FA-70149 / Map projection transforms / Member archive

Spherical Albers equal-area conic forward: northing orientation · case 04

The map is flipped north-south about the origin.

Member previewVariant 4 · 3 implementations · 8 checks per implementation

Case contract

Input [lon, lat, lon0, lat0, lat1, lat2] degrees, sphere R = 6371000, lat1 != -lat2. n = (sin p1 + sin p2)/2; C = cos(p1)**2 + 2*n*sin(p1); rho = R*sqrt(C - 2*n*sin(phi))/n; rho0 likewise at lat0; theta = n*dlon with dlon wrapped to [-180, 180). x = rho*sin(theta), y = rho0 - rho*cos(theta), rounded to 2 decimals.

Why this case matters

Continental statistics maps use Albers because it preserves area; a wrong constant silently biases area totals.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
control #3	`[601606.86, 1530950.07]`	`[601606.86, -1530950.07]`	Failed

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