geo-viewport

Turns bounding boxes / extents into centerpoint & zoom combos for static maps.

Works in node.js and browsers, via browserify or a script tag.

Install

npm install --save geo-viewport

Or use a plugin:

<script src='//api.tiles.mapbox.com/mapbox.js/plugins/geo-viewport/v0.2.1/geo-viewport.js'></script>

The script-tag include exports an object called geoViewport, with methods bounds and viewport documented below.

Example

Live example with Mapbox Static Map API

With Node

var geoViewport = require('geo-viewport');
 
geoViewport.viewport([
    5.668343999999995,
    45.111511000000014,
    5.852471999999996,
    45.26800200000002
], [640, 480])
 
// yields
// {
//     center: [
//         5.7604079999999955,
//         45.189756500000016
//     ],
//     zoom: 11
// }

In a browser:

<script src='//api.tiles.mapbox.com/mapbox.js/plugins/geo-viewport/v0.1.1/geo-viewport.js'></script>
<script>
var bounds = geoViewport.viewport([
    5.668343999999995,
    45.111511000000014,
    5.852471999999996,
    45.26800200000002
], [640, 480]);
 
var center = geoViewport.bounds(
  [-75.03,
  35.25],
  14,
  [600, 400]);
 
console.log(bounds);
console.log(center);
</script> 

api

viewport(bounds, dimensions, minzoom, maxzoom, tileSize)

Given a WSEN array of bounds and a [x, y] array of pixel dimensions, return a { center: [lon, lat], zoom: zoom } viewport.

Example:

// first argument is the bounds, and the image is 640x480
geoViewport.viewport([
    5.6683, 45.111, 5.8524, 45.268
], [640, 480])

bounds(viewport, zoom, dimensions, tileSize)

Given a centerpoint as [lon, lat] or { lon, lat }, a zoom, and dimensions as [x, y], return a bounding box.

Example:

geoViewport.bounds([-75.03, 35.25], 14, [600, 400])

tile sizes

Be aware that these calculations are sensitive to tile size. The default size assumed by this library is 256x256px; however, Mapbox Vector Tiles are 512x512px.

For example, to calculating a bounding box for a classic raster-based 256x256 tile:

geoViewport.bounds([-75.03, 35.25], 14, [600, 400], 256)
 
// since 256 is default, you can omit the tileSize param
geoViewport.bounds([-75.03, 35.25], 14, [600, 400])

To calculate a bounding box for a Mapbox vector tile source, such as an image from the Mapbox Static Image API:

geoViewport.bounds([-75.03, 35.25], 14, [600, 400], 512)

There's a handy blog post discussing the issue here.