diff --git a/src/org/oscim/renderer/overlays/BasicOverlay.java b/src/org/oscim/renderer/overlays/BasicOverlay.java
index 742c5527..b63f0760 100644
--- a/src/org/oscim/renderer/overlays/BasicOverlay.java
+++ b/src/org/oscim/renderer/overlays/BasicOverlay.java
@@ -43,6 +43,9 @@ public abstract class BasicOverlay extends RenderOverlay {
 		layers = new Layers();
 	}
 
+	/**
+	 * use synchronized when modifying layers
+	 */
 	@Override
 	public synchronized void render(MapPosition pos, float[] mv, float[] proj) {
 		setMatrix(pos, mv);
diff --git a/src/org/oscim/renderer/overlays/ExtrusionOverlay.java b/src/org/oscim/renderer/overlays/ExtrusionOverlay.java
index 2549bfbe..1ab18cf3 100644
--- a/src/org/oscim/renderer/overlays/ExtrusionOverlay.java
+++ b/src/org/oscim/renderer/overlays/ExtrusionOverlay.java
@@ -62,7 +62,7 @@ public class ExtrusionOverlay extends RenderOverlay {
 	private int mTileCnt;
 
 	@Override
-	public synchronized void update(MapPosition curPos, boolean positionChanged,
+	public void update(MapPosition curPos, boolean positionChanged,
 			boolean tilesChanged) {
 
 		mMapView.getMapViewPosition().getMapPosition(mMapPosition, null);
@@ -160,7 +160,7 @@ public class ExtrusionOverlay extends RenderOverlay {
 	private final float[] mVPMatrix = new float[16];
 
 	@Override
-	public synchronized void render(MapPosition pos, float[] mv, float[] proj) {
+	public void render(MapPosition pos, float[] mv, float[] proj) {
 		// TODO one could render in one pass to texture and then draw the texture
 		// with alpha... might be faster.