wrap latitude and longitude values around the poles in order to normalize their ranges

wrap.js

/**
  normalize co-ordinates that lie outside of the normal ranges.

  longitude wrapping simply requires adding +- 360 to the value until it comes
  in to range.

  for the latitude values we need to flip the longitude whenever the latitude
  crosses a pole.
**/


function wrap( lat, lon ){

  var point = { lat: lat, lon: lon };
  var quadrant = Math.floor( Math.abs( lat ) / 90) % 4;
  var pole = ( lat > 0 ) ? 90 : -90;
  var offset = lat % 90;

  switch( quadrant ){
    case 0:
      point.lat = offset;
      break;
    case 1:
      point.lat = pole - offset;
      point.lon += 180;
      break;
    case 2:
      point.lat = -offset
      point.lon += 180;
      break;
    case 3:
      point.lat = -pole + offset;
      break;
  }

  if( point.lon > 180 || point.lon < -180 ){
    point.lon -= Math.floor(( point.lon + 180 ) / 360) * 360;
  }

  return point;
}

module.exports = wrap;

wrap_test.js

var wrap = require('./wrap');

module.exports.tests = {};

module.exports.tests.control = function(test, common) {
  test('control - no wrapping required', function (t) {
    var norm = wrap(55.555, 22.222);
    t.equal(norm.lat, 55.555);
    t.equal(norm.lon, 22.222);
    t.end();
  });
};

module.exports.tests.latitude_positive = function(test, common) {
  test('positive latitude wrapping - 1 degree', function (t) {
    var norm = wrap(1, 0);
    t.equal(norm.lat, 1);
    t.equal(norm.lon, 0);
    t.end();
  });
  test('positive latitude wrapping - 91 degrees', function (t) {
    var norm = wrap(91, 0);
    t.equal(norm.lat, 89);
    t.equal(norm.lon, 180);
    t.end();
  });
  test('positive latitude wrapping - 181 degrees', function (t) {
    var norm = wrap(181, 0);
    t.equal(norm.lat, -1);
    t.equal(norm.lon, 180);
    t.end();
  });
  test('positive latitude wrapping - 271 degrees', function (t) {
    var norm = wrap(271, 0);
    t.equal(norm.lat, -89);
    t.equal(norm.lon, 0);
    t.end();
  });
  test('positive latitude wrapping - 361 degrees', function (t) {
    var norm = wrap(361, 0);
    t.equal(norm.lat, 1);
    t.equal(norm.lon, 0);
    t.end();
  });
  test('positive latitude wrapping - 631 degrees', function (t) {
    var norm = wrap(631, 0);
    t.equal(norm.lat, -89);
    t.equal(norm.lon, 0);
    t.end();
  });
  test('positive latitude wrapping - 721 degrees', function (t) {
    var norm = wrap(721, 0);
    t.equal(norm.lat, 1);
    t.equal(norm.lon, 0);
    t.end();
  });
};

module.exports.tests.latitude_negative = function(test, common) {
  test('negative latitude wrapping - 1 degree', function (t) {
    var norm = wrap(-1, 0);
    t.equal(norm.lat, -1);
    t.equal(norm.lon, 0);
    t.end();
  });
  test('negative latitude wrapping - 91 degrees', function (t) {
    var norm = wrap(-91, 0);
    t.equal(norm.lat, -89);
    t.equal(norm.lon, 180);
    t.end();
  });
  test('negative latitude wrapping - 181 degrees', function (t) {
    var norm = wrap(-181, 0);
    t.equal(norm.lat, 1);
    t.equal(norm.lon, 180);
    t.end();
  });
  test('negative latitude wrapping - 271 degrees', function (t) {
    var norm = wrap(-271, 0);
    t.equal(norm.lat, 89);
    t.equal(norm.lon, 0);
    t.end();
  });
  test('negative latitude wrapping - 361 degrees', function (t) {
    var norm = wrap(-361, 0);
    t.equal(norm.lat, -1);
    t.equal(norm.lon, 0);
    t.end();
  });
  test('negative latitude wrapping - 631 degrees', function (t) {
    var norm = wrap(-631, 0);
    t.equal(norm.lat, 89);
    t.equal(norm.lon, 0);
    t.end();
  });
  test('negative latitude wrapping - 721 degrees', function (t) {
    var norm = wrap(-721, 0);
    t.equal(norm.lat, -1);
    t.equal(norm.lon, 0);
    t.end();
  });
};

module.exports.tests.longitude_positive = function(test, common) {
  test('positive longitude wrapping - 1 degree', function (t) {
    var norm = wrap(0, 1);
    t.equal(norm.lat, 0);
    t.equal(norm.lon, 1);
    t.end();
  });
  test('positive longitude wrapping - 181 degrees', function (t) {
    var norm = wrap(0, 181);
    t.equal(norm.lat, 0);
    t.equal(norm.lon, -179);
    t.end();
  });
  test('positive longitude wrapping - 271 degrees', function (t) {
    var norm = wrap(0, 271);
    t.equal(norm.lat, 0);
    t.equal(norm.lon, -89);
    t.end();
  });
  test('positive longitude wrapping - 361 degrees', function (t) {
    var norm = wrap(0, 361);
    t.equal(norm.lat, 0);
    t.equal(norm.lon, 1);
    t.end();
  });
  test('positive longitude wrapping - 631 degrees', function (t) {
    var norm = wrap(0, 631);
    t.equal(norm.lat, 0);
    t.equal(norm.lon, -89);
    t.end();
  });
  test('positive longitude wrapping - 721 degrees', function (t) {
    var norm = wrap(0, 721);
    t.equal(norm.lat, 0);
    t.equal(norm.lon, 1);
    t.end();
  });
};

module.exports.tests.longitude_negative = function(test, common) {
  test('negative longitude wrapping - 1 degree', function (t) {
    var norm = wrap(0, -1);
    t.equal(norm.lat, 0);
    t.equal(norm.lon, -1);
    t.end();
  });
  test('negative longitude wrapping - 181 degrees', function (t) {
    var norm = wrap(0, -181);
    t.equal(norm.lat, 0);
    t.equal(norm.lon, 179);
    t.end();
  });
  test('negative longitude wrapping - 271 degrees', function (t) {
    var norm = wrap(0, -271);
    t.equal(norm.lat, 0);
    t.equal(norm.lon, 89);
    t.end();
  });
  test('negative longitude wrapping - 361 degrees', function (t) {
    var norm = wrap(0, -361);
    t.equal(norm.lat, 0);
    t.equal(norm.lon, -1);
    t.end();
  });
  test('negative longitude wrapping - 631 degrees', function (t) {
    var norm = wrap(0, -631);
    t.equal(norm.lat, 0);
    t.equal(norm.lon, 89);
    t.end();
  });
  test('negative longitude wrapping - 721 degrees', function (t) {
    var norm = wrap(0, -721);
    t.equal(norm.lat, 0);
    t.equal(norm.lon, -1);
    t.end();
  });
};

module.exports.all = function (tape, common) {
  function test(name, testFunction) {
    return tape('SANTIZE wrap ' + name, testFunction);
  }

  for( var testCase in module.exports.tests ){
    module.exports.tests[testCase](test, common);
  }
};

Thank you for your gist. The code let me understand the meaning of wrap !

thanks.

You're welcome ;)

I'm not sure how people are finding this gist but I should mention that this was merged into the Pelias geocoder codebase.

:) I believe it showed up in a google search on wrapping lat lon...

The "negative latitude wrapping - 721 degrees" test is not given. Twice the positive latitude wrapping - 721 degrees test is given.

This is great! I ported it to Rust: https://gist.github.com/austenadler/ad22593c9492820a59217b6c84d301d6

Thanks @missinglink. This was really helpful :)

Was very useful, as @rjdverbeek-tud mentioned, there is a test missing / improperly copied.

Oops, I've updated the incorrect test case, thanks for spotting that.