When drawing a graphicslayer with points, the marker symbols are created at the position of the points.By default, if you symbol is not centered, the top-left corner of the symbol will be at the position of the point. That explains the shift you pointed out.To center the symbol, the easier way is to apply an offset equal to half the size by using the MarkerSymbol OffsetX and OffsetY properties.In your case, the symbol size is 8, so this should do the trick:<esri:MarkerSymbol x:Key="SelectMarkerSymbol" OffsetX="4" OffsetY="4" >
<esri:MarkerSymbol.ControlTemplate>
<ControlTemplate>
<Ellipse x:Name="Element" Width="8" Height="8" Fill="blue" Stroke="blue" HorizontalAlignment="Center" VerticalAlignment="Center">
<......