There are various formulae for the area depending on what degree of accuracy you require, and how many width measurements you want to make in addition to the usual tip-mid-tail data.
First, estimate the contact length, LC. If you have the skis available, you can just put the two skis bottom to bottom, and measure the distance between the contact points. If you don't have the skis available to measure, you can estimate the contact length as the published (chord length) minus about 15 cm.
Next, you need some width data. Lets assume you don't have access to the skis and just have the usual three published widths.
To a very good approximation, you can approximate the load bearing area of the ski as the sum of the areas of three rectangles:
1) a tip region which is about LC/4 long by W_tip wide;
2) a middle region which is about LC/2 long by W_mid wide; and
3) a tail region which is about LC/4 long by W_tail wide.
Thus, the entire formula is:
A_total = LC * (W_tip/4 + W_mid/2 + W_tail/4)
This formula is of course approximate, but it should give you absolute area numbers good to a few percent, and, perhaps even more importantly, allow you to get relative areas of one ski vs another to a very high degree of precision.
Hope this helps,
PS - I just noticed your last sentence: "...Which would feel longer.."
The load bearing area of a ski really is important only in powder. The load bearing area determines the pressure the ski exerts on the snow, and hence how much you will compress (ie, sink into) soft snow.
Other quantities such as polar moment of inertia, sidecut, flex and flex distribution, initial camber, etc. are much more important in determining how "turny" a given ski feels in all conditions (incl powder).
There are, of course, ways to estimate all of these quantities, but they all require measurements that most private individuals would not be able to make, and/or detailed data which unfortunately is not published.
[This message has been edited by PhysicsMan (edited June 22, 2001).]</FONT>