IGPP Global Earth Free-Air Anomaly

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_images/GMT_faa.jpg

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., 2012]. Polar regions beyond ±80˚ are not covered and are set to NaN.

Usage

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):

IGPP Global Earth Free-Air Anomaly. An asterisk denotes tiled datasets.

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 dataset (@earth_faa.cpt) will be used.

Technical Information

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

  1. Sandwell, D.T., Harper, H., Tozer, B. and Smith, W.H., 2019: [https://doi.org/10.1016/j.asr.2019.09.011].

  2. Pavlis, N.K., Holmes, S.A., Kenyon, S.C. and Factor, J.K., 2012: [https://doi.org/10.1029/2011JB008916].