Leaflet: Fit polyline in view

I’ve been playing with the Leaflet.js library over the Christmas holidays to visualise running routes drawn onto the map using a Polyline and I wanted to zoom the map the right amount to see all the points.

Pre requisites

We have the following HTML to define the div that will contain the map.

We also need to import the following Javascript and CSS files:

  
  
 
  
  

Polyline representing part of a route

The following code creates a polyline for a Strava segment that I often run.

var map = L.map('map');
L.tileLayer('http://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png', {maxZoom: 18,}).addTo(map);
 
var rawPoints = [
  { "latitude": 51.357874010145395, "longitude": -0.198045110923591 },
  { "latitude": 51.3573858289394, "longitude": -0.19787754933584795 },
  { "latitude": 51.35632791810057, "longitude": -0.19750254941422557 },
  { "latitude": 51.35553240304241, "longitude": -0.197232163894512 },
  { "latitude": 51.35496267279901, "longitude": -0.1970247338143316 },
  { "latitude": 51.35388700570004, "longitude": -0.19666483094752069 },
  { "latitude": 51.3533898352570, "longitude": -0.1964976504847828 },
  { "latitude": 51.35358452733139, "longitude": -0.19512563906602554 },
  { "latitude": 51.354762877995036, "longitude": -0.1945622934585907 },
  { "latitude": 51.355610110109986, "longitude": -0.19468697186046677 },
  { "latitude": 51.35680377680643, "longitude": -0.19395063336295112 },
  { "latitude": 51.356861596801075, "longitude": -0.1936180154828497 },
  { "latitude": 51.358487396611125, "longitude": -0.19349660642888197 }
];
 
var coordinates = rawPoints.map(rawPoint => new L.LatLng(rawPoint["latitude"], rawPoint["longitude"]))
 
let polyline = L.polyline(
    coordinates,
    {
        color: 'blue',
        weight: 3,
        opacity: .7,
        lineJoin: 'round'
    }
);
 
polyline.addTo(map);

I wanted to centre the map around the polyline and initially wrote the following code to do this:

let lats = rawPoints.map(c => c.latitude).reduce((previous, current) => current += previous, 0.0);
let longs = rawPoints.map(c => c.longitude).reduce((previous, current) => current += previous, 0.0);
 
const position = [lats / rawPoints.length, longs / rawPoints.length];
map.setView(position, 17);

This works fine but the zoom factor was wrong when I drew longer polylines so I needed a better solution.

I should have RTFM because there’s a much simpler way to do this. I actually found the explanation in a GitHub issue from 2011 ! We can replace the previous snippet with this single line of code:

map.fitBounds(polyline.getBounds());

And this is how it looks on the screen:

Mark Needham稿源:Mark Needham (源链) | 关于 | 阅读提示

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