Volume Calculations from RTK point data... surface volume tool and reference plane height

[CaseyLangstroth]

I have RTK field data that I am using to calculate the volume of some landslide deposits. I think because I am recording an orthometric height, not a simple depth from the surface, I am running into some issues in calculating the volume.

What I've been doing is making a TIN from the surface points and a TIN from the depth points. I then convert those each to a raster, adjusting the number of observations as needed. I use raster calculator to subtract the surface from the depth, and get a difference raster.

If I just take the sum of the raster values and multiply it by the cell area, my volumes are generally too low. I found a solution which involves using the difference raster in the SurfaceVolume tool, and I can't seem to wrap my head around exactly why it works.

My solution is to take the number of cells in the difference raster (count) - sum of values in the difference raster. What I can't seem to justify or explain next is that if I divide this number by 1000 and enter this as my reference plane height in the SurfaceVolume tool, I get a volume (below the plane) that is virtually perfect to what is expected for the site.

Why do I need to divide by 1000? Count is unitless, sum would be meters. So count-sum is a value in meters. The input coordinate system for my difference raster is UTM, so the reference plane height is being read as meters. If I don't divide this by 1000, my volumes come out in the millions of cubic meters (which is not accurate at all). Is there anyone that has any ideas of why this seems to work? I am also keeping my z factor at 1.

An example with numbers:

Difference raster sum: 2406.5

Cell area = 4 m3

Sum * cell area = 8046.3 (reasonable but low).

or

Difference raster count: 3192

3192 - 2406.5 = 785.5

785.5/1000 = 0.785 or 0.8

if I put the difference raster in as my surface and I calculate volume below the plane height of 0.8, my volume is 11660, much more realistic.

if I leave the plane height as 785.5, my volume is 11374824. Massive!!! (again though if you divide this by 1000 it seems to be reasonable...?)

So why do I need to divide 785.5/1000 when it should already be in meters? What is the SurfaceVolume tool actually doing that makes this work?

[ChrisLandvogt · Accepted solution]

Hi Casey, I know this isn't directly addressing your question, but what happens when you run the cut fill tool https://pro.arcgis.com/en/pro-app/latest/tool-reference/3d-analyst/cut-fill.htm. Use this on the before (depth elevations) and after (RTK surface) rasters. It should give you a per-cell cut/fill volume that you can sum and work with. (it's basically doing the hard work you described in your post for you). Would that be a simpler solution?

I'm not sure the reference plane volume tool is what you're looking for, unless the z value (which would be z=0). for it is coincident with the base of normalised surface you've constructed.

Chris

[CaseyLangstroth]

Cut and fill works pretty well for most of my inputs, so thank you!

I am still a bit curious about the original question. One thing I found is that the points that make up my TINs and rasters have an XY and Z tolerance of 0.001 meters.Do you know if there is any significance to this or a relationship to dividing by 1000 on my plane height?