88 lines
2.7 KiB
Markdown
88 lines
2.7 KiB
Markdown
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---
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title: Feature
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date: 2020-10-10
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author: ac
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tags:
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- OpenLayers
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- Feature
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categories:
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- GIS
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---
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## 1. Feature
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> 我们把现实世界中实体，如：道路、行政边界、电线杆等等，抽象为要素（`Feature`）。
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一个要素（`Feature`）包含一个几何对象（`Geometry`）来表述物体的空间几何信息和属性（`Attributes`）来描述物体的其他信息。
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```
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Feature
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↙ ↘
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Geometry Attributes
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↙ ↓ ↘
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Point Polyline Polygon
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```
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## 2. ol中的Feature
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`ol`中`ol.Feature`作为一个矢量对象存储矢量数据，包含几何对象（`geometry`）和其他自定义的属性信息，即 `{k:v}`，其中有个geometry的key，其他都是自定义的key。
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`Feature`中常用的方法：
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- `setStyle(style)`:为要素渲染样式,style可以是单个也可以是数组；
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- `getGeometry()`：获取要素中的几何对象；
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- `setGeometryName()`：当要素在创建时的对象中有多个value值是几何对象时，可以通过该方法指定要渲染的几何对象。默认是key为geometry的几何对象。
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```javascript
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var feature = new Feature({
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geometry: new Polygon(polyCoords),
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labelPoint: new Point(labelCoords),
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name: 'My Polygon'
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});
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// get the polygon geometry
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var poly = feature.getGeometry();
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// Render the feature as a point using the coordinates from labelPoint
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feature.setGeometryName('labelPoint');
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// get the point geometry
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var point = feature.getGeometry();
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```
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创建`Feature`实例除了使用构造器以外，针对一些特殊格式（`XML`、`WKT`、`TopoJSON`、`KML`、`GML`等）的数据可以使用`format`解析器创建。
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```javascript
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var wktString = "POLYGON((12724298.889 2576595.308,12721929.341 2568340.109,12716960.934 2564900.445,12724298.889 2576595.308))";
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var wtkFeature = (new ol.format.WKT()).readFeature(wktString,{dataProject:'EPSG:3857',featureProject:'EPSG:3857'});
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```
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> `TopoJSON`是`GeoJSON`按拓扑学编码后的扩展形式。使用点、弧（有向线）来表示的图形
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## 3. 创建方式
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方式一：使用 Feature 构造器创建
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```javascript
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var pointFeature = new ol.Feature({
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geometry: new ol.geom.Point([12703928.421,2575539.521]),
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name: '点要素'
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});
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```
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方式二：使用 format 解析器创建，针对一些特殊格式的数据
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```javascript
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var wktString = "POLYGON((12724298.889 2576595.308,12721929.341 2568340.109,12716960.934 2564900.445,12724298.889 2576595.308))";
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//创建解析wkt格式的解析器对象
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var wktformat = new ol.format.WKT();
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//读取wkt文本
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var wtkFeature = wktformat.readFeature(wktString,{dataProject:'EPSG:3857',featureProject:'EPSG:3857'});
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```
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