Deck surface/heads? Surface finnish?

How is the RA "roughness average" determined? I see head gaskets companys saying that you need a RA 30 or a RA 50. I know it relates to how "flat" and "smooth" the surfaces are, but how is it measured and with what instrament?
Thanks, Brian

I had the same basic Q a couple of weeks ago ... and the cure-all(I think) .. is to get a set of copper head gaskets.

I have a 383 block that I am putting into a boat that will have 11 to 1 compression PLUS ... and the deck surface has some lite rust on it. I don't want to deck the block any further(don't want any more CR) ...so I think that the copper-gaskets will be the plan.

Mine are fine. I was just wondering how its measured? And with what?
Brian

here is a meter i found with a quick search.
http://www.tradekey.com/product_view/id/1090455.htm

Its done with a profilometer, but most machine shops know the rate of feed, RPM surface speed of their machines and can make a pretty good guess on how to surface either the head or the block.