Magnetic Variation

A Python3 module to calculate magnetic variation, based on WMM valid until 2025.

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This module provides the function

magnetic_variation(date, latitude, longitude, elevation)

date must be a float - the Julian date. Example: 2022.1 is in February.

latitude is in radians - positive for north, negative for south.

longitude is in radians - positive for east, negative for west.

elevation is in meters.

This returns what the geo-scientists would call ‘declination’ but what aviators call ‘magnetic variation.’

The result is in radians, and is negative for EAST “east is least” and positive for WEST.

For example:

>>> from geomag import magnetic_variation
>>> from math import degrees, radians

>>> degrees(magnetic_variation(2022.05,
                               radians(25.7953611),
                               radians(-80.2901158),
                               3)
6.9474...

This shows the magnetic variation for the Miami International Airport (KMIA) as 6.9 degrees WEST, which matches the sectional chart for January 2022.

Building from COF files

This distribution includes WMM2010.COF, WMM2015.COF, and WMM2020.COF.

These files were downloaded from the National Centers for Environmental Information (NCEI), formerly known as the National Geophysical Data Center (NGDC).

https://www.ngdc.noaa.gov/geomag/WMM/DoDWMM.shtml

The file wmm.c contains static data compiled from these .COF files.

To generate the file from scratch, use the ‘compile-cof’ script:

$ ./compile-cof WMM2010.COF WMM2015.COF WMM2020.COF >wmm.c

This incorporates all models, so you can get the magnetic variation for any latitude and longitude from any date from January 1, 2010.

If you're tight on space, and only need the current magnetic model:

$ ./compile-cof WMM2020.COF >wmm.c

Testing the Software

The ‘test’ subdirectory has test values from the NCEI.

Test the Python:

$ cd test
$ python3 test_wmm2020_test_values.py

Test the C code:

$ cd test
$ make
$ ./test_geomag