OpenLayer OpenStreetMap矢量大小独立于Zoom

Question

我的OSM上有一个VectorLayer，我已经在Map上成功绘制了一个多边形。然而，正在发生的事情是，当我放大时多边形的尺寸变得太大或者尺寸越来越小。有没有办法保持多边形的大小独立于缩放？ 这是我绘制圆圈的代码

map = new OpenLayers.Map("container");
var mapnik = new OpenLayers.Layer.OSM();
epsg4326 = new OpenLayers.Projection("EPSG:4326");
var zoom           = 6; 
map.addLayer(mapnik);
map.addControl(new OpenLayers.Control.DragPan());
projectTo = map.getProjectionObject();
var lonLat = new OpenLayers.LonLat(-82.56,43.67611).transform(epsg4326, projectTo);
    map.setCenter(lonLat, zoom);
var styleMap = new OpenLayers.StyleMap(OpenLayers.Util.applyDefaults(
    {fillColor: "black", fillOpacity: 0.5, strokeColor: "black"},
OpenLayers.Feature.Vector.style["default"]));
var vectorLayer = new OpenLayers.Layer.Vector("Overlay", {styleMap: styleMap});
var point = new OpenLayers.Geometry.Point(lonLat.lon, lonLat.lat);
var mycircle = OpenLayers.Geometry.Polygon.createRegularPolygon
(
point,
50000,
50,
0
);
var featurecircle = new OpenLayers.Feature.Vector(mycircle);
vectorLayer.addFeatures([featurecircle]);
map.addLayer(vectorLayer);