地震数据聚类【译】

原文链接及内容

原文链接：https://openlayers.org/en/latest/examples/earthquake-clusters.html

此示例解析一个 KML 文件，并将要素以聚类形式渲染到矢量图层上。本示例的样式设计较为复杂。单个地震点（渲染为五角星）的大小与其震级相关。聚类点的透明度与聚类中包含的要素数量相关，其大小反映了组成聚类的要素的地理范围。当点击或悬停在聚类点上时，会显示组成该聚类的单个要素。

为实现这一效果，我们广泛使用了样式函数。

main.js代码如下：

import KML from 'ol/format/KML.js';
import Map from 'ol/Map.js';
import View from 'ol/View.js';
import {
  Circle as CircleStyle,
  Fill,
  RegularShape,
  Stroke,
  Style,
  Text,
} from 'ol/style.js';
import {Cluster, Stamen, Vector as VectorSource} from 'ol/source.js';
import {
  Select,
  defaults as defaultInteractions,
} from 'ol/interaction.js';
import {Tile as TileLayer, Vector as VectorLayer} from 'ol/layer.js';
import {createEmpty, extend, getHeight, getWidth} from 'ol/extent.js';

const earthquakeFill = new Fill({
  color: 'rgba(255, 153, 0, 0.8)',
});
const earthquakeStroke = new Stroke({
  color: 'rgba(255, 204, 0, 0.2)',
  width: 1,
});
const textFill = new Fill({
  color: '#fff',
});
const textStroke = new Stroke({
  color: 'rgba(0, 0, 0, 0.6)',
  width: 3,
});
const invisibleFill = new Fill({
  color: 'rgba(255, 255, 255, 0.01)',
});

/**
 * 为单个地震点生成五角星样式，基于震级调整大小。
 */
function createEarthquakeStyle(feature) {
  /**
   * 2012_Earthquakes_Mag5.kml 在每个 Placemark 中的非标准<magnitude>标签中存储了每次地震的震级。
   * 我们从 Placemark 的名称中提取这些信息。
   */
  const name = feature.get('name');
  // magnitude是震级是意思
  const magnitude = parseFloat(name.substr(2));
  //根据震级确定圆的半径
  const radius = 5 + 20 * (magnitude - 5);

  return new Style({
    geometry: feature.getGeometry(),
    image: new RegularShape({
      // 当提供 radius 时将生成正多边形，同时提供 radius 和 radius2 时则生成星形。
      radius: radius,
      radius1: 3,
      points: 5,
      angle: Math.PI,
      fill: earthquakeFill,
      stroke: earthquakeStroke,
    }),
  });
}

let maxFeatureCount;
let vector = null;
/**
 * 计算聚类点的半径和最大点要素数量，基于地图分辨率。
 */
const calculateClusterInfo = function (resolution) {
  maxFeatureCount = 0;
  const features = vector.getSource().getFeatures();
  let feature, radius;
  for (let i = features.length - 1; i >= 0; --i) {
    feature = features[i];
    const originalFeatures = feature.get('features');
    const extent = createEmpty();
    let j, jj;
    for (j = 0, jj = originalFeatures.length; j < jj; ++j) {
      extend(extent, originalFeatures[j].getGeometry().getExtent());
    }
    maxFeatureCount = Math.max(maxFeatureCount, jj);
    radius = (0.25 * (getWidth(extent) + getHeight(extent))) / resolution;
    feature.set('radius', radius);
  }
};

let currentResolution;
/**
 * 为聚类点或单个点生成样式，动态调整透明度和半径。
 */
function styleFunction(feature, resolution) {
  if (resolution != currentResolution) {
    calculateClusterInfo(resolution);
    currentResolution = resolution;
  }
  let style;
  const size = feature.get('features').length;
  if (size > 1) {
    style = new Style({
      image: new CircleStyle({
        radius: feature.get('radius'),
        fill: new Fill({
          color: [255, 153, 0, Math.min(0.8, 0.4 + size / maxFeatureCount)],
        }),
      }),
      text: new Text({
        text: size.toString(),
        fill: textFill,
        stroke: textStroke,
      }),
    });
  } else {
    const originalFeature = feature.get('features')[0];
    style = createEarthquakeStyle(originalFeature);
  }
  return style;
}

/**
 * 为选中（悬停或点击）的点生成样式，显示所有原始点。
 */
function selectStyleFunction(feature) {
  const styles = [
    new Style({
      image: new CircleStyle({
        radius: feature.get('radius'),
        fill: invisibleFill,
      }),
    }),
  ];
  const originalFeatures = feature.get('features');
  let originalFeature;
  for (let i = originalFeatures.length - 1; i >= 0; --i) {
    originalFeature = originalFeatures[i];
    styles.push(createEarthquakeStyle(originalFeature));
  }
  return styles;
}

vector = new VectorLayer({
  source: new Cluster({
    distance: 40,
    source: new VectorSource({
      url: 'data/kml/2012_Earthquakes_Mag5.kml',
      format: new KML({
        extractStyles: false,//忽略 KML 文件中的样式
      }),
    }),
  }),
  style: styleFunction,
});

const raster = new TileLayer({
  source: new Stamen({
    layer: 'toner',
  }),
});

const map = new Map({
  layers: [raster, vector],
  interactions: defaultInteractions().extend([
    new Select({
      condition: function (evt) {
        return evt.type == 'pointermove' || evt.type == 'singleclick';
      },
      style: selectStyleFunction,
    }),
  ]),
  target: 'map',
  view: new View({
    center: [0, 0],
    zoom: 2,
  }),
});

界面布局文件index.html代码如下：

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <title>Earthquake Clusters</title>
    <link rel="stylesheet" href="node_modules/ol/ol.css">
    <style>
      .map {
        width: 100%;
        height: 400px;
      }
    </style>
  </head>
  <body>
    <div id="map" class="map"></div>
    <!-- Pointer events polyfill for old browsers, see https://caniuse.com/#feat=pointer -->
    <script src="https://cdn.jsdelivr.net/npm/elm-pep@1.0.6/dist/elm-pep.js"></script>
    <script type="module" src="main.js"></script>
  </body>
</html>