从osm Overpass API读取和分析JSON-获取折线

Question

with Overpass API I get this (polyline) data from OSM as JSON file:

{
  "version": 0.6,
  "generator": "Overpass API",
  "elements": [
{
  "type": "node",
  "id": 308240514,
  "lat": 52.7074546,
  "lon": 7.1369361
},
{
  "type": "node",
  "id": 308729130,
  "lat": 52.6934662,
  "lon": 7.1353250
},
......
.......
.......
{
  "type": "way",
  "id": 99421713,
  "nodes": [
    1149813380,
    2103522316,
    2103522207,
    2103522202,
    2103522201,
    .....
    ....
    ],
      "tags": {
    "admin_level": "2",
    ......
  }
},
{
  "type": "way",
  "id": 99421718,
  "nodes": [
    647317213,
    2103495916,
    2103495906,
    2103495902,
    2103495901,
    ....
    ....
    ....
    ]

For printing the polylines (ways) in a mapping application like Google Maps API I need to get the coordinates (lat,lon in JSON) assigned to the ways (type:way) by the numbers in the nodes array - these numbers are the id's of the coordinates. As result I need something like this:

"coords":{
"way1" : [(37.772323, -122.214897), (21.291982, -157.821856),(-18.142599, 178.431),(-27.46758, 153.027892)],
"way2" : [(37.772323, -122.214897),...........] 

I used jquery to get the JSON file and then loop through the data, so I could get the coordinates, but not assigned to the ways and also not in the right order like the nodes array.

$.getJSON(url, function(data) {
   $.each(data.elements, function(i,data){
      var coords = (data.lat,data.lon);
      .........

Anyone has an idea how to solve my problem? Is jquery solutions for this or it is better to use native javascript?

... 2 days later:

After a few hours of testing and trying at least I found a solution for my problem. Here is the javascript-code:

$.getJSON('test.js', function(data) {

var ways = [];
var way_nodes = [];
var inhalt = [];

for (var x in data.elements) {
    if (data.elements[x].type == "way") {
        var way_tmp = data.elements[x].nodes;
        ways.push(way_tmp);
    }
    if (data.elements[x].type == "node") {
        inhalt = data.elements;
    }
}

for (var h in ways) {
    var mypath = [];
    way_nodes = ways[h];
    for (var k in way_nodes) {
        for (var x in inhalt) {
            if (way_nodes[k] == inhalt[x].id) {
                var coords = new google.maps.LatLng(inhalt[x].lat,inhalt[x].lon);  
                mypath.push(coords);
            }
        }
    }

    var polyline = new google.maps.Polyline({
        path: mypath,
        strokeColor: "#FF0000",
        strokeOpacity: 0.6,
        strokeWeight: 5
    });

    var poly_points = polyline.getPath();
    for (var i = 0; i < poly_points.length; i++) {
        bounds.extend(poly_points.getAt(i));
    }   
    polyline.setMap(map);
}
    map.fitBounds(bounds);
});

And here is a link to a working example displaying with the Google Maps API: http://www.ralf-wessels.de/test/apiv3/json/04map_osm_viele_polylines_structured.html# I don't know if this is the smartest way to solve the problem, specially if I work with big data. If someone knows a better way, I'm interested in this.

Answer 1

For your data manipulation needs, I'd suggest looking into functional libraries like Lodash . Or better yet, Ramda . Lodash is more popular, Ramda is more convenient with emphasis on currying and function composition. Both share the upside of breaking things down to small, easily manageable parts.

There's a bit of a learning curve, but after learning such a tool, you'll find just how much of a pain data manipulation with for-loops was.

For example with Ramda, the same functionality could be achieved like this:

var parseWaysFromResponse = (function () {
    // function [{id:1, key1:val1 ...}, {id:2, key2:val2}]
    //    -> {1:{key1:val1 ...}, 2:{key2:val2 ...}}
    var generateIdToNodeMap = R.compose(
        R.mapObj(R.head),
        R.groupBy(R.prop('id'))
    );

    // Filter array of objects based on key "type"
    var elementTypeIs = function(typeVal) {
      return R.propEq('type', typeVal);
    }

    // Create {id:{values}} from the apiResponse elements
    var getNodes = R.compose(
      generateIdToNodeMap,
      R.filter(elementTypeIs('node'))
    );

    // Api elements -> [[way1 node-id1, id2, ...], [way 2 node-id1, ...]]
    var getWayNodes = R.compose(
      R.pluck('nodes'),
      R.filter(elementTypeIs('way'))
    );

    // Map generated by getNodes, node id -> [lat, lon] of given node id
    var linkNodeIdToCoords = R.curry(function (nodes, id) {
        return R.props(['lat', 'lon'], nodes[id])
    });

    return function (apiResponse) {
        var nodes = getNodes(apiResponse.elements);
        var getAllWays = R.compose(
            R.map(R.map(linkNodeIdToCoords(nodes))),
            getWayNodes
        );
        return getAllWays(apiResponse.elements)
    }
})();