May 25, 2023 Remade the east and north VD files with a 10 km low-pass filter instead of a 16 km low-pass filter. east_32.1_10km.img north_32.1_10km.img August 3, 2022 This version 32 includes an additional 12 months of Altika, Cryosat LRM, Cryosat SAR and Sentinel-3A/B. Waveform retracking was performed on the following altimeter data Jason-1/2, Envisat, Altika, and Cryosat-LRM. This was done to reduce the wavelength of the SWH smoothing filter from 45 km to 30 km. In addition, we solved for the optimal sea state bias for all the altimeters to reduce the SWH noise in the > 30 km wavelength band. Finally, we implemented a mask to remove all data within 3 km of any land. This helps to reduce land-contaminated waveforms. A precise land mask has been applied to all products to further reduce land/ocean edge effects. A description of each file follows. curv_32.1.img - ocean surface curvature = vertical gravity gradient (VGG), Eotvos * 10, read with img2grd only curv_32.1.nc - ocean surface curvature = vertical gravity gradient (VGG), Eotvos, netcdf, lon-lat coordinates grav_32.1.img - free-air gravity anomaly, milliGal * 10, read with img2grd only grav_32.1.nc - free-air gravity anomaly, milliGal, netcdf, lon-lat coordinates east_32.1.img - deflection of the vertical = sea surface slope, microradian * 10, read with img2grd only east_32.1.nc - deflection of the vertical = sea surface slope, microradian, netcdf, lon-lat coordinates north_32.1.img - deflection of the vertical = sea surface slope, microradian * 10, read with img2grd only north_32.1.nc - deflection of the vertical = sea surface slope, microradian, netcdf, lon-lat coordinates grav_error_32.1.img - uncertainty in gravity anomaly and sea surface slope, milligal or microradian * 10, read with img2grd only grav_error_32.1.nc - uncertainty in gravity anomaly and sea surface slope, milligal or microradian, netcdf, lon-lat coordinates gravity_attribution.doc - recommended citation and attribution for commercial use make_NETCDF.com - example of how to translate the img files to netcdf src_img - source code for reading the img files in fortran or C August 4, 2021 This version 31 includes an additional 9 months of Altika, Cryosat LRM, Cryosat SAR and Sentinel-3A/B. The Altika data beginning on cycle 127, February 4, 2019 were retracked to better accommodate the loss in amplitude associated with poor control in off-nadir pointing. This increased the number of Altica observations by about 50%. Also we had not kept up with the Cryosat SAR data because the format changed to D (netcdf) in 2017. We processed 3-years of additional SAR data. October 9, 2020 This version 30 includes an additional year of Altika, Cryosat, and Sentinel-3A/B data. In addition, the wavelength of the shallow water filter was increased from 12,13 km to 14,14 km as follows. # depth filter filename # 0 14 grav0.img # 1 14 grav1.img # 2 14 grav2.img # 3 14 grav3.img # 4 15 grav4.img # 5 16 grav5.img # 6 17 grav6.img # November 22, 2019 This version 29 includes 2 years of data from Sentinel-3A/B satellites on a 54-day repeat ground track. In addition Yao Yu re-designed all the low-pass filters to be consistent. See filter_design.docx June 28, 2019 This version 28 includes more data from Cryosat-2, Jason-2 and Altika. Also the grids have been converted to Cartesian coordinates and stored as NETCDF March 11, 2019 The latest version of the V27 grid has a better masking of raw altimeter data at the coastline. This removes a few coastal gravity artifacts. October 4, 2018 This directory contains the V27 gravity and vertical gravity gradient grids. The main improvement of V27 with respect to V26 is 12 months of retracked Jason-2 data were added as well as 2 more months of AltiKa data. June 21, 2018 This directory contains the V26 gravity and vertical gravity gradient grids. The main improvement of V26 with respect to V25 is 6 more months of CryoSat-2 LRM data and 6 more months of Altika data. In addition, there is about 2 X more Cryosat SAR data than previous versions which helps in the Arctic. November 11, 2017 This directory contains the V25 gravity and vertical gravity gradient grids. The main improvement of V25 with respect to V24 is 12 more months of CryoSat-2 LRM data were added. More important 31 additional months of CryoSat-2 SAR data were used and this versions now includes 13 months of Altika data. June 23, 2016 This directory contains the V24 gravity and vertical gravity gradient grids. The main improvement of V24 with respect to V23 is 12 more months of CryoSat-2 data were added. 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 img2grd curv.img.23.1 -R-5/5/-10/10 -Gcurv.grd -D -T1 -m1 -E -S0.1 -V M 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