The facts on this site are drawn from standard, publicly available references: publications of the International Bureau of Weights and Measures (BIPM) on the SI and the kelvin, International Earth Rotation and Reference Systems Service (IERS) material on reference meridians, national mapping and geodetic agency documentation for vertical datums and kilometre-zero markers, and standard histories of mathematics for the development of zero.
Readers who want to go deeper should consult the proceedings of the 1884 International Meridian Conference, BIPM's SI Brochure, and scholarly histories of the number zero in Babylonian, Maya and Indian traditions. Where sources disagree on details — such as the precise offset between the Airy line and the IERS Reference Meridian — this site follows the values most commonly cited by geodetic authorities.