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Webmercator, geojson and 2D canvas
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license: gpl-3.0 | |
height: 600 | |
scrolling: no | |
border: no |
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<!DOCTYPE html> | |
<html lang="en"> | |
<head> | |
<meta charset="UTF-8"> | |
<meta name="viewport" content="width=device-width, initial-scale=1.0"> | |
<meta http-equiv="X-UA-Compatible" content="ie=edge"> | |
<title>Mercator projection example</title> | |
<style> | |
body { | |
margin: 0; | |
padding: 0; | |
} | |
canvas { | |
display: block; | |
margin: auto; | |
} | |
; | |
</style> | |
</head> | |
<body> | |
<script> | |
// Used to generate colors | |
let i = 0; | |
// Adjust the map size to the window size | |
const MAP_SIZE = Math.min(window.parent.window.innerHeight, window.parent.window.innerWidth); | |
// Set the projection Radius, force the diameter to be equal to map size. | |
const R = MAP_SIZE / (2 * Math.PI); | |
// Diameter of the globe | |
const D = R * 2 * Math.PI; | |
// Create a canvas element | |
const canvas = document.createElement('canvas'); | |
// Set canvas size | |
canvas.width = MAP_SIZE; | |
canvas.height = MAP_SIZE * 0.7; | |
// Add the canvas to the DOM | |
document.body.appendChild(canvas); | |
// Save a drawing context into the map variable | |
const map = canvas.getContext('2d'); | |
// Get the data from a json and draw it | |
fetch('world.json') | |
.then(response => response.json()) | |
.then(drawGeoJson); | |
/** | |
* Draw a geoJson object, it only supports polygons and MultiPolygons | |
*/ | |
function drawGeoJson(geoJsonData) { | |
geoJsonData.features.forEach(feature => { | |
if (feature.geometry.type === 'Polygon') { | |
drawPolygon(map, feature.geometry.coordinates[0]); | |
} | |
if (feature.geometry.type === 'MultiPolygon') { | |
feature.geometry.coordinates.forEach(coords => drawPolygon(map, coords[0])); | |
} | |
}); | |
} | |
/** | |
* Given a list of coordinates, draw into a rendering context a polygon. | |
*/ | |
function drawPolygon(ctx, coords) { | |
ctx.fillStyle = getColor(); | |
ctx.beginPath(); | |
const initialPoint = project(coords.shift()); | |
ctx.moveTo(initialPoint.x, initialPoint.y); | |
coords.map(project).forEach(point => ctx.lineTo(point.x, point.y)); | |
ctx.closePath(); | |
ctx.fill(); | |
} | |
/** | |
* Webmercator projection. | |
* Since D === MAP_SIZE this function already returns pixel coordinates in the canvas. | |
*/ | |
function project([lon, lat]) { | |
const sinlat = Math.sin(lat * Math.PI / 180.0); | |
const x = D * lon / 360.0; | |
const y = R / 2 * Math.log((1 + sinlat) / (1 - sinlat)); | |
return { x: (D / 2 + x), y: (D - (D / 2 + y)) } | |
} | |
/** | |
* Generate a color | |
*/ | |
function getColor() { | |
return `rgb(${i++ % 255}, ${180}, ${255})`; | |
} | |
</script> | |
</body> | |
</html> |
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