3D Analyst/Surface Analyst/Area and Volume Statistics inconsistent why!?

[Scott_Atherton]

I am using ArcMap 9.3.1, 3D Analyst/Surface Analyst/Area and Volume Statistics to calculate some statistics on a TIN we have for a lake. The two options for calculating statistics are “Calculate Statistics above plane” and “Calculate Statistics below plane”. It says my Z max is 14.19 and my Z factor is 1. When I enter my Height of plane as 0 and calculate the statistics above the plane it gives me these results:

And when I enter my Height of plane as 14.19 and hit calculate statistics below plane I get these results:

These numbers should be the same should they not be? am I missing something here? Any explanation would be greatly appreciated,
Thanks in advance!

[jbswain]

Please take a look at this help documentation on Surface Volume. When you are changing the plane, then you have to consider what is being asked. The reason your values are different is that there are vastly different areas being surveyed depending on your data.

[Scott_Atherton]

Thank you for your response, Ok that explains why the area and volume are different, what I was really struggling with was why is the 2d surface area not the same. When I calculate these statistics for the lake (depth expressed in positive numbers) the plane would be set to 0 and I would be “calculating above the line”, however, that 2d surface area does not match up with the area calculated with the field calculator, it is the “calculate below the line” 2d surface area that matches up with the field calculator. In the image you posted above the blue line would represent your 2d surface area and it remains consistent regardless of the above/below calculation, or am I mistaken on this point?

[jbswain]

Ok so that I am following along, you are comparing the 2d Surface Area did not match the calculated geometry of the shape, the raster was built on? But only for the 'below' calculation, not the 'above.' Is that correct?

In the screenshots, the plane height is changing so it will definitely not match. I would believe if you have selected 0 for the height in each case the 2d area should match, but will not if you change the plane height, since the cross section you are comparison will change.