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GGMplus read-me.dat V1.0
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GGMplus V1.0 - May 30, 2013
Christian Hirt
Western Australian Centre for Geodesy
Curtin University Perth
Citation
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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
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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
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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
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Functional Folder Suffix Format Unit No-data value
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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
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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
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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.