moving a circle in google maps api

Question

Ok so I'm learning the google maps API and I'm trying to get a point to move real time based on the long and lat(exactly how google maps works with the blue dot)

No unless I'm mistaken it looks like I have to draw my own circle and update it's position.

The problem is I just can't get the circle to get updated.

here is my code:

var citymap = {};
citymap['chicago'] = {
  center: new google.maps.LatLng(41.878113, -87.629798),
  population: 2842518
};

var cityCircle;

function initialize() {
  // Create the map.
  var mapOptions = {
    zoom: 4,
    center: new google.maps.LatLng(37.09024, -95.712891),
    mapTypeId: google.maps.MapTypeId.TERRAIN
  };

  var map = new google.maps.Map(document.getElementById('map-canvas'),
      mapOptions);

  // Construct the circle for each value in citymap.
  // Note: We scale the population by a factor of 20.
  for (var city in citymap) {
    var populationOptions = {
      strokeColor: '#fff',
      strokeOpacity: 0.8,
      strokeWeight: 2,
      fillColor: '#fff',
      map: map,
      center: citymap[city].center,
      radius: 50000,

    };
    // Add the circle for this city to the map.
    cityCircle = new google.maps.Circle(populationOptions);
    var a=0;
    window.setInterval(function(){
        var p = populationOptions.center;
        var g = p.lat()+500;
        var m = p.lng()+500;    
        cityCircle.setCenter(new google.maps.LatLng(g,m));
        cityCircle.setRadius(a+=10);
    }, 50);
  }
}

I was actually quite surprised google's api doesn't just expose that blue marker and you can just move it when you need to.

Answer 1

Your update function is incorrect. Any valid latitude + 500 will be off the map. A more reasonable number would be 0.1 degrees. Also, instead of populationOptions.center you should use cityCircle.getCenter().

// Add the circle for this city to the map.
cityCircle = new google.maps.Circle(populationOptions);
var a=50000;
window.setInterval(function(){
    var p = cityCircle.getCenter();
    var g = p.lat()+0.1;
    var m = p.lng()+0.1;    
    cityCircle.setCenter(new google.maps.LatLng(g,m));
    cityCircle.setRadius(a+=10);
}, 500);

working example

code snippet:

html,
body,
#map-canvas {
  height: 100%;
  margin: 0px;
  padding: 0px
}

<script src="https://maps.googleapis.com/maps/api/js?key=AIzaSyCkUOdZ5y7hMm0yrcCQoCvLwzdM6M8s5qk"></script>
<div id="map-canvas"></div>
<script>
  var citymap = {};
  citymap['chicago'] = {
    center: new google.maps.LatLng(41.878113, -87.629798),
    population: 2842518
  };

  var cityCircle;

  function initialize() {
    // Create the map.
    var mapOptions = {
      zoom: 4,
      center: new google.maps.LatLng(37.09024, -95.712891),
      mapTypeId: google.maps.MapTypeId.TERRAIN
    };

    var map = new google.maps.Map(document.getElementById('map-canvas'),
      mapOptions);

    // Construct the circle for each value in citymap.
    // Note: We scale the population by a factor of 20.
    for (var city in citymap) {
      var populationOptions = {
        strokeColor: '#FF0000',
        strokeOpacity: 0.8,
        strokeWeight: 2,
        fillColor: '#FF0000',
        fillOpacity: 0.35,
        map: map,
        center: citymap[city].center,
        radius: 50000

      };
      // Add the circle for this city to the map.
      cityCircle = new google.maps.Circle(populationOptions);
      // move the circle
      var a = 50000;
      window.setInterval(function() {
        var p = cityCircle.getCenter();
        var g = p.lat() + 0.1;
        var m = p.lng() + 0.1;
        cityCircle.setCenter(new google.maps.LatLng(g, m));
        cityCircle.setRadius(a += 10);
      }, 500);
    }
  }

  google.maps.event.addDomListener(window, 'load', initialize);
</script>