FA-69979 / Map projection transforms / Member archive

Web mercator ground resolution and scale denominator: zoom exponent · case 04

Every zoom level reports half the correct resolution.

Member previewVariant 4 · 3 implementations · 8 checks per implementation

Case contract

Input [lat, z, tile_size, dpi]. Ground resolution in metres per pixel is 2*pi*6378137*cos(lat)/(tile_size*2**z) with the equatorial circumference computed exactly. The map scale denominator is resolution*dpi/0.0254. Return [resolution rounded to 6 decimals, scale denominator rounded to the nearest integer] computed from the unrounded resolution.

Why this case matters

Scale bars, level-of-detail choice and print exports depend on these values.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
control #3	`[18.672958, 220547]`	`[37.345917, 441094]`	Failed

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