将折线转换为 GeoJSON

Question

我从 Google Maps / Open Street Maps 等服务中捕获了一系列（编码或解码）折线。

例如：

var polylines = ["kclaG|i_qLe@i@{AgBu@_AsCyCiBmBMMW[eCiC}A_BEEeBiB{@}@MW]a@a@e@]a@IGGG_AgAm@i@MOYYMEQSCCi@k@KKu@y@{A}Ay@cA{@{@eBiBOMe@k@_@a@e@e@QQY[cAkAUWMOUWu@w@AACCYY?AMKUUSSSQ]]GGECCCECA?AAA?A?C?G?WB"
       ,"yfnaGld}pL?e@?]AuDAgA?KAkBGqG?{C?_B?S?aC?[?]A}A?wAASAQ"
       ,"qmnaGlxxpLn@tEL~@"]

我想将这些存储为 GeoJSON。我找到了很多通过 Leaflet 之类的工具呈现 GeoJSON 的包，但是我很难找到可以将折线导出到 GeoJSON 的包。

是否存在这样的解决方案，或者是否有我应该研究的 GeoJSON 标准的特定子集，以便我可以编写工具 myset？

旁注：我最终想要一个 KML 或 SHP 文件。我打算使用http://ogre.adc4gis.com/从 GeoJSON 转换为 SHP。

Answer 1

您正在寻找 GeoJSON LineString 几何类型。虽然算法有点复杂，但Github 上有一个项目提供了 JavaScript 中的功能。您应该能够轻松地将其移植到您选择的语言。我还将该功能封装到 JSFiddle 中，以便您可以将其用作在线工具。

您可以在https://jsfiddle.net/ivansams/tw7qLvh4/2/找到该工具- 您可以将编码的折线粘贴到“编码折线”中，然后单击“解码”，这将返回 GeoJSON LineString。下面复制了代码，因为 StackOverflow 的规则要求 JSFiddle 附带代码，但可以在 Github 项目站点获取最新代码。

function decode() {
  var encodedPolyline = document.getElementById("encoded-polyline").value;
  document.getElementById("geojson").value = encodedPolyline;
  var coords = polyline.toGeoJSON(encodedPolyline);
  document.getElementById("geojson").value = JSON.stringify(coords);
}

var polyline = {};

function encode(current, previous, factor) {
  current = Math.round(current * factor);
  previous = Math.round(previous * factor);
  var coordinate = current - previous;
  coordinate <<= 1;
  if (current - previous < 0) {
    coordinate = ~coordinate;
  }
  var output = '';
  while (coordinate >= 0x20) {
    output += String.fromCharCode((0x20 | (coordinate & 0x1f)) + 63);
    coordinate >>= 5;
  }
  output += String.fromCharCode(coordinate + 63);
  return output;
}

/**
 * Decodes to a [latitude, longitude] coordinates array.
 *
 * This is adapted from the implementation in Project-OSRM.
 *
 * @param {String} str
 * @param {Number} precision
 * @returns {Array}
 *
 * @see https://github.com/Project-OSRM/osrm-frontend/blob/master/WebContent/routing/OSRM.RoutingGeometry.js
 */
polyline.decode = function(str, precision) {
  var index = 0,
    lat = 0,
    lng = 0,
    coordinates = [],
    shift = 0,
    result = 0,
    byte = null,
    latitude_change,
    longitude_change,
    factor = Math.pow(10, precision || 5);

  // Coordinates have variable length when encoded, so just keep
  // track of whether we've hit the end of the string. In each
  // loop iteration, a single coordinate is decoded.
  while (index < str.length) {

    // Reset shift, result, and byte
    byte = null;
    shift = 0;
    result = 0;

    do {
      byte = str.charCodeAt(index++) - 63;
      result |= (byte & 0x1f) << shift;
      shift += 5;
    } while (byte >= 0x20);

    latitude_change = ((result & 1) ? ~(result >> 1) : (result >> 1));

    shift = result = 0;

    do {
      byte = str.charCodeAt(index++) - 63;
      result |= (byte & 0x1f) << shift;
      shift += 5;
    } while (byte >= 0x20);

    longitude_change = ((result & 1) ? ~(result >> 1) : (result >> 1));

    lat += latitude_change;
    lng += longitude_change;

    coordinates.push([lat / factor, lng / factor]);
  }

  return coordinates;
};

/**
 * Encodes the given [latitude, longitude] coordinates array.
 *
 * @param {Array.<Array.<Number>>} coordinates
 * @param {Number} precision
 * @returns {String}
 */
polyline.encode = function(coordinates, precision) {
  if (!coordinates.length) {
    return '';
  }

  var factor = Math.pow(10, precision || 5),
    output = encode(coordinates[0][0], 0, factor) + encode(coordinates[0][1], 0, factor);

  for (var i = 1; i < coordinates.length; i++) {
    var a = coordinates[i],
      b = coordinates[i - 1];
    output += encode(a[0], b[0], factor);
    output += encode(a[1], b[1], factor);
  }

  return output;
};

function flipped(coords) {
  var flipped = [];
  for (var i = 0; i < coords.length; i++) {
    flipped.push(coords[i].slice().reverse());
  }
  return flipped;
}

/**
 * Encodes a GeoJSON LineString feature/geometry.
 *
 * @param {Object} geojson
 * @param {Number} precision
 * @returns {String}
 */
polyline.fromGeoJSON = function(geojson, precision) {
  if (geojson && geojson.type === 'Feature') {
    geojson = geojson.geometry;
  }
  if (!geojson || geojson.type !== 'LineString') {
    throw new Error('Input must be a GeoJSON LineString');
  }
  return polyline.encode(flipped(geojson.coordinates), precision);
};

/**
 * Decodes to a GeoJSON LineString geometry.
 *
 * @param {String} str
 * @param {Number} precision
 * @returns {Object}
 */
polyline.toGeoJSON = function(str, precision) {
  var coords = polyline.decode(str, precision);
  return {
    type: 'LineString',
    coordinates: flipped(coords)
  };
};

if (typeof module === 'object' && module.exports) {
  module.exports = polyline;
}