[AS3]Splitting bitmapData into tiles

Hi everyone:hugegrin:

Im pretty sure this is easy, but i will ask anyway:P
I have a big movieClip that contains a level for my game.
I use bitmapData.draw() to draw it into a bitmap and use bitmapData.hitTest() to check for collisions with walls etc.
Now if i draw a realy high/wide level flash throws this error:

ArgumentError: Error #2015: Invalid BitmapData.
at flash.display::BitmapData()

or the whole level dosen`t show up:P

So i thougt the solution would be to split up the bitmapData in tiles.
Something like this:

(The red squares are tiles…)

How would i do this? And would it ruin my bitmapData.hitTest() ??

Thanks in advance:D