Difference between ABOVE and BELOW Reference Plane in Surface Volume tool

[LouisChora]

I'm calculating the volume of a lake TIN using the Surface Volume tool. Specifically, I'm calculating the volume at every 0.5 metres and then calculating the volume at 1 metre bands down to the bottom (i.e. volume between 20 m and 20.5 m; volume between 20.5 m and 21 m, etc).

I wasn't clear which reference plane to use, so I chose both in separate analyses. My assumption was that it wouldn't matter because the volume should be same (we're talking about the same 0.5 metre band). But the results were very different. Using the BELOW reference plane for 20/20.5 m I got approximately 85,000 m³ and using the ABOVE reference plane for the same band I got approximately 1.3 million m³.

Can someone explain this? I've looked at Surface Volume ESRI help file, but still I can't get my head around it.

Any help would be much appreciated!

Louis

[CamPatterson]

Louis,
Try using the Polygon Volume tool with the BELOW option to compute the water volume in your lake at various elevations. Create your shoreline polygons for the various elevations by creating contours, then convert selected closed contours to polygons. The tool will add volume and surface area attributes to your lake elevation polygons.
-Cam

[CamPatterson]

Louis,
To answer your original question, the ABOVE option measure the volume of subsurface earth between your reference elevation and the subject surface model above; the BELOW option measure the volume of air or water between your reference elevation and the surface below. To use the Surface Volume tool rather than the Polygon Volume tool, you need to use the BELOW option, and make sure that the only part of the surface below your reference plan is inside the lake... in other words there can't be areas of sub-water level elevation outside the lake in your overall surface.
-Cam