LatLonSpherical.intersection() - rounding errors lead to φ3=NaN

[henrythasler]

Action

Add the following test case to the intersection group in latlon-spherical-tests.js:

test('φ3=NaN', () => should.equal(LatLon.intersection(new LatLon(-77.6966041375563, 18.28125000000003), 179.99999999999994, new LatLon(89, 180), 180), null));

Expected behavior

I don't know... something around the south pole or no solution at all?

Observed behavior

φ3=NaN

  1 failing

  1) latlon-spherical
       intersection
         rounding errors, φ3=NaN:
     TypeError: invalid lat ‘NaN’
      at new LatLonSpherical (latlon-spherical.js:49:31)
      at Function.intersection (latlon-spherical.js:442:16)
      at Context.test (test/latlon-spherical-tests.js:183:81)
      at process.topLevelDomainCallback (domain.js:126:23)

Workaround

(...)
const φ3 = Math.asin(Math.sin(φ1) * Math.cos(δ13) + Math.cos(φ1) * Math.sin(δ13) * Math.cos(θ13));
if (isNaN(φ3)) {
    return null;
}
(...)

[chrisveness]

I think a better option would be to clamp the value supplied to Math.asin() in the calculation of φ3:

const φ3 = Math.asin(Math.min(Math.max(Math.sin(φ1)*Math.cos(δ13) + Math.cos(φ1)*Math.sin(δ13)*Math.cos(θ13), -1), 1));

Test:

test('rounding: φ3 requires clamp', () => LatLon.intersection(new LatLon(-77.6966041375563, 18.2812500000000), 179.99999999999995, new LatLon(89, 180), 180).toString().should.equal('90.0000°S, 163.9902°W'));

As far as I can see, this gives correct values.

[henrythasler]

Ok, that works for me. Thanks!

[chrisveness]

Fixed in v2.2.1.