Author: Shahryar P. Monajjemi
Date: 08-21-2019
Hello Everyone!
In a series of posts, we would like to discuss all details; technical and non-technical aspects of 3D laser scanning technology, which is also called LiDAR, 3D Scanning, 3D Surveying, and Laser Surveying technologies in today’s industry.
Let’s start this topic with 3 keywords in the construction industry: time, money and quality. Laser scanning technologies can save money, accelerate the design and construction process and improve the design quality, especially in renovation projects. But how does it work?
3D laser scanner devices emit millions of laser lights and by timing their return, can accurately and precisely calculate their spatial locations. These devices designed to produce multiple high-speed scans in one system. They are small, user-friendly, and easy to operate even in the active construction sites. They work by digitally capturing the dimensions and spatial relationships of objects using the reflection of the laser light.
Laser scanning technology has a wide array of design and construction industry, including:
Precise 3D survey of existing condition (pre-construction phase)
Building Information Modeling (BIM)
Virtual Design Construction (VDC)
Schedule and progress control during construction
Contract document versus as-built verification and comparison
When the existing condition of the sites are scanned, millions of data points from the surfaces of the objects are captured and the result is an accurate point combination of the scanned areas. A combination of all these points is called a “Point Cloud”. In general, the tolerance of laser scanning point cloud data from a terrestrial scanning method is up to 1/8-inch in 35 feet.
Terrestrial scanning? What is this?! Well, in terrestrial laser scanning methods, depending on site conditions, several stations need to be captured. Why? Imagine a very simple situation. You want to scan a cubical island in your kitchen. The laser scanner devices can capture ONLY the visible things. Whatever our eyes can see, they can see that too. Not more! So if you want to capture the entire island, you need to capture four stations from four corners of the cubical object.
After completion of the job site scanning, these stations need to be stitched together. This process is called point cloud registration or alignment. Many of you probably saw white spheres or checkerboard papers in the job sites while performing laser scanning tasks. These objects or papers are called targets. Targets are used to register the point cloud and place the different parts of the puzzle in their correct locations. There are also other methods to register point cloud data, depending on the device and various software programs used.
Recently, manufacturers and software developers of laser scanning technologies developed an “on-site registration technique”. Simply, while performing the scanning from various stations, the scanner can be connected to a work-station (high-performance laptop), and the point cloud processing software can do the scan registration automatically. But, in my own experience, this method is not the best one in large scale projects such as infrastructure facilities. But, it is a time-saving method in the projects which require less than 20 scan stations.
The point cloud data can be geo-referenced. Which means the data can be tied into the local surveying plane on its real location. The minimum survey points required for geo-referencing point cloud data is 3 points. The points shall be indifferent Easting, Northing, and Elevation (X, Y, Z). Geo-reference point clouds give millions of survey points to engineers and users which can minimize traditional surveying tasks in the projects and improved the accuracy of the data.
Let’s go back again and talk a little bit more about targets in the laser scanning operations. First of all, like survey control points, targets shall not be placed in the same X, Y, and Z locations. Consider a rotated triangle in the air. The rotation point of this triangle is its center. Corners of the triangle are the best locations for targets. Second, if you are using sphere targets, make sure to fully capture it. If one side of the sphere target hides, for example behind the door, that will cause inaccuracy in the registration. Third, do not put sphere targets very close (like a foot or two) to the laser scanner device. Again, in my own experience sometimes it makes trouble in the registration process. Four, if you are using checkerboards, make sure the angle between scanner and checkerboard is not too sharp, let say not less than 30-degrees. Five, do not break sphere targets! They are damn expensive for no reason!
Question: How far should we place the targets from the laser scanner?
Answer: This is really experience-based, depends on many factors, site condition (e.g. if there is too much vibration in the job site, farther targets may not be detected during registration), laser scanner type, and the quality/resolution of the laser scanner. But in general, I myself try not to place targets more than 30-feet far from the scanner.
I will continue the rest in the next posts. Please, if you have any questions or comments, feel free to send us an email (blog@bimnyc.com) or comment here.
At BIMNYC, we are using cutting edge technologies to suit our clients needs and budget. Our team of engineers and designers are trained, educated and experienced in Building Information Modeling (BIM), Virtual Design and Construction (VDC), 3D Laser Scanning, and Digital Construction Management techniques, and their related technologies such as remote sensing, computational design, and asset management technologies.
Our goal is to share what we know and help you, as the client, to achieve your goals. If you are interested in collaborating with BIMNYC, give us a call at 212-433-3553 or shoot us an email at info@bimnyc.com
Cheers,
Shahryar