We just completed a project for a private landfill here in FL to help settle a contractor dispute about how much dirt was moved/removed from a retention pond. The problem stemmed from the fact that the design engineer estimated the volume as one amount of cubic yards and the earthworks guys sent a bill for twice that amount!
We thought it would be easy by collecting it with airborne LiDAR as part of our flight testing, but then realized that the area in question was a pond that was under water! So, back to the drawing board…
Back in my RCID/Disney days, I worked with some smart people and we learned how to integrate GPS and Bathymetric sensors to map the Hydrilla in their lakes. We also gathered some useful Bathymetric data that could be used to determine target concentrations of herbicides based on a specific dilution factor. The most important part of that equation was knowing the amount of water in the lake and it was a math formula from there on forward. Divide the volume by the target concentration level and you had the amount of herbicide needed to make the brew.
So, we went old school and used our RTK rover to supply a GPS location and the Bathymetric sensor to grab the Z (depth) values for the lake in question. The collection took about an hour and we had a processed and calibrated bathymetric surface before leaving the project site. From there, we integrated the bathy data with the airborne LiDAR to get a continuous representation of the underwater surface.
There was a small discrepancy between the water elevation on the day of airborne collection and the bathy collection. This was handled by surveying the water elevation on the day of the bathy collection and then adjusting all of the depths to this elevation (corrected for the transducer offset which was about 0.1 foot). This gave us the correct elevations relative to the airborne LiDAR data set.
We determined that the volume of dirt removed was the same as the yield as determined by the design engineer. It turns out that the contractor might have to come to the table to prove that they moved more material then the design engineer predicted and we confirmed with this cool project!