OpenLayers Measure

var options_M = {'displayClass': 'olControlDrawFeature',title:'测距',drawFeature: myDrawFeature_M};
var M = new OpenLayers.Control.DrawFeature(measureVector, OpenLayers.Handler.Path,options_M);
function myDrawFeature_M(geometry)
{
var feature_M = new OpenLayers.Feature.Vector(geometry);
var diam = 6370;
var consta=2;
var grad=360;
var result;
var dist = feature_M.geometry.getLength();
result = ( diam * consta * Math.PI ) * (dist / grad );
$('info').innerHTML = "距离是: " + result.toFixed(2) + " Km";
feature_M.destroy();
measureVector.addFeatures([feature_M]);
M.featureAdded(feature_M);
}