A project I am currently involved with is to ascertain the volume in various soil bunds on a residential construction site. The bunds are ideal for a UAV survey as they are irregular in shape making a conventional survey difficult. In addition the volumes were required quite quickly and using a UAV the initial work was completed in 48 hours, a conventional surevy was estimated at 6/7 days.
With this project I planned to use Ground Control Points as there is a need to compare the bunds with levels prior to the development commencing. I planned to fly the 9ha site using my DJI Inspire 1 Pro using an X5 camera with the standard DJI 15mm lens. The flight was set to fly at 75m above take off point and I opted for 80% side overlap and 75% front overlap. The flight took approximately 13 minutes. Weather conditions were overcast with a wind speed of 6-10 mph. I set out 11 ground control points (GCP's) which is more than I would usually use on a 9ha site. I decided to use more in light of the underlating terrain. These were all surveyed in using GNSS/RTK.
The following day I was on site again filming other parts of the development and had time to fly the site a second time. I decided to use a different camera and used the X3. To add more variables I flew this mission at 65m and using the same overlaps. In light of the lower altitude the overall flight time was longer at 20 minutes so two batteries were used. The weather was sunny and wind speed was 5-6 mph.
The two sets of data were proceesed using ESRI's Drone2Map. Drone2Map is a relatively new package and fully integrages with ESRI's suite of products. The software used PIX4D as it's processing engine. To undertake volumetric analysis you do need a licence for 3D Analyst which is one of many extensions available for the core software ArcMap. The imagery was processed to create an orthomosic, DTM, DSM and a point cloud.
The first data set (X5 using the GCP's) achieved X/Y accuracy of circa 35mm and Z of <20mm. The calculated volume of this survey was 65,575 cubic metres. The volume of the X3 survey calculated at 65,803 cubic metres. The variance was therefore a nominal 288 cubic metres.
Whilst the survey with ground controls does allow that data to be used in other applications and most importantly "true" geographically the experiment does demonstrate that a survey without GCP's still provides accurate volumes.