Why is mouse.GetPostion returning the same value everywhere on canvas?

I'm coding a fixed-size map on a canvas that is built from polygon user controls that then has another user control for game counters added on top. As I play around with different user interface options, the first thing I wanted to do was get the Mouse position when the left button is clicked. When the mouse is clicked, regardless of where on the canvas the mouse is I get the same x and y values returned. Also, the canvas is inside a scroll viewer as the Canvas/map is larger than the window, when I move the window, the returned value changes dependent on where the top left hand corner of the windo is, but doesn't chnage if I click different points within the canvas/ I'm obviously missing something with my implimentation. XAML and C# code below, any advice would be appreciated.

XAML:

<DockPanel>
        <ToolBarTray DockPanel.Dock="Top">
            <ToolBar >
                <Menu>
                    <MenuItem Header="Turn Options">
                        <MenuItem Header="Advance Phase"/>
                        <MenuItem Header="End Turn" />
                        <Separator/>
                    </MenuItem>
                        <MenuItem Header="Map Options">
                        <MenuItem Header="Load Map"/>
                    </MenuItem>
                    <Separator />
                    <MenuItem Header="Designer Mode"/>
                    <Separator/>
                    <MenuItem Header="Economics">
                        <MenuItem Header="Open Economics"/>
                    </MenuItem>
                    <Separator/>
                    <MenuItem Header="Game Stats"/>
                </Menu>
            </ToolBar>
        </ToolBarTray>
        <ScrollViewer DockPanel.Dock ="Top" VerticalScrollBarVisibility="Visible" HorizontalScrollBarVisibility="Visible" >
        <Canvas x:Name="MainMap" Background="Black" Height="1740" Width="4740" IsHitTestVisible="true" MouseDown="MainMap_MouseClick" >
            
        </Canvas>
        </ScrollViewer>
    </DockPanel>

private void MainMap_MouseClick(object sender, MouseButtonEventArgs e)
{
    try
    {
        Control src = e.Source as Control;

        if (e.Source is GameCounter)
        {
            if (src != null)
            {
                switch (e.ChangedButton)
                {
                    case MouseButton.Left:
                        GameCounter clickedGameCounter = (GameCounter)e.Source;
                        Point p = Mouse.GetPosition(MainMap );
                        MessageBox.Show("X = " + p.X + " Y = " + p.Y);
                        break;
                }
            }
        }
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
    }
}