IGPP 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. The latest version 32 is 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 Earth Free-Air Anomaly. An asterisk denotes tiled datasets.

Code	Dimensions	Reg	Size	Description
01d	360 x 180	g,p	101 KB	1 arc degree global FAA (FAA @ 315 km)
30m	720 x 360	g,p	349 KB	30 arc minute global FAA (FAA @ 157 km)
20m	1080 x 540	g,p	750 KB	20 arc minute global FAA (FAA @ 105 km)
15m	1440 x 720	g,p	1.3 MB	15 arc minute global FAA (FAA @ 79 km)
10m	2160 x 1080	g,p	2.8 MB	10 arc minute global FAA (FAA @ 52 km)
06m	3600 x 1800	g,p	7.4 MB	6 arc minute global FAA (FAA @ 32 km)
05m*	4320 x 2160	g,p	10 MB	5 arc minute global FAA (FAA @ 26 km)
04m*	5400 x 2700	g,p	16 MB	4 arc minute global FAA (FAA @ 21 km)
03m*	7200 x 3600	g,p	27 MB	3 arc minute global FAA (FAA @ 16 km)
02m*	10800 x 5400	g,p	59 MB	2 arc minute global FAA (FAA @ 11 km)
01m*	21600 x 10800	p	219 MB	1 arc minute global FAA (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].