Autodesk University 2015 had many classes about Reality Computing, including this one: Using Drones to Control a Site: A Study of UAVs for Project Control and Quantification (CS11327), presented by Andrew Thoma from HITT Contracting and Peter Marchese from Microdesk. For anyone in the construction industry interested in using UAVs and Reality Computing, this class is an excellent primer based on a real project. You can play back the entire class online by clicking on the link above* and below is a summary of their presentation.
Virginia-based HITT is currently working for a private owner to develop a 12-building campus (constructing 8 new buildings and renovating 4 existing buildings) on a 200-acre site. 120 acres of the site will be affected by the construction. The construction/renovation of the buildings are happening concurrently, but county laws limit ground disturbance to just 18 acres at a time unless there is strict erosion control measures. There are 25 major trades working on the project and Building Information Modeling (BIM) is being used for all project features and systems—down to the landscaping and site utilities.
Given the large number of trades and laborers involved, the concurrent building construction and requisite material layout yards, and the county’s ground disturbance limits, HITT needs to carefully calculate how much of the site is exposed on any given day, constantly track and verify earthworks operations, and closely coordinate work among various trades.
As Thoma described it in his AU class, “to find a solution we look to the sky”. HITT uses a UAV quadcopter to regularly perform photometric surveys of the project. These surveys enable the firm to carry out:
- area calculations to understand how much of the site is exposed at one time
- volume calculations of earthworks by converting photos of the construction site into geolocated surface models and comparing those ‘digital terrain models’ or DTMs to the proposed model-based DTM to assess the work being performed
Area calculation of disturbances
In the past, HITT calculated area disturbance by hiring an aerial photography company to fly over the site once a month and take pictures. The firm would import the images, visually identify disturbance areas, and then use the software’s area measurement tool to add up the areas and check if the project was within the 18-acre disturbance limit.
With UAVs, HITT is using roughly the same process (using digital photographs and takeoff area tools to calculate areas). But there are a couple of big differences. The aerial photography is on demand and acquired at minimal cost. The digital photographs are imported into software to create a composite image of the site that is then used to visually identify and quickly measure disturbance areas.
Volume calculations of material
HITT’s traditional method of calculating how much earth was being moved around on a site is to count the number of trucks moving dirt around or in and out of the site, and estimate what percentage of the truck was filled. From those records (kept by the truck drivers) and knowing the volume of an individual truck, HITT would calculate total volume of earth. This is a completely manual data gathering effort and its accuracy requires the drivers to keep good records and make good estimates.
Alternatively, HITT uses ground surveys to measure volumes. But this would be too slow on this particular project because it requires multiple survey setups around all of the buildings under construction. And to accurately measure volumes, the surveyors have to fully capture the terrain, which requires a lot of survey shots around each building—too many for daily data collection. Laser scanning would be a lot quicker and more accurate. But on this project, HITT didn’t have a laser scanner or an experienced crew, so it would have been too costly to do the work internally and outsourcing the effort was likewise cost prohibitive.
So HITT is using its UAV to shoot images of the construction site and then using photogrammetry technology to quickly create a 3D surface of the current construction from those images. That surface is imported into Civil 3D and geolocated using known surface control points. From there, HITT uses the software’s volume comparison tools to measure the volume difference between the day’s construction surface or DTM, and the model-based, to-be DTM. Compared to the traditional approaches outlined above, this process is faster (it takes only 30 minutes for the UAV flyover of the site), repeatable, more accurate, and requires less man-hours (and therefore less expensive).
The aerial site photographs captured daily also helps HITT control and coordinate trades and movement on the site. For example, the company uses the images to track materials on site or plan for upcoming material delivery and laydown needs. By draping the images on top of the Civil 3D design model, HITT can visually check work progress. UAVs are also used to easily inspect hard to reach areas that would otherwise require lifts or other specialty equipment. Video cameras mounted to the UAV are also used to record existing conditions. For example, a new sewer main on the project required HITT to dig up an existing private road. By capturing the road before the work was started, they had a way to record the pre-construction condition of the road in case of post-construction complaints.
Thoma and Marchese then tag-teamed the rest of the presentation (although the sound recording at some points are an issue—you’ll have to crank the volume on your computer). They describe what construction companies will need to fly UAV aerial surveys—from equipment to insurance and certifications to software—and they ballpark the costs of the equipment as well as the software. They also outline the legalities of flying a UVA, what’s involved in flying one, and some safety tips. In the final part of the class, Thoma and Marchese walk you through the workflows (including tips and tricks) for using UAVs for aerial surveying.
As mentioned earlier, this is an excellent overview for anyone considering the use of UAVs and Reality Computing for construction!
* You will need a login & password to download material from au.autodesk.com, but you can get those for free just by registering at the site.
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