwaites Glacier in the Amundsen Sea Embayment of West Antarctica was collected and processed primarily by the Univeristy of Texas at Austin, Institute for Geophysics (UTIG) in the austral summer of 2004-2005. The survey was named AGASEA (Airborne Geophysics of the Amundsen Sea Embayment, Antarctica) and we used a Micro-G LaCoste Air/Sea II gravimeter. Five flights of data were collected during that same Antarctic field season by the British Antarctic Survey and are included in these gridded gravity anomalies and crossover differences. This dataset covers an irregularly-shaped area between Longitudes 135 deg W and 93 deg W and Latitudes 72 deg S and 82 deg S. The data lines were flown at an angle to the northing and easting lines of a polar stereographic projection with true scale at 71 deg S (called ps71). Flight line spacing was a 15 km regular grid with extra lines of data collected at angles to the grid as the opportunity arose (draped lines following glacier flow lines were flown but not included in this gravity dataset because gravity could not be recovered during draped flying). Files available: 1. utig_agasea_freeairgrv.llg -Data file of free-air gravity anomalies after standard data processing (Diehl et al., 2008a) and gridding in GMT. Anomalies are referenced to WGS-84 sea level, upward continued to 3600 m altitude, have a 9km spatial resolution, and have a 2.3 mGal RMS error (though spatially variable). We recommend interpreting these anomalies in the context of the crossover differences provided in file #2 (utig_agasea_freeairgrv_crossover_diffs.dat). -Gridding parameters: ps71 coordinates (in kilometers, W/E/S/N): -1650/-900/-900/100 Line data averaged in 5 km bins Spline surface fit (tension 0.25; 2 km interval) Surface masked to remove interpolated areas outside of original data -File Columns: 1 Longitude in decimal degrees (western hemisphere negative) 2 Latitude in decimal degrees (southern hemisphere negative) 3 Free-Air Gravity value in mGals 2. utig_agasea_freeairgrv_crossover_diffs.dat Data file of crossover differences in the line-based free-air data, calculated before gridding for two points near each other on crossing lines (see Diehl et al. (2008a) for further information). -File Columns: 1 Longitude for point 1 in decimal degrees (western hemisphere negative) 2 Latitude for point 1 in decimal degrees (southern hemisphere negative) 3 Free-Air Gravity value in mGals for point 1 4 Longitude for point 2 in decimal degrees (western hemisphere negative) 5 Latitude for point 2 in decimal degrees (southern hemisphere negative) 6 Free-Air Gravity value in mGals (point 2) 7 Free-Air Gravity Difference for crossing lines (ie. crossover difference) in mGals (absolute value of column 3 minus column 6) 3. utig_agasea_freeairgrv.jpg JPEG image of the data from file #1, projected into polar stereographic 71. 4. utig_agasea_freeairgrv_crossover_diffs.jpg JPEG image of data from file #2, projected into polar stereographic 71 and plotted using the location of point 1 (ie. location in columns 1 and 2 and data in column 7). Contours are ice surface elevations from Liu et al. (2001). References: Diehl, T.M., J.W. Holt, D.D. Blankenship, D.A. Young, T.A. Jordan, and F. Ferraccioli. 2008a. “First airborne gravity results over the Thwaites Glacier Catchment, West Antarctica.” Geochem. Geophys. Geosyst., 9(4), Q04011, doi: 10.1029/2007GC001878. Diehl, T.M., J.W. Holt, D.D. Blankenship, and D.A. Young. 2008b. “Free-Air gravity anomalies over the Thwaites Glacier Catchment, West Antarctica.” http://www.ngdc.noaa.gov/mgg/gravity/utig_agasea.html, National Geophysical Data Center, Boulder, Colo. Liu, H., K. C. Jezek, B. Li, and J. L. Zhao (2001), RADARSAT Antarctic Mapping Project digital elevation model version 2, Natl. Snow and Ice Data Cent., Boulder, Colo. (Available at http://nsidc.org/data/nsidc-0082.html) This work funded by: - The National Science Foundation, grant OPP-0230197 (http://www.nsf.gov/awardsearch/showAward.do?AwardNumber=0230197) - The Jackson School of Geosciences and the Geology Foundation at the University of Texas at Austin - The G. Unger Vetleson Foundation - The UK Natural Environment Research Council For more information, contact: Theresa M. Diehl University of Texas at Austin Institute for Geophysics 10100 Burnet Rd. (R2200) Austin, TX 78758 Email: theresa@ig.utexas.edu (At The National Geodetic Survey in Silver Spring, MD as of September 2008)