Anders Kristoffersson - 1 year ago 90

Javascript Question

I want to show specific information depending on where i am.

I have five cities with different information, and i want to show that city(information) that i'm closest to.

How to i do that the simplest way, using javascript.

Ex.

If i store the cities lat, long in an array

`var cities = [`

['new york', '111111', '222222', 'blablabla']

['boston', '111111', '222222', 'blablabla']

['seattle', '111111', '222222', 'blablabla']

['london', '111111', '222222', 'blablabla']

]

And with my current location(lat, long) i want the city that i'm closet to.

Answer Source

Here is a basic code example using HTML5 geolocation to get the user's position. It then calls nearestcity() and calculates the distance (km) from the location to each city. I passed on using the Haversine formulae and instead used the simpler Pythagoras formulae and an equirectangular projection to adjust for the curvature in longitude lines.

```
<html>
<head>
</head>
<script>
// Get User's Coordinate from their Browser
window.onload = function () {
// HTML5/W3C Geolocation
if ( navigator.geolocation )
{
navigator.geolocation.getCurrentPosition( UserLocation );
}
// Default to Washington, DC
else
NearestCity( 38.8951, -77.0367 );
}
// Callback function for asynchronous call to HTML5 geolocation
function UserLocation( position )
{
NearestCity( position.coords.latitude, position.coords.longitude );
}
// Convert Degress to Radians
function Deg2Rad( deg ) {
return deg * Math.PI / 180;
}
function PythagorasEquirectangular( lat1, lon1, lat2, lon2 )
{
lat1 = Deg2Rad(lat1);
lat2 = Deg2Rad(lat2);
lon1 = Deg2Rad(lon1);
lon2 = Deg2Rad(lon2);
var R = 6371; // km
var x = (lon2-lon1) * Math.cos((lat1+lat2)/2);
var y = (lat2-lat1);
var d = Math.sqrt(x*x + y*y) * R;
return d;
}
var lat=20; // user's latitude
var lon=40; // user's longitude
var cities= [
["city1", 10, 50, "blah" ],
["city2", 40, 60, "blah" ],
["city3", 25, 10, "blah" ],
["city4", 5, 80, "blah" ]
];
function NearestCity( latitude, longitude )
{
var mindif=99999;
var closest;
for (index = 0; index < cities.length; ++index) {
var dif = PythagorasEquirectangular( latitude, longitude, cities[ index ][ 1 ], cities[ index ][ 2 ] );
if ( dif < mindif )
{
closest=index;
mindif = dif;
}
}
// echo the nearest city
alert( cities[ closest ] );
}
</script>
<body>
</body>
</html>
```