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d3 / d3-geo-projection

Extended geographic projections for d3-geo.
https://observablehq.com/collection/@d3/d3-geo-projection
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Correct HEALPix cap parallel #172

Closed Portponky closed 5 years ago

Portponky commented 5 years ago

The parallel was off by half a degree due to incorrect number of (angular) minutes.

Fil commented 5 years ago

Yes.

Furuti: “The boundary parallels, approximately 41°48'37" N and S, are chosen in order to make the triangular regions cover 1/3 of the total area.” https://web.archive.org/web/20170218212050/http://www.progonos.com/furuti/MapProj/Normal/ProjPCyl/ProjPCH/

The exact angle given in https://fits.gsfc.nasa.gov/wcs/mhg_20050615.pdf is asin(1 - 1/K) — in our case we have fixed K=3. I think it's better to use the formula.

To complete this pull-request we'll also have to update img/healpix.png with the new result of the yarn prepublishOnly command (optimized with optipng).

Fil commented 5 years ago

Merged in https://github.com/d3/d3-geo-projection/commit/fc45c3d545a62988e476eb1dd544f97256d2332d & https://github.com/d3/d3-geo-projection/commit/d6977314132c35c465fe9838090de97de983143f

Thank you!