I can think of a few methods, such as:

Put CDS cells on a grid beneath a table of plexiglass, and shine light from
above.

Use strips of metal cross-wise to each other with conductive foam between them,
and scan it like a keypad with an a/d converter

Shine lasers across the surface(about 2-5mm above the surface) in a grid pattern

If the objects have or can have magnetic properties, place a grid of coils
beneath the surface

Place a video camera above the surface, and make sure the colors contrast
between the surface and the objects

I hope this helps!

-Adam

MacBrick wrote:
>
> Hi,
>
> I am looking for suggestions to accomplish the following.
> I have a board of about 4 by 3 feet. Subjects in a study are t place objects
> on this board. I need to know the location where they pick up an object and
> the location where they place it. The resolution is not that important, 16 x
> 32 squares or similar is sufficient.
>
> Is there a simple way to accomplish this? I have constructed a similar
> set-up but on larger scale (50 x 100 feet or so) using a matrix made up of
> steel strips covered with a carpet. Something similar, but way smaller would
> be nice.
> I have also thought of a touch screen as used in PDA's. Does anyone know how
> to construct something like that on a larger scale?
>
> Other suggestions/experiences are welcome,
>
> Ronald Leenes