FA-70044 / Map projection transforms / Member archive

Equirectangular projection with standard parallel: true-scale parallel · case 04

Horizontal scale is correct only when the origin latitude equals the standard parallel.

Member previewVariant 4 · 3 implementations · 8 checks per implementation

Case contract

Input [mode, a, b, lon0, lat0, lat1] on a sphere R = 6371007. mode "fwd": a=lon, b=lat; dlon = lon-lon0 wrapped into [-180, 180); x = R*rad(dlon)*cos(rad(lat1)), y = R*rad(lat-lat0). mode "inv": a=x, b=y; lon = lon0 + deg(x/(R*cos(rad(lat1)))) wrapped into [-180, 180), lat = lat0 + deg(y/R). Forward results round to 2 decimals, inverse results to 6.

Why this case matters

Plate carree style grids with a true-scale parallel are common for regional rasters and simple web previews.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
control #3	`[-1158318.82, -7314632.7]`	`[-918956.11, -7314632.7]`	Failed

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