I need an Arrow to be at the Top Center of my Android Screen

For whatever reason the following layout places the arrow in the top left of the screen and I would like it centered horizontally and at the top

<LinearLayout
  xmlns:android="http://schemas.android.com/apk/res/android"
  android:layout_width="fill_parent"
  android:layout_height="fill_parent">
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        <RelativeLayout
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:layout_gravity="center_horizontal"  
        >   

          <com.google.android.maps.MapView
                android:id="@+id/map2"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                android:apiKey="0bjRuqXOuK5dP5guXo0LV5aUbn_s-aYzsmeuawA"
                android:clickable="true"
           />


            <edu.elon.ecs.GreenArrow
                android:id="@+id/greenarrow"
                android:layout_width="wrap_content" 
                android:layout_height="wrap_content" 
                android:gravity="center"                
                android:src="@drawable/greenarrow"
            />
        </RelativeLayout> 

</LinearLayout>

try this

<edu.elon.ecs.GreenArrow
                android:id="@+id/greenarrow"
                android:layout_width="wrap_content" 
                android:layout_height="wrap_content" 
                android:layout_centerHorizontal="true"
            android:layout_alignParentTop="true"
                android:gravity="center"                
                android:src="@drawable/greenarrow"
            />