IGPP Global Earth Free-Air Anomaly
Both geodynamic and geophysical studies have an interest in the study of the Earth’s gravity field and are often used in conjunctions with bathymetric or topographic relief to assess the degree of isostasy. David Sandwell and colleagues have provided altimetry-based marine FAA and similar grids since 1995 and the latest versions are now available for remote use in GMT. Over land these grids are filled in by the EGM2008 model [Pavlis et al., 2008]. Polar regions beyond ±80˚ are not covered and are set to NaN.
Usage - Global Earth Free-Air Anomaly Grid
You access a global free-air (faa) grid by specifying the special name
@earth_faa_[rru[_reg]]
The following codes for rru and the optional reg are supported (dimensions are listed for pixel-registered grids; gridline-registered grids increment dimensions by one):
Code |
Dimensions |
Reg |
Size |
Description |
|---|---|---|---|---|
01d |
360 x 180 |
g,p |
106 KB |
1 arc degree global FAA (1 min @ 111 km) |
30m |
720 x 360 |
g,p |
368 KB |
30 arc minute global FAA (1 min @ 55 km) |
20m |
1080 x 540 |
g,p |
785 KB |
20 arc minute global FAA (1 min @ 37 km) |
15m |
1440 x 720 |
g,p |
1.3 MB |
15 arc minute global FAA (1 min @ 28 km) |
10m |
2160 x 1080 |
g,p |
2.9 MB |
10 arc minute global FAA (1 min @ 18 km) |
06m |
3600 x 1800 |
g,p |
7.5 MB |
6 arc minute global FAA (1 min @ 10 km) |
05m |
4320 x 2160 |
g,p |
11 MB |
5 arc minute global FAA (1 min @ 9 km) |
04m |
5400 x 2700 |
g,p |
16 MB |
4 arc minute global FAA (1 min @ 7.5 km) |
03m |
7200 x 3600 |
g,p |
28 MB |
3 arc minute global FAA (1 min @ 5.6 km) |
02m |
10800 x 5400 |
g,p |
60 MB |
2 arc minute global FAA (1 min @ 3.7 km) |
01m |
21600 x 10800 |
p |
220 MB |
1 arc minute global FAA (1 min original) |
See GMT remote dataset usage for when resolution codes are optional or required.
All of these data will, when downloaded, be placed in your ~/.gmt/server directory, with
the earth_faa files being placed in an earth/earth_faa sub-directory. If you do not
specify a CPT, the default CPT for this data set will be used (@earth_faa.cpt)
Technical Information - Global Earth Free-Air Anomaly Grid
We scale and reformat the original data to take up very little space so that downloads from the servers are as fast as possible. For the FAA grid this means we chose 0.025 mGal as the smallest data unit, which is well below the uncertainties in the model. Data are scaled and shifted to fit in a short integer grid that is highly compressed by netCDF lossless compression and chunking. The data are reported in mGal.
Data References - Global Earth Free-Air Anomaly Grid
Sandwell, D.T., Harper, H., Tozer, B. and Smith, W.H., 2019: [https://doi.org/10.1016/j.asr.2019.09.011].
Pavlis, N.K., Holmes, S.A., Kenyon, S.C. and Factor, J.K., 2012: [https://doi.org/10.1029/2011JB008916].