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point-within-polygon

v1.0.8

Published

基于射线法实现快速判定点是否在面内

Downloads

4

Readme

point-within-polygon

基于射线法快速实现判别点在面内,在批量点和复杂面的情况下优化非常明显,通常有近20倍的性能提升。支持判别点在Polygon,点在MultiPolygon内的批量判断计算。

原理说明

默认射线法是通过比较点和segment的交点数量来确定点是否在面内,假设面有m个顶点(m-1个segment),有n个需要判别的点,则计算量为(m-1)*n,当m特别大时,面异常复杂,可能还会存在大量的孔洞,此时计算量会非常大,性能很低,如下图: 复杂面

本方案通过对面的segment建立rtree索引,从而避免逐线段比较,通过索引快速过滤出若干segment,导致计算量大大减少:

segement建立rtree索引

安装

npm install point-within-polygon --save

使用

支持在nodejs中使用,同时支持以es6形式在前端使用。

point_within_polygon(ptFeatures,pgFeature,withIndexs=1)

ptFeatures:GeoJSON的feature数组。

pgFeature:GeoJSON的Polygon或MultiPolygon面数据。

withIndexs:是否使用索引查询,默认1是基于Segment的rtree索引查询。

使用参考如下示例:

const point_within_polygon = require('point-within-polygon');
const pgFeature = require('./test.json');

//区间内随机生成1万个点
let pts = [];
for (let i = 0; i < 10000; i++) {
    const lon = 117.5 + (120.5 - 117.5) * Math.random();
    const lat = 31.5 + (33.5 - 31.5) * Math.random();
    pts.push({
        'type': 'Feature',
        'properties': {
            'id': i
        },
        'geometry': {
            'type': 'Point',
            'coordinates': [ lon, lat ]
        }
    })
}

console.time('基于索引查询');
const result1 = point_within_polygon(pts, pgFeature,1);
console.timeEnd('基于索引查询');
console.log('选择的要素数量:'+result1.length);


console.time('不基于索引查询');
const result2 = point_within_polygon(pts, pgFeature,0);
console.timeEnd('不基于索引查询');
console.log('选择的要素数量:'+result2.length);

性能比较

turf里也有点在面内的查询方法,详细参考:http://turfjs.org/docs/#pointsWithinPolygon

在性能比较时,1万点在面内计算耗时,执行测试:

npm run test
基于索引查询: 367.796ms
选择的要素数量:1718
不基于索引查询: 3.663s
选择的要素数量:1718
turf查询: 4.285s
选择的要素数量:1718

带索引的查询耗时远远优于默认的射线法耗时,尤其在点足够多,面足够复杂的情况下提升更明显,turf性能比无索引查询性能更慢。