Aerial Data Capturing

Photography & Filming

The use of drones for stockpile surveys and volume calculations

Practical Drone Solutions Ltd recently had the opportunity to perform some drone aerial surveying tests and demonstrations to ADBLY Construction Ltd on one of their sites in Bedfordshire. The reason for these surveys were to show ADBLY Construction Ltd the ease of use of deploying a drone for the purpose of assisting in stockpile volume calculations, as well as to see if the results from the drone survey would fall within ADBLY’s allowable tolerances.

It must be noted though, that using a drone, does NOT cut out the role of the traditional surveyor, but instead, assists and adds an additional layer of data to the already captured data acquired by the traditional surveying method. Where the use of drones becomes beneficial, is when there are large areas to be surveyed and when there are areas to be surveyed that are awkward to gain access to. By using drones to survey these areas, it can be done in a much shorter time and in a much safer manner.

The following measurements can be taken from a processed Orthomosaic map:

  • volume calculations
  • area calculations
  • straight line calculations (showing side profile of whatever the line is drawn across)
  • distance measurements
  • elevations

What is an Orthomosaic map?

Unlike an uncorrected aerial photograph, an orthophotograph can be used to measure true distances, because it is an accurate representation of the Earth’s surface, having been adjusted for topographic relief, lens distortion, and camera tilt.

There are two types of results that can be obtained from an Orthomosaic map, when surveying an area –

  • Relative accuracy – No GCP’s (ground control points)
  • Global accuracy – using GCP’s (ground control points)

Relative accuracy

The horizontal accuracy within a map largely depends on the Ground Sampling Distance (GSD, i.e. number of pixels per centimetre) of your data. You can expect the local error to be around 1 to 3 times the average GSD of the data.

Global accuracy

Global accuracy is when all of the images captured by the drone are processed as normal, but with the addition of data collected from well positioned GCP’s (ground control points) used in the surveyed area. The captured GCP data will help “tie-in” the generated Orthomosaic map to where it actually is on planet earth. The GCP points should be recorded by a qualified surveyor using professional measuring equipment. Ground Control Points processing adds another layer of location data to the map, rather than relying solely on the GPS of the drone. With added GCPs and Checkpoints, it is possible to expect 1-5 centimetres of accuracy but will be greatly dependent on your Ground Sampling Distance, i.e. the number of pixels/cm.

At this moment in time, Practical Drone Solutions Ltd are only able to supply the results of the data processed, where GCP data has NOT been integrated with the images during the processing stage. We will write another blog in the near future showing the results with the GCP data incorporated into the processing stage.

PROCEDURE

To give us the best data possible, Practical Drone Solutions performed 3 different surveys of the same stockpiles (one large and one small, both covered in the same flight, each time). The reason for the 3 different surveys, was to see how the different parameters influenced the outcome of the results. The 3 different parameter settings were as follows:

TEST FLIGHT No1

  • Drone = Phantom 4 Pro
  • Height = 50m
  • Speed = 2m/s
  • Front lap = 80%
  • Side lap = 80%
  • Technique = Cross hatch
  • Camera position = -90°
  • Camera Aperture = F3.2 Camera ISO = 400
  • Camera Shutter Speed = 1/240
  • GSD = 1.2cm/pixel

TEST FLIGHT No2

  • Drone = Phantom 4 Pro
  • Height = 30m
  • Speed = 2m/s
  • Front lap = 75%
  • Side lap = 75%
  • Technique = Cross hatch
  • Camera position = -90°
  • Camera Aperture = F4.5
  • Camera ISO = 400
  • Camera Shutter Speed = 1/240
  • GSD = 0.7cm/pixel

TEST FLIGHT No3

  • Drone = Phantom 4 Pro
  • Height = 30m
  • Speed = 2m/s
  • Front lap = 80%
  • Side lap = 80%
  • Technique = Terrain mode
  • Camera position = -90°
  • Camera Aperture = F4.0
  • Camera ISO = 400
  • Camera Shutter Speed = 1/240
  • GSD = 0.7cm/pixel

ADBLY Construction Ltd personnel took accurate readings of each GCP point using professional measuring equipment, in this case it was the Leica robotic 1205. The drone “take-off” point was allocated to the GCP1 position. Once the map had been processed, it was then re-calibrated by Practical Drone Solutions, by assigning the measured height of GCP1 that was given to us from ADBLY, to the GCP1 “take-off” location, and in this case that height was 28.41m.

Now that we had re-calibrated the map, it was possible to use the “location annotation” tool within our software, to click anywhere within the generated map, thus giving us the exact coordinates and elevation of that specific location that was clicked on. We then went on to click on each GCP point to see what the resulting measurements would be. The table below, shows the differences between the measurements acquired by Practical Drone Solutions Ltd (The Drone) in relation to the results supplied by ADBLY Construction Ltd.

Table showing results of height differences at each GCP location

Table showing results of height differences at each GCP location

Table showing stockpile volume measurement differences for both the large and small stockpiles

CONCLUSION

This has been a very interesting and exciting project so far for Practical Drone Solution Ltd as it has given us the opportunity to work and test “drone aerial surveying” in a real-life scenario. It is interesting to see that in table No1, “flight No3 – Terrain mode @30m” gives the best results when it comes to elevation measurements. Unfortunately, at this time, the results are still inconclusive, but as we have said earlier, once we have the maps re-processed using the GCP data, and we have the data back from ADBLY Construction Ltd with regards to their traditional survey method, it will be interesting to see the final results.

The company that processes our data, states that it is not necessary (within reason) to use GCP’s for the purpose of “volume calculations”, as the data generated within a drone mapped area, can be accurate enough. However, this will all depend on your client and the types of tolerances that they will accept.

What Practical Drone Solution Ltd are trying to show here, is that there are many different factors that need to be considered when doing a drone aerial survey. It’s not as simple as getting a drone up into the air, taking some photographs and then spitting out some data for your client.

Just some of the things that need to be taken into consideration are as follows:

  • Camera settings – ISO, aperture, shutter speed, manual or auto settings
  • Weather – cloudy, sunny, windy (this will affect your camera settings and flight time)
  • Drone speed – (this can affect the quality of the images taken as well as the size of the area to be surveyed)
  • Altitude (GSD) cm/pixel – (this again can affect the quality of the images taken as well as the size of the area to be surveyed)

Only by doing these test surveys and demonstrations, is it possible to get a clearer picture of what parameters and techniques should be applied for any particular aerial survey. Here at Practical Drone Solution Ltd, we want to be able to provide our customers/clients with the best and most reliable data possible, and by continuing to practice and by doing these test surveys, we hope to improve on the quality and service that we provide.

Practical Drone Solution Ltd hopes that you have enjoyed this blog so far and we look forward to being able to give a final conclusion once we have completed the next phase.