How to Tie Point Cloud Data to Survey Control Points
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The accurate as-built information provided by point cloud data has transformed the way architecture, engineering, and construction projects are planned and managed.
Yet, some projects require accurate survey control points, so all team members including contractors, engineers, architects, and operations managers can confidently move forward knowing the 3D models and measurements provided are precise and reliable.
WHAT ARE SURVEY CONTROL POINTS?
Survey control points are accurately positioned reference points, marked with positioning or scanning targets to ensure that scanned data aligns precisely within a spatial coordinate system.
These control points are determined by licensed surveyors who conduct land surveys by using techniques like Real-Time Kinematic (RTK) Positioning, GPS Positioning, and total stations.
GPRS does not conduct Land Surveys or Subsurface Utility Engineering Surveys (SUE). Our work, however, can support both sets of surveying professionals by providing SUE level B locating and mapping quality. And, in the case of reality capture via LiDAR 3D laser scans, our rate of accuracy is between 2 and 6 mm.
RTK positioning is a protocol that utilizes existing GNSS (global navigation satellite system) technology to provide accurate geolocation within centimeters in some conditions. Determining control points with GPS technology is similar as it also relies on satellite signals. However, RTK uses mobile receivers that can communicate with the RTK station, and satellites, which leads to more precise reference points.
A total station is a surveying device that combines an electronic theodolite and an electronic distance meter (EDM). A theodolite is used for measuring angles and the EDM is used to measure distances. By combining these tools, precise coordinates can be calculated and stored for future reference.
Once the points are established, they serve as fixed anchors for aligning, registering, and geo-referencing the 3D laser scans across the scanned areas.
THE PURPOSE OF SURVEY CONTROL POINTS IN 3D LASER
Survey control points play a crucial role in certain 3D laser and reality capture projects, so it is important to know what scenarios do and do not require them.
Phillip Smith, the GPRS Technology Solutions Lead, has been with Existing Conditions, now a GPRS company, for over a decade.
In a recent interview, Smith detailed one of the main circumstances survey control points are needed:
“One of the main reasons for [survey control points] is because when we go out and scan a site, we can't see property lines. That is a legal description that we cannot see. ‘So, how far is my building from the property line?’ Well, we cannot tell you that. We don't know – it's not a physical object in the world.
So, it all comes back to that property line and how do we get our drawings to be lined up with the property lines in the most accurate way possible? And clients need to know what their offsets are for various code enforcement or zoning or whatever else – that's all the reason for that.”
Smith also shared what kind of projects require survey control points and how the control points aid those projects:
We need control points if the client needs to coordinate drawings between multiple different members, so there is coordination between multiple different disciplines within the AEC industry.
We also need control points if the project is so large and we need to make sure everything connects together well and accurately. Or if the client wants to scan a large property in segments at a time. We want control points in that site so that we could do a portion and tie it into control, take the next portion and tie it into that same control without having to reprocess all the data together so it saves time.”
HOW TO TIE IT ALL TOGETHER
Once the scans are completed, the raw point cloud data is taken through the registration process. During registration, the data collected from multiple laser scans on site are compiled and accurately aligned into a single dataset.
The use of survey control points ensures that the different scans fit together precisely and are geographically accurate.
For jobs done by Existing Conditions, our Mapping & Modeling Team processes the point cloud data and control points as described above, while also removing any noise, duplicate points, or unnecessary objects from the scans. After the unified dataset has been finalized, it is used to generate accurate 3D models, 3D meshes, and 2D CAD drawings.
One example of Existing Conditions utilizing survey control points to deliver data their client can rely on is their work on 101 Federal Street in Boston, Massachusetts.
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101 Federal Street stands at 381 feet and has over 800,000 s.f. of office space. Existing Conditions was hired to conduct a full 3D scan of the structure. A new construction project was in close proximity to the building, so the data provided could also help assess any damage after the construction concluded.
By linking the survey control points to the point cloud data, our Mapping & Modeling Team created a complete and accurate dataset and a 3D Revit model of 101 Federal Street which were delivered to the client before the nearby construction began.
Once the neighboring construction was done, Existing Conditions was asked to complete a second 3D scan to compare the pre and post-construction data, a prime example of how Existing Conditions helps our clients’ projects to Start Accurate and Stay Accurate.
EXISTING CONDITIONS 3D LASER SCANNING SERVICES
Over the past 27 years, Existing Conditions has measured, documented, and modeled over 10,000 buildings spanning over 700 million square feet across the United States, establishing ourselves as an industry leader in 3D laser scanning. Architects, builders, engineers, developers, facility managers, trust us to deliver digital representations of the as-built environment with unparalleled accuracy and efficiency. Our services, including 3D laser scanning, 3D photogrammetry, and BIM modeling, Intelligently Visualize The Built World® to enable architects to make informed design decisions based on the most accurate data available.
What can we help you visualize?
FAQs
How Accurate is the Point Cloud and BIM Model?
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Point clouds are a digital representation of spatial information, stored as a collection of points with three spatial coordinates. They are millimeter-accurate and are scanned in the level of detail required for your project. BIM model features are created to your project-specific scope, LOD specifications can vary throughout the model. By specifying scope and project details, BIM models can be customized to your project.
What are the Benefits of 3D Laser Scanning?
3D laser scanning captures millions of 3D data points per second for each scan location, providing incredibly rich data of a building or project site. Sites are captured in high detail the first time, eliminating disruption and the need for return visits. Datasets are dimensionally accurate, measurable and shareable, expediting project planning and execution. Accurate design plans are produced from the start expediting field work and reducing change orders, delays and costs. Communication is improved, teams can discuss plans while each has access to the same information, creating a more dynamic working environment.
