Function to create a semicircle with Google Maps API v3. Coordinated and azimuth data are required. This is an adaptation of https://groups.google.com/forum/#!topic/google-maps-api/rKefiTZPXE8

drawSector

function drawSector(lat, lng, r, azimuth, width) { 
  var centerPoint = new google.maps.LatLng(lat, lng); 
  var PRlat = (r/3963) * (180 / Math.PI); // using 3963 miles as earth's radius 
  var PRlng = PRlat/Math.cos(lat*((Math.PI / 180))); 
  var PGpoints = []; 
  PGpoints.push(centerPoint); 
 
  with (Math) { 
    lat1 = lat + (PRlat * cos( ((Math.PI / 180)) * (azimuth  - width/2 ))); 
    lon1 = lng + (PRlng * sin( ((Math.PI / 180)) * (azimuth  - width/2 ))); 
    PGpoints.push( new google.maps.LatLng(lat1,lon1)); 
 
    lat2 = lat + (PRlat * cos( ((Math.PI / 180)) * (azimuth  + width/2 ))); 
    lon2 = lng + (PRlng * sin( ((Math.PI / 180)) * (azimuth  + width/2 ))); 
 
    var theta = 0; 
    var gamma = ((Math.PI / 180)) * (azimuth  + width/2 ); 
 
    for (var a = 1; theta < gamma ; a++ ) { 
    theta = ((Math.PI / 180)) * (azimuth  - width/2 +a); 
    PGlon = lng + (PRlng * sin( theta )); 
    PGlat = lat + (PRlat * cos( theta )); 
 
    PGpoints.push(new google.maps.LatLng(PGlat,PGlon)); 
  }
 
  PGpoints.push( new google.maps.LatLng(lat2,lon2)); 
  PGpoints.push(centerPoint);
} 
 
  var poly = new google.maps.Polygon({
    path:PGpoints,
    strokeColor: '#4B0082',
    strokeOpacity: .2,
    map: map
  });
  
  poly.setMap(map);
  return poly; 
}