FA-69864 / Map projection transforms / Member archive

Spherical web mercator forward projection: northing sign convention · case 04

The map is mirrored north-south.

Member previewVariant 4 · 3 implementations · 8 checks per implementation

Case contract

Input [lon, lat] in degrees. Longitude is first wrapped into the half-open range [-180, 180); latitude is clamped to +/-85.05112878. Return [x, y] metres on a sphere of radius 6378137 with y growing northward, each rounded to 3 decimals.

Why this case matters

Web map tiles, vector tile encoders and map viewers all start from this forward transform; small mistakes shift features by kilometres or mirror them across the equator.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
regression #4	`[-19480910.889, -557305.257]`	`[-19480910.889, 557305.257]`	Failed

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