May 3, 2014 This directory contains the V23 gravity and vertical gravity gradient grids. The VGG file has the same format as the gravity. The units are eotvos units times 10. The main improvement of V23 with respect to V22 is 9 more months of CryoSat-2 data were added. November 3, 2013 This directory now contains the final version of the V22 gravity grid. There are two changes with respect to the September 30, beta version. 1) The low-pass filter that is applied to the gravity depends on depth as follows. # depth filter filename # 0 13km grav0.img # 1 14 grav1.img # 2 15 grav2.img # 3 16 grav3.img # 4 17 grav4.img # 5 18 grav5.img # 6 20 grav6.img 2) A slope correction was applied to the altimeter profiles to account for the offset of the footprint away from NADIR due to a high geoid slope. September 30, 2013 David T. Sandwell Note these are beta versions of V22 as presented in the IAG meeting in Potsdam. Global gravity grids. The V21 gravity models as published in: http://topex.ucsd.edu/sandwell/publications/142.pdf http://topex.ucsd.edu/sandwell/publications/144.pdf HOW TO ACCESS THE GRID WITH GMT If you are a GMT user there are two programs that enable you to access these files. 1) img2grd - this one reads the special Mercator format and dumps a subgrid. # # script to cut a small area and make a straight lon/lat grid # img2grd grav.img.23.1 -R-5/5/-10/10 -Ggrav.grd -D -T1 -m1 -E -S0.1 -V 2) The other really important, but cryptic, program will interpolate the Mercator grid at any desired point. So if you have a file of ship data and want to add a column of gravity or predicted depth with no interpolation errors try the following. grdtrack -Ggrav.img.23.1,.1,0,80.738 < rc2507.xyg > rc2507.xygs