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    [OC] XKCD 3207: When did the largest share of the population live within 5° of zero magnetic declination?
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    [OC] XKCD 3207: When did the largest share of the population live within 5° of zero magnetic declination?


    I got nerd sniped by the title text of XKCD 3207:

    'The zero line in WMM2025 passes through a lot of population centers; I wonder what year the largest share of the population lived in a zone of less than 5° of declination,' he thought, derailing all other tasks for the rest of the day.

    With some help from Claude Code, I built an interactive visualization to answer the question.

    Data sources and code.

    Posted by ahogue

    View 6 Comments

    6 Comments

    1. ahogue on

      Data sources:

      [](https://github.com/awhogue/zero-declination#data-sources)

      * **Magnetic declination (1590–1985):** [gufm1](https://royalsocietypublishing.org/doi/10.1098/rsta.2000.0569) model via `pymagglobal`
      * **Magnetic declination (1990–2025):** [IGRF-14](https://www.ngdc.noaa.gov/IAGA/vmod/igrf.html) via `ppigrf`
      * **Population (1600–2020):** [HYDE 3.3](https://public.yoda.uu.nl/geo/UU01/94FNH0.html) gridded historical population

      Code: [https://github.com/awhogue/zero-declination](https://github.com/awhogue/zero-declination)

      Tools: Claude Code (Opus 4.6 medium effort)

    2. Icing-Egg on

      Thank you for your service

      Instead of an xkcd being for something, there’s now something for an xkcd

    4. ThongsGoOnUrFeet on

      Zero magnetic declination means there is no angle difference between True North and Magnetic North at a specific location.

    6. Pryymal on

      I would like to see it on a globe, or another set of projections that better visualizes the areas near the poles