In June 2014, Landair Surveys measured stockpile volumes at a site in Brooklyn in Melbourne’s western suburbs. The site chosen for the research was a small quarry and materials recycling facility. The size of the site was 220m x 500m. The 26 stockpiles ranged from 35m³ to 29,000m³. We applied the following surveying methods:
- Aerial survey (also called photogrammetry) using a calibrated aerial camera from a fixed wing plane. Five existing photo control points were used to control the photography. These points were external to the site.
- GPS land survey. Three teams of surveyors were required to survey all stockpiles.
- Laser scanning. A Trimble TX5 static laser scanner was used for accuracy comparison.
- Drone with a camera. A SenseFly eBee unmanned aerial vehicle was used with a small camera. Nine photo control points had to be placed on a 200m grid within the site.
We conducted this research to compare:
- Accuracy.
- Time required on site.
- Cost.
- Number of sites per day that could be surveyed using the chosen method.
ACCURACY
Conventionally, it is assumed by industry that a GPS ground survey is the most accurate. {{image2-A:R-w:250}}
On this site we discovered that the difference in total volumes between GPS and both the drone and aerial survey was less than one per cent.
For both the aerial survey and the drone, individual larger stockpiles averaged accuracies of two per cent. We concluded that the results between traditional aerial surveying, drone photography and GPS were very consistent.
A laser scanner can only measure what is in the line of sight from the scanner, so dips, holes or valleys are not measured, therefore in this instance laser scanning rated low on accuracy. On this site the laser scanner gave 12 per cent greater volumes than the other three methods (see Figure 1).
TIME SPENT ON-SITE
The time spent on-site is important for safety reasons.
Obviously, the less time surveyors are on-site the better it is from a safety point of view, and there are less interruptions to your normal operations. An induction process is required for GPS, laser scanning and drone surveys.
To survey the stockpiles on the Brooklyn site, the GPS survey required three teams working all day. GPS surveyors also need to climb all over stockpiles. {{image3-A:L-w:250}}
Mobile laser scanners drive all around site to measure stockpiles from all possible angles, interacting with mobile plant and other vehicles.
A drone survey also requires a lot of time on-site. Civil Aviation Safety Authority regulations state that the operator needs to see the drone at all times. At the Brooklyn site, the drone survey took about two hours for the flying plus two hours of surveying the ground photo control point grid, which was quite dense. Drones require more ground control points than aerial survey.
In this case, an aerial survey required zero time on this site, as the photo control points external to the site were placed five years beforehand.
The amount of time spent measuring stockpiles is also important, because on some sites stockpile volumes can change rapidly. Aerial surveys allow a snapshot to be taken of stockpiles and all volumes calculated for the same moment in time (see Figure 2).
COST
The GPS survey was the most expensive option for measuring the volumes of the stockpiles. Costs for mobile laser scanning vary between suppliers; sometimes laser scanning could be more expensive than GPS, sometimes a little bit less. {{image4-A:R-w:250}}
For this site, aerial surveying was the least expensive option, and the drone survey turned out to be slightly more expensive.
SITES PER DAY
Given GPS, laser scanning and drone surveys require site inductions, time spent on-site doing the survey and travel, time between sites could quickly add up. This limits the number of sites that can be surveyed in one day.
Aerial surveying allows for surveying up to 12 sites per day, where drone and mobile laser scanning surveys could complete two or three sites per day. GPS surveys require multiple teams just to complete one site in a day.
THINGS TO KEEP IN MIND
As GPS and mobile laser scanning surveys prove to be expensive and require significantly more time on the ground, many quarry operators are looking at aerial surveying or drones to carry out their surveys.
Drones are able to fly below clouds but can be affected by high winds. Drones can provide aerial photographs and 3D models of the land but are not able to directly map individual features such as the tops of batters. Drones are best suited to smaller areas.
Aerial surveys can be affected by low clouds but provide an instantaneous snapshot of a site. Aerial surveys provide aerial photographs, 3D models and the ability to directly map individual site features.
SATISFYING BUSINESS GOALS
Our research showed that drone surveys (with many photo control points) and aerial surveys are just as accurate as GPS surveys. The research further showed that aerial surveying was the safest and most cost-effective way to determine the volumes of stockpiles.
Technology can be fast, accurate, bright and shiny but it is important to know what to do with the data produced, the limitations of the data and the most efficient way of satisfying the initial business requirements.