------------------------- ERTM2160 read-me.dat V1.1 ------------------------- ERTM2160 V1.1 - Oktober 08, 2014 Christian Hirt Western Australian Centre for Geodesy Curtin University Perth List of changes --------------- - Terrain heights now available in same format as ERTM2160 (Oct 08 2014) - Matlab access software ( ERTM2160_2013_v2) modified to allow access to terrain heights (Oct 08 2014) ERTM2160 provides computerised maps of Earth's short-scale gravity field at 0.002 degree (7.2 arc-seconds, or ~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, with the exception of Greenland. The spectral band-width of ERTM2160 is ~10 km (harmonic degree 2160) to ~250 m. The four gravity field functionals provided are - 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 - Quasi/geoid effects Every ERTM2160 computation point (without exception) is based on numerical integration of topographic/bathymetric mass-effects within a cap of 200 km radius. For the downward-continuation of ERTM2160 gravity functionals, terrain heights are provided in folder /dem in the same data format as the gravity functionals. The terrain heights provided are from SRTM, with ocean heights set to zero. Citation -------- Hirt C., M. Kuhn, S.J. Claessens, R. Pail, K. Seitz, T. Gruber (2014), Study of Earth's short-scale gravity field using the high-resolution SRTM topography model, Computers & Geosciences, 73, 71-80, doi: 10.1016/j.cageo.2014.09.00. Data access and file formats ---------------------------- The ERTM2160 gravity field model is freely available via http://ddfe.curtin.edu.au/gravitymodels/ERTM2160 Due to its total size of 44 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. All data is stored in 2-byte signed integer big-endian format (int16, ieee-be), and the file size per tile is 12,208 KB. 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 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 Quasi/geoid height /geoid .ha int16 1 mm -2^15 --------------------------------------------------------------------------- Terrain elevation /dem .dem int16 1 m n/a --------------------------------------------------------------------------- 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.dg contains gravity disturbances 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 ERTM2160 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/ERTM2160 1. ERTM2160_tilelist_public.dat: ASCII list of tile names, tile boundaries, and number of data/ no-data points within the tile 2. ERTM2160_2013_v2.m: Matlab function for seamless access of the ERTM2160 binary files 3. test_access_ERTM2160.m: Matlab test driver script that uses ERTM2160_2013_v2.m to access ERTM2160 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. (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 ultra-high resolution picture of Earth's gravity field, Geophysical Research Letters, 40(16), 4279-4283, doi: 10.1002/grl.50838. Hirt C., M. Kuhn, S.J. Claessens, R. Pail, K. Seitz, T. Gruber (2014), Study of Earth's short-scale gravity field using the high-resolution SRTM topography model, Computers & Geosciences, 73, 71-80, doi: 10.1016/j.cageo.2014.09.00. 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 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.