Use Google maps API to draw a polyline that changes color

I'm working on a project that plots a track from GPX file onto google maps, and color-code it with an attribute (let's call it Score). I've done some editing that now there is an additional tag Score for each coordinate in this GPX file. Here is a snippet of my modified GPX file.

<trkpt lat="50.834048" lon="-0.127354"> <ele>31.0</ele>
<Score>-0.64730385</Score>
<time>2016-07-07T14:31:51Z</time>
</trkpt>

<trkpt lat="50.833833" lon="-0.127600"> <ele>31.0</ele> 
<Score>-0.647203</Score>
<time>2016-07-07T14:32:09Z</time>
</trkpt>

I'm learning Google maps' javascript API to do the plotting part. My question is, how to create a ployline that changes color based on Score? I understand that I can draw thousands of mini polylines with just two points, and set a color for each of them. But it feels very primitive. Are there better ways of doing it? Also, I could write my own function to parse this GPX file, but why doesn't google maps support GPX?

If it helps to illustrate my purpose, this is what I try to achieve: enter image description here

Answers:

Answer

One option would be to make a separate polyline for each line segment and assign it a color based on the Score in the first point of that segment.

for (var i=0;i<(trkpts.length-1);i++) {
  var coord1 = new google.maps.LatLng(parseFloat(trkpts[i].getAttribute("lat")),
                                      parseFloat(trkpts[i].getAttribute("lon")));
  var coord2 = new google.maps.LatLng(parseFloat(trkpts[i+1].getAttribute("lat")),
                                      parseFloat(trkpts[i+1].getAttribute("lon")));
  var score = parseFloat(nodeValue(trkpts[i].getElementsByTagName("Score")[0]));
  var polyline = new google.maps.Polyline({
    map: map,
    path: [coord1, coord2],
    strokeColor: rgb(-1,1,score),
    strokeWeight: 4,
    strokeOpacity: 1.0
  });
}

proof of concept fiddle

picture of fiddle

code snippet:

var map;
var bounds = new google.maps.LatLngBounds();

function initialize() {
  map = new google.maps.Map(
    document.getElementById("map_canvas"), {
      center: new google.maps.LatLng(34, 108),
      zoom: 13,
      mapTypeId: google.maps.MapTypeId.ROADMAP
    });
  var gpxStr = '<gpx><trk><trkseg><trkpt lat="50.834048" lon="-0.127354"> <ele>31.0</ele><Score>-0.64730385</Score><time>2016-07-07T14:31:51Z</time></trkpt><trkpt lat="50.833833" lon="-0.127600"> <ele>31.0</ele><Score>-0.647203</Score><time>2016-07-07T14:32:09Z</time></trkpt><trkpt lat="50.833715" lon="-0.127768"> <ele>31.0</ele><Score>0.647203</Score><time>2016-07-07T14:32:50Z</time></trkpt><trkpt lat="50.833171" lon="-0.128468"> <ele>31.0</ele><Score>0.99609375</Score><time>2016-07-07T14:32:50Z</time></trkpt><trkpt lat="50.832951" lon="-0.128771"> <ele>31.0</ele><Score>-0.5</Score><time>2016-07-07T14:32:50Z</time></trkpt></trkseg></trk></gpx>';

  var xml = parseXml(gpxStr);
  var trkpts = xml.getElementsByTagName("trkpt");
  var bounds = new google.maps.LatLngBounds();
  for (var i = 0; i < (trkpts.length - 1); i++) {
    var coord1 = new google.maps.LatLng(parseFloat(trkpts[i].getAttribute("lat")),
      parseFloat(trkpts[i].getAttribute("lon")));
    bounds.extend(coord1);
    var coord2 = new google.maps.LatLng(parseFloat(trkpts[i + 1].getAttribute("lat")),
      parseFloat(trkpts[i + 1].getAttribute("lon")));
    bounds.extend(coord2);

    var score = parseFloat(nodeValue(trkpts[i].getElementsByTagName("Score")[0]));
    var polyline = new google.maps.Polyline({
      map: map,
      path: [coord1, coord2],
      strokeColor: rgb(-1, 1, score),
      strokeWeight: 4,
      strokeOpacity: 1.0
    })

  }
  map.fitBounds(bounds);
}
google.maps.event.addDomListener(window, "load", initialize);

function rgb(minimum, maximum, value) {
  var ratio = 2 * (value - minimum) / (maximum - minimum);
  b = Math.floor(Math.max(0, 255 * (1 - ratio)));
  r = Math.floor(Math.max(0, 255 * (ratio - 1)));
  g = 255 - b - r;

  var hexStr = ("00" + r.toString(16)).slice(-2);
  hexStr += ("00" + g.toString(16)).slice(-2);
  hexStr += ("00" + b.toString(16)).slice(-2);
  hexStr = "#" + hexStr;
  return hexStr
}
function parseXml(str) {
    if (window.ActiveXObject) {
      var doc = new ActiveXObject('MicrosoftXMLDOM');
      doc.loadXML(str);
      return doc;
    } else if (window.DOMParser) {
      return (new DOMParser()).parseFromString(str, 'text/xml');
    }
  }
  //nodeValue: Extract the text value of a DOM node, with leading and trailing whitespace trimmed

function nodeValue(node, defVal) {
  var retStr = "";
  if (!node) {
    return (typeof defVal === 'undefined' || defVal === null) ? '' : defVal;
  }
  if (node.nodeType == 3 || node.nodeType == 4 || node.nodeType == 2) {
    retStr += node.nodeValue;
  } else if (node.nodeType == 1 || node.nodeType == 9 || node.nodeType == 11) {
    for (var i = 0; i < node.childNodes.length; ++i) {
      retStr += arguments.callee(node.childNodes[i]);
    }
  }
  return retStr;
};
html,
body,
#map_canvas {
  height: 100%;
  width: 100%;
  margin: 0px;
  padding: 0px
}
<script src="https://maps.googleapis.com/maps/api/js"></script>
<div id="map_canvas" style="border: 2px solid #3872ac;"></div>

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