Name: Magnetic Declination
Rating: 4.6 (19 reviews)
Author: simonletts

*** Now updated to WMM 2025 ***

This widget shows the magnetic declination at the current location and time.

When the widget is launched it displays the last value that was calculated (if available) while it acquires a GPS position to calculate the up-to-date value. This is shown in grey rather than the normal white.

If there is screen space, modern watches that support the Glance mode of launching widgets will also display the last value that was calculated.

Depending on when & where you last ran the app the previous value could be different from the current one.

It’s well known that a magnetic compass doesn’t point exactly towards True North – it points to Magnetic North. The difference is called the magnetic declination. It varies both around the world and over time but can be calculated for any given time and place using the World Magnetic Model. This was developed by the National Centers for Environmental Information and the British Geological Survey (https://www.ncei.noaa.gov/products/world-magnetic-model). The model is updated every 5 years. The current one is WMM2025 covering 2025-2029. Hopefully, I’ll update the widget at the end of 2029 when the next model is released!!

The code is based on WMM_Tiny by John Blaiklock (https://github.com/miniwinwm/WMM_Tiny/) which is itself a cut down version of the original source code available from NOAA. WMM_Tiny, and therefore this work, is released under the MIT licence (https://opensource.org/licenses/MIT).

The icons are based on the image https://upload.wikimedia.org/wikipedia/commons/c/c2/Magnetic_declination.svg which is licensed under the Creative Commons licence CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/).

The app has been tested on my vívoactive® HR, a fenix 5, and other devices in the simulator. Please contact me if you have any problems, comments or requests. Don't forget to include your email address if you would like a reply. 

Thank you to Peter Bleakley for suggestion this widget and for helping test it. 

N.B. The widget is still a work in progress, it has occasionally crashed on some models in the simulator. Please let me know if it fails for you.