Different results: Surface volume vs Cut fill

[SebastianWesth]

Hi there, 

I've been trying to calculate the volume between a bathymetry surface (between - 15 and - 6 m, mostly around -9 m) and a certain depth, in this case -8 m, in order to estimate the amount of sediment that can be filled at the site before reaching the -8 m limit. 

I've used both Surface volume and Cut fill, where I get two different results for the volume, 399,329 m3 and 402,170 m3, respectively. When using the Cut fill I used a raster with a constant value of -8m, and then I summed first the negative values and then the positive ones. The volume above the reference plane is 5,190 m3 and 5,286 m3, respectively for the two tools. Also different volumes? 

This confuses me, am I doing something wrong? 

I've tried looking into different pixel types, thought that it may could be due to rounding of the values, but this doesn't seem to be the case, as all rasters are float. I see that some people have had the exact same problem before but I didn't find a satisfying answer when searching on the topic.

Hope that someone can help me. 

[MeredithPayne1]

Hello Sebastian,

You may be getting different volumes because the two tools you are using don't interpret your input data in the same way. Cut Fill was designed and created for raster processing with Spatial Analyst (its licensing is shared with 3D Analyst). Raster processing uses the area of the cell as the fundamental unit. Therefore, Cut Fill calculates volume based on cells. On the other hand, Surface Volume views a raster as a regularly spaced set of points (a lattice of points), where the cell centers are the points. A surface made from a lattice of points is ½ cell less around the perimeter than a cell based interpretation of the same dataset. "The resulting analysis will decrease the data area of the raster by half a cell relative to the data area displayed for the raster." (https://pro.arcgis.com/en/pro-app/latest/tool-reference/3d-analyst/surface-volume.htm). This might account for the difference in the volume calculation you are seeing between the two tools.

Sincerely,

Meredith