Upload an Equirectangular base map and a Mercator-projected overlay. The overlay is first mathematically de-projected to Equirectangular using the Mercator formulas from chrisjones.id.au/Projection, then you can drag / scale / rotate it slightly to fine-tune alignment on the base map.
The base map's own projection (equirectangular) is linear, so for every base pixel we get λ, φ directly. We then use the forward relation above on φ to compute exactly which row of the Mercator overlay it corresponds to (columns map linearly to λ). This removes the north/south Mercator stretching automatically — no manual point-matching needed.
Change these only if your base map does not cover the full globe linearly.
Set these to the actual latitude/longitude bounds your Mercator overlay image covers.
⌨️ On any bound field: ↑/↓ = ±0.01 | Shift + ↑/↓ = ±1 | Enter = run conversion