EMAG2 Global Earth Magnetic Anomaly Model

_images/GMT_mag.jpg

The US National Environmental Information Center Office of Geomatics provide EMAG2, the Earth Magnetic Anomaly Model at 2 arc-minute resolution based on satellite, ship, and airborne magnetic measurements. The latest EMAG2 version 3 [Meyer et al., 2017] grid is now available for remote use in GMT.

Usage

You access a global EMAG2 grid by specifying the special name

@earth_mag[_rru[_reg]]

This version is observed at sea level over oceanic regions and have no data over land. For a version where all observations are relative to an altitude of 4 km above the geoid and includes data over land, use instead

@earth_mag4km[_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). The sizes refers to the earth_mag4km version (the oceanic files are ~60% smaller):

EMAG2 Global Earth Magnetic Anomaly Model. An asterisk denotes tiled datasets.

Code

Dimensions

Reg

Size

Description

01d

360 x 180

g,p

94 KB

1 arc degree global EMAG2 (2 min @ 111 km)

30m

720 x 360

g,p

329 KB

30 arc minute global EMAG2 (2 min @ 55 km)

20m

1080 x 540

g,p

695 KB

20 arc minute global EMAG2 (2 min @ 37 km)

15m

1440 x 720

g,p

1.2 MB

15 arc minute global EMAG2 (2 min @ 28 km)

10m

2160 x 1080

g,p

2.5 MB

10 arc minute global EMAG2 (2 min @ 18 km)

06m

3600 x 1800

g,p

5.7 MB

6 arc minute global EMAG2 (2 min @ 10 km)

05m*

4320 x 2160

g,p

9.3 MB

5 arc minute global EMAG2 (2 min @ 9 km)

04m*

5400 x 2700

g,p

14 MB

4 arc minute global EMAG2 (2 min @ 7.5 km)

03m*

7200 x 3600

g,p

24 MB

3 arc minute global EMAG2 (2 min @ 5.6 km)

02m*

10800 x 5400

p

52 MB

2 arc minute global EMAG2 (2 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_mag files being placed in an earth/earth_mag sub-directory and the earth_mag4km files in an earth/earth_mag4km sub-directory. If you do not specify a CPT, the default CPT for this dataset (@earth_mag.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 EMAG2 grid this means we chose 0.2 nTesla as the smallest data unit, which is well below the uncertainties in the model (0.4 nT for the 4 km altitude version). 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 nT.

Data References

  1. Meyer, B., Saltus, R. and Chulliat, A., 2017: [https://doi.org/10.7289/V5H70CVX].