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# healpix

convert between lonlat and HEALPix coordinates

HEALPix coordinates are an equal area projection very similar to cube mapping. This coordinate system is used heavily in cosmology, where equal-area and uniform point distribution are important for density comparisons and binning.

The HEALPix algorithm is described in the paper Mapping on the HEALPix grid

# example

## lon/lat to x/y

``````var healpix = require('healpix')(4,3)
var phi = Number(process.argv[2]) / 180 * Math.PI
var theta = Number(process.argv[3]) / 180 * Math.PI
var out = healpix.ang2xy([], phi, theta)
console.log(out.join(' '))
``````

output:

``````\$ node lonlat2xy.js -148 65
-2.476485479234211 -1.1544044416499766
``````

## x/y to lon/lat

``````var healpix = require('healpix')(4,3)
var PI = Math.PI
var x = Number(process.argv[2])
var y = Number(process.argv[3])
var out = healpix.xy2ang([], x, y)
console.log(out[0]*180/PI, out[1]*180/PI)
``````
``````\$ node xy2lonlat.js -2.476485479234211 -1.1544044416499766
-148.00000000000003 -65
``````

# api

``````var healpix = require('healpix')
``````

## var hp = healpix(H, K)

Create a healpix projection parameterized by `H,K`.

## hp.ang2xy(out, phi, theta)

Calculate `[x,y]` on the healpix projection given:

• phi - longitude in radians
• theta - latitude in radians

Set `[x,y]` and return `out`.

## hp.xy2ang(out, x, y)

Calculate `[phi,theta]` from `[x,y]` on the healpix projection.

Set `[phi,theta]` and return `out`.

# install

``````npm install healpix
``````

BSD

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