How do I align the matrix used in SpriteBatch and the matrix used in BasicEffect?

I'm writing a Windows tool that uses XNA and allows the user to place textures on the screen.

I'm using a 'camera' for the SpriteBatch used to draw the textures, e.g:

Matrix matrixForSpriteBatch = Matrix.CreateTranslation(    -_cameraPosition.X, -_cameraPosition.Y, 0.0f) *Matrix.CreateRotationZ(_rotation) *Matrix.CreateScale(_scale) *Matrix.CreateTranslation(     GraphicsDevice.Viewport.Width / 2,      GraphicsDevice.Viewport.Height / 2, 0.0f);

This is working fine - the user can drop a texture onto the canvas and move, rotate and size it. They can also move the camera around to see more things.

What I now want to do is let the users click around to define points of a polygon. These points are then sent to my DrawPolygonFilled method:

public void DrawPolygonFilled( SpriteBatch spriteBatch,         Vector2[ ] worldPoints,         Color color ){    Matrix matrixForVertices =        Matrix.CreateTranslation( _cameraPosition.X, -_cameraPosition.Y, 0.0f ) *            Matrix.CreateRotationZ( -rotation ) *                Matrix.CreateScale( camera.Scale ) ;    Vector2[] transformedPoints = worldPoints.Transform( matrixForVertices ) ;    var vertices = new VertexPositionColor[ transformedPoints.Length ] ;    for (int i = 0; i < transformedPoints.Length; i++)    {        vertices[ i ] = new VertexPositionColor( new Vector3( transformedPoints[ i ], 0 ), color ) ;    }    Vector2[ ] outputVertices ;    short[ ] outputIndicies ;    Vertices.Triangulate( transformedPoints, Vertices.WindingOrder.Clockwise, out outputVertices, out outputIndicies ) ;    if (outputIndicies.Length > 0)    {        foreach( EffectPass pass in _basicEffect.CurrentTechnique.Passes )        {            pass.Apply( ) ;            spriteBatch.GraphicsDevice.DrawUserIndexedPrimitives<VertexPositionColor>(                Microsoft.Xna.Framework.Graphics.PrimitiveType.TriangleList,                vertices,                0,                vertices.Length,                outputIndicies,                0,                outputIndicies.Length /3 ) ;        }    }}

This isn't displaying anything as the client space for BasicEffect is 0,0 at top-left, and 1-1 at bottom-right.So I added this to BasicEffect:

_basicEffect = new BasicEffect( GraphicsDevice )    {        VertexColorEnabled = true    } ;_viewMatrix = Matrix.CreateLookAt(    new Vector3( 0, 0, GraphicsDevice.Viewport.Width ),    Vector3.Zero,    Vector3.Up    ) ;_projectionMatrix = Matrix.CreateOrthographic(    GraphicsDevice.Viewport.Width,    GraphicsDevice.Viewport.Height,    .1f,    GraphicsDevice.Viewport.Width ) ;_basicEffect.Projection = _projectionMatrix ;_basicEffect.View = _viewMatrix ;

With this, the polygon is displayed but it's upside-down. Also, when the position of the polygon changes (the user can drag the polygon around with the mouse), the polygon goes up when the user moves the mouse down!

I'm a bit thick (but trying to become less thick) when it comes to matrices and geometry. Am I missing something? I tried changing Down to Up in CreateLookAt but it still didn't draw correctly and now when the user mouses left, then polygon goes right!

Here's a screen-shot when drawing a polygon at 0,0:

The points on the white path are where the user clicked (and are drawn with SpriteBatch) and the yellow polygon is what's drawn by my method above.

Here's another screen-shot that show what happens when I move the polygon to 20,20:

The white path is drawn starting from 20,20, but the filled polygon is starting from 20,-20 and is drawn upside-down

I think this is a matrix issue between the matrix used by the SpriteBatch (to draw the white paths) and the matrix used by the BasicEffect (to draw the filled polygon).