Measuring Curvature

Put it on a flat plate and use a vernier gauge to measure how high the centre lifts off the surface?

Can we ask why?

Virtually all thin plastic sheet will have a 'grain' and a curve where it is rolled onto a reel.

I wanted flat sheet to build a variable orifice flap valve...even shim steel was curved.

So I built an automated shim straightener!

The most accurate method would be a contact-less system such as laser or optical measuring if you have access to it. If it's a one off check & you don't have such systems, ask some companies in your area. You will find most are happy to help out if they have the right equipment.

Get some chrome spray paint (the good stuff used in custom automotive) and chrome out that sucker. Then get a laser pointer and a tripod like structure. Put the film on something that you can move in one direction in certain distance increments. Make yourself a screen of some sort where you can project the reflected beam. Scan your film. With the incoming and outgoing beam angles you have the angle of tangency at each point. Get the idea?

This worked 4 me for "scanning" the hull curvature of a RC-boat couple years ago.

Disclaimer: This might not work 4 u. I don't know the dimensions of the object your trying to analyze.

Good luck!