I created a 6-grid raster (download here, Fig.3) to show my question. Each raster is 10m*10m size. I used "Surface Volume" in "3D Analyst Tools" to calculate the area below 0m (Fig.1).
Since all six grids have a z value less than 0, the area should be 6 * 10 * 10 = 600 m2. However, the result using "Surface Volume" is 200 m2 (Area_2D = 200) (Fig.2)!
Why does this unexpected result (200 instead of 600) happen?
It is how the area of the surface is calculated by the tool.
Per the docs on Surface Volume tool (it is Pro but should be the same)
A raster surface is evaluated using the extent of the center point of each cell, as opposed to the extent of the entire cell area. The resulting analysis will decrease the data area of the raster by half a cell relative to the data area displayed for the raster.
If you look at your dataset, you will see that your raster only contains 6 cells in a 2x3 arrangement.
This means the tool is only using 1x2 cells for area calculations. It is discarding the outer half-cells in its calculation, hence your 200m^2 result.
