ardupilot/libraries/AP_Declination/generate/README.md

# Declination Table Generator

To run the generator you need the igrf module. Install like this:

  python3 -m pip install --user igrf

Note that python3 must be used.

Then cd to libraries/AP_Declination directory, and run:

 python3 generate/generate.py

it will updates the tables.cpp code