openlayers是开源webgis，
先去官网下载 OpenLayers - Get the Code
新建html，写入如下代码：
<head>
<script src = "../build/ol.js"></script>
</head>
<body>
<div id="map" class="map"></div>
<script>
var map = new ol.Map({
layers: [
new ol.layer.Vector({
//这是一个能选择的地图源
source: new ol.source.Vector({
url: 'geojson/countries.geojson',
projection: 'EPSG:3857',
format: new ol.format.GeoJSON()
})
})
],
target: 'map',
view: new ol.View({
center: [0, 0],
zoom: 2
})
});
</script>
</body>
这里先加入ol.js，建立ol.Map类，三要素，layers，target，view
ol.layers.Vector为矢量图层，这里有个坑，
会报错：from origin null has been blocked by cors policy: cross origin requests are only supported for protocol schemes:
需要做处理 在chrome浏览器快捷方式， 点击右键属性，快捷方式目标填入
-allow-file-access-from-files
然后，就行了