geojson 数据抽稀

接上一篇手绘地图实现，其中通过借助工具将 svg 导出 geojson 数据，但是通常数据量非常大，而且有很多重复的点数据/非常相近的数据

这样的数据，虽然可以用，但是考虑到前端性能问题，需要将 geojson 数据进行点抽稀
目前有很多的抽稀算法，比如道格拉斯-普克算法、垂距限值法，这里暂时不做讨论

这里可以借助另外一个宝藏工具Turf（https://github.com/Turfjs/turf/）

Turf 的作用就是，对 geojson 数据进行抽稀

接下来我们对黄色板块进行数据抽稀操作

代码如下：

const turf = require('@turf/turf');
const fs = require('fs');
// geo数据
const coordinates = [
  [
    [119.75217242139553, 30.189759022525294],
    [119.75329376495, 30.191341051272516],
    [119.75388138579581, 30.193125439402152],
    [119.75389083220648, 30.19498061401667],
    [119.75364757017621, 30.19682967368732],
    [119.75340430814606, 30.198678727588927],
    [119.75316107959941, 30.200527746745593],
    [119.75291781756914, 30.202376731190043],
    [119.75267455549948, 30.20422565194926],
    [119.75243129346927, 30.206074566932116],
    [119.75218806492273, 30.20792344716493],
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    [119.74928406155641, 30.228207868281288],
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];

let geojson = turf.polygon(coordinates);
var options = { tolerance: 0.001, highQuality: false };
var simplified = turf.simplify(geojson, options);
console.log(
  '输入长度：',
  geojson.geometry.coordinates[0].length,
  simplified,
  '\n输出长度：',
  simplified.geometry.coordinates[0].length
);
fs.writeFile('./output.json', JSON.stringify(simplified.geometry), (err) => {
  if (err) {
    console.error(err);
    return;
  }
  console.log('写入文件成功');
  //文件写入成功。
});

如下，我们运行该 node 脚本查看输出，原 geojson 数据从 604 个点减少到了 55 个点

$ node utils.js
输入长度： 604 {
  type: 'Feature',
  properties: {},
  geometry: { type: 'Polygon', coordinates: [ [Array] ] }
}
输出长度： 55
写入文件成功

可能遇见的问题 First and last Position are not equivalent

/Users/aiyouwei/Desktop/test/node_modules/@turf/helpers/dist/js/index.js:238
                throw new Error("First and last Position are not equivalent.");
                ^

Error: First and last Position are not equivalent.
    at Object.polygon (/Users/aiyouwei/Desktop/test/node_modules/@turf/helpers/dist/js/index.js:238:23)

因为我这里的数据设定的是turf.polygon，所以有一些数据的coordinates数组中，首位可能有问题，所以需要手动修改一下原先的mapData，将coordinates[0]数组中的第一项拷贝一分放到末尾项