# IGPP Earth East-West Deflection
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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](https://topex.ucsd.edu/marine_grav/mar_grav.html)
have provided altimetry-based marine FAA and similar grids since 1995. The latest version 33 is now
available for remote use in GMT. Over land these grids are filled in by the EGM2008 model
[Pavlis et al., 2012]. This dataset contains the east-west deflection of the vertical
in micro-radians. Polar regions beyond ±80˚ are not covered and are set to NaN.

## Usage

You access a global east-west deflection (edefl) grid by specifying the special name

> @earth_edefl[\_*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):

(tbl-earth_edefl)=

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

Note that resolutions 05m and higher are tiled.
See {gmt-docs}`Remote Datasets <datasets/remote-data.html>` for more information.

See {gmt-docs}`GMT remote dataset usage <datasets/remote-data.html#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_edefl files being placed in an `earth/earth_edefl` sub-directory. If you do not
specify a CPT, the default CPT for this dataset (*@earth_defl.cpt*) will be used.

## Examples

To create a PDF map of the east-west deflection of the vertical at 1 arc degree resolution, try:

```bash
gmt grdimage @earth_edefl_01d -Baf -pdf earth_edefl
```

## 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 EDEFL grid this means
we chose 1/32 micro-radians 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 micro-radians, where
positive (negative) values indicate a deflection to the east (west).

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