I figured out the meaning of the GeoJSON data in "mw.json" (D3-Celestial, Olaf Frohn).

There are 5 groups (brightness levels, 1 darkest to 5 brightest) of coordinates describing polygons that delineate the Milky Way on the sky dome : 10 polygons for brightness level 1, 113 at level 2, 46 when level 3, 27 with level 4, and 6 polygons for level 5.

The coordinates [longitude,latitude] are in degrees with up to 3 decimals.

Right ascension ($\alpha$, [0°,360°[) equals positive 'longitude', otherwise add 360° to the 'longitude' to obtain $\alpha$. Divide $\alpha$ (in degrees) by 15 to obtain it in hours.

Declination ($\delta$, [+90°,-90°]) always equals the 'latitude'.

Equatorial coordinates $\alpha$ and $\delta$ are given for epoch J2000.

The plotted Milky Way should look like this.

I figured out the meaning of the GeoJSON data in "mw.json" (D3-Celestial, Olaf Frohn) describing the Milky Way boundary.

There are 5 groups (brightness levels, 1 darkest to 5 brightest) of coordinates describing polygons that delineate the Milky Way on the sky dome : 10 polygons for brightness level 1, 113 at level 2, 46 when level 3, 27 with level 4, and 6 polygons for level 5.

The coordinates [longitude,latitude] are in degrees with up to 3 decimals.

Right ascension ($\alpha$, [0°,360°[) equals positive 'longitude', otherwise add 360° to the 'longitude' to obtain $\alpha$. Divide $\alpha$ (in degrees) by 15 to obtain it in hours.

Declination ($\delta$, [+90°,-90°]) always equals the 'latitude'.

Equatorial coordinates $\alpha$ and $\delta$ are given for epoch J2000.

The plotted Milky Way should look like this.