------------------------ GGMplus read-me.dat V1.0 ------------------------ GGMplus V1.0 - May 30, 2013 Christian Hirt Western Australian Centre for Geodesy Curtin University Perth Citation -------- Hirt, C., S.J. Claessens, T. Fecher, M. Kuhn, R. Pail, M. Rexer (2013), New ultrahigh-resolution picture of Earth's gravity field, Geophysical Research Letters, Vol 40, doi: 10.1002/grl.50838 General ------- GGMplus provides computerised gravity field maps at 0.002 deg (7.2 arc-seconds, ~220 m in latitude direction) resolution for all land areas of Earth within 60 geographic latitude, and an adjoining ~10 km marine zone along the coast lines, The five gravity field functionals provided are - Free-fall gravity accelerations - Gravity disturbances (radial derivatives of the disturbing potential) - North-South deflection of the vertical in Helmert definition - East-West deflection of the vertical in Helmert definition - Molodenski quasigeoid heights All gravity field functionals provided refer to the Earth's surface. The underlying Geodetic Reference System is GRS80. The zero- and first-degree harmonic coefficients in the synthesis were assumed to be zero. The functionals are provided in the zero-tide system. The short-scale component of every GGMplus computation point (without exception) is based on numerical integration of topographic mass-effects within a cap of 200 km radius. Data access and file formats ---------------------------- The GGMplus gravity field model is freely available via http://ddfe.curtin.edu.au/gravitymodels/GGMplus Due to its total size of 74 GB, the model is partitioned and distributed in terms of 881 binary files of 5 deg x 5 deg size for each functional. Each 5 deg x 5 deg tile contains 2500 x 2500 grid points in centre-of-cell representation (grid points are not situated on integer meridians and parallels). The grid resolution is 0.002 deg (7.2 arc seconds) with the grid equally spaced in terms of geodetic (GRS80) latitude and longitude. Depending on the functional, data is stored either in 2-byte signed integer big -endian format (int16, ieee-be), or 4-byte signed integer big-endian format (int32, ieee-be). Vertical deflections (xi, eta) and gravity disturbances (dg) are in 2-byte format, quasigeoid heights and gravity accelerations in 4-byte format. The file sizes are 12,208 KB and 24,416 KB, respectively. Table 1 provides details on the file formats, suffixes and units of storage, and no- data values indicating offshore areas. Table 1 --------------------------------------------------------------------------- Functional Folder Suffix Format Unit No-data value --------------------------------------------------------------------------- Gravity acceleration /ga .ga int32 0.1 mGal -2^31 Gravity disturbance /dg .dg int16 0.1 mGal -2^15 North-South DoV /xi .xi int16 0.1 arc-sec -2^15 East-West DoV /eta .eta int16 0.1 arc-sec -2^15 Quasigeoid height /geoid .ha int32 1 mm -2^31 --------------------------------------------------------------------------- The integer meridian and parallel located closest to the South-Westernmost data point of each 5 deg x 5 deg determines the filename, and the functional determines the suffix. For instance the file N50E005.ga contains gravity accelerations over the area 50.001 to 54.999 degree geodetic latitude, and 5.001 to 9.999 degree geodetic longitude. Records proceed along meridians from South to North and columns proceed from West to East. The first record is the South-West corner (50.001 deg latitude, 5.001 deg longitude in the example), and the last record is the North-East corner (54.999 deg latitude, 9.999 deg longitude). No-data values (see Table) flag offshore areas (i.e. about 10 km or further away from the nearest land point). Note that the gravity data is held in 0.1 mGal, 0.1 arc-sec and mm, respectively in the binary files. Accordingly, conversion factors (1000 in case of quasigeoid heights, 10 for the other functionals) must be applied to scale the data to basic units mGal, arc-sec and metres. When extracting GGMplus gravity field functions with the Matlab scripts provided, the conversion factors are automatically taken into account. For convenient use, the following files are available at http://ddfe.curtin.edu.au/gravitymodels/GGMplus 1. GGMplus_tilelist_public.dat: ASCII list of tile names, tile boundaries, and number of data/ no-data points within the tile 2. ggmplus2013_v4.m: Matlab function for seamless access of the GGMplus binary files 3. test_access_ggmplus.m: Matlab test driver script that uses ggmplus2013_v4.m to access GGMplus binary files. References ---------- Becker et al. (2009) Global Bathymetry and Elevation Data at 30 Arc Seconds Resolution: SRTM30_PLUS. Marine Geodesy 32(4), 355-371. Forsberg R. (1984) A study of terrain reductions, density anomalies and geophysical inversion methods in gravity field modelling. Report 355, Department of Geodetic Science and Surveying, Ohio State University, Columbus. Hirt C. (2012) Efficient and accurate high-degree spherical harmonic synthesis of gravity field functionals at the Earth's surface using the gradient approach. Journal of Geodesy 86(9), 729-744. Hirt C. (2013) RTM gravity forward-modeling using topography/bathymetry data to improve high-degree global geopotential models in the coastal zone, Marine Geodesy 36(2), 1-20. Hirt, C., S.J. Claessens, T. Fecher, M. Kuhn, R. Pail, M. Rexer (2013), New ultrahigh-resolution picture of Earth's gravity field, Geophysical Research Letters, Vol 40, doi: 10.1002/grl.50838 Jarvis A, Reuter HI, Nelson A Guevara E (2008) Hole-filled SRTM for the globe Version 4. Available from the CGIAR-SXI SRTM 90m database: http://srtm.csi.cgiar.org Mayer-Guerr T, E Kurtenbach, A Eicker (2010) The ITG GRACE 2010 model, http://www.igg.uni-bonn.de/apmg/index.php?id=itg-grace2010. Pail, R. et al. (2011) First GOCE gravity field models derived by three different approaches, Journal of Geodesy 85(11), 819-843. Pavlis, N.K., S.A. Holmes, S.C. Kenyon, and J.K. Factor (2012) The development and evaluation of the Earth Gravitational Model 2008 (EGM2008). Journal Geophysical Research 117, B04406. Disclaimer ---------- Neither Curtin University nor any of its staff accept any liability in connection with the use of data and models provided here. Neither Curtin University nor any of its staff make any warranty of fitness, completeness, usefulness and accuracy of the data and models for any intented or unintended purpose.