Imagine that you are about to place concrete for an industrial floor that is 8 inches thick, post-tensioned, and has specified requirements for levelness and flatness. But before you place any concrete, you first create a “spatial image” or “digital scan” of the area, plotting three-dimensional (3-D) points every couple of millimeters. After you place and finish the concrete, you do another scan of the finished floor. Here's the information that now is available to you: the precise location of all the reinforcement, the thickness of the floor
at all locations, the total amount of concrete placed, and the levelness and flatness of the entire floor. And if the image is compared to the specified allowances for the project, it's possible to map all out-of-spec locations on the floor.
In the near future you will probably be able to use the image to automatically direct a grinder to each “must grind” area, bringing the floor flatness into tolerance. It only takes 5 to 20 minutes, depending on the interval between points selected, to complete the scan. This may sound like something from the future—maybe something you hoped never to see—but the technology is here today and increasingly is being used by surveyors now, with contractors starting to invest in it too.
The origins of the technology go back 20 years with the decommissioning of a nuclear power facility in France. An accurate as-built drawing was needed before work began and a small French company that was owned by the nuclear power company invented the imaging technology needed to do the job. The company was later sold to Trimble, Sunnyvale, Calif. In 1998, the technology was offered commercially to the open by several vendors. In the past 10 years significant improvements have made the system much easier to use and prices for equipment and software now are much more affordable. So even though this may be considered new technology, it's been used long enough and improved to make its use easy and reliable.
Leica Geosystems HDS, San Ramon, Calif., and Trimble are the leading surveying instrument companies that provide this equipment to the construction and surveying industry. Trimble refers to this technology as “spatial imaging” and Leica uses the term “high-definition surveying” (HDS). You also will find it referred to as “3-D imaging,” “terrestrial scanning,” LIDAR (when images are taken from aircraft), and LADAR (ground-based systems). The generic term is “3-D laser scanning.”
Types of instruments
There are many features available but the one common component is lasers that locate points in the X, Y, and Z directions. You can choose between two types of lasers to provide this information: “time of flight” and phase
lasers. Without getting too technical, “time of flight” laser systems are the ones most used in the construction industry. They are much slower than phase lasers, shooting approximately 5000 points per second compared to 500,000 points for phase lasers, but they can record information as far away as 900 feet or more, compared to just 250 feet with phase lasers. Geoff Jacobs, senior vice president of strategic marketing for Leica, says that the lasers are similar to the ones used in total stations. “It's a reflector-less total station on steroids,” he adds.
A second component included in about half the scanners sold today is a high-resolution camera that captures a photographic image in either black and white or color. This helps you visualize the area and interpret the laser point information more easily. You also can add a third component, a global positioning system (GPS) which
provides location information. Omar Soubra, portfolio manager for spatial imaging at Trimble, says that a motor in the instrument rotates it in a circle, making 360-degree images possible.
How it works
When a surveying instrument such as a total station is used onsite, the purpose is to capture or plot a few points that are relevant to a project. But instead of locating just those few points, what if you could plot millions of points in a very short amount of time and use that data to provide much more accurate information? That's the idea behind digital scanning.
3-D laser scanners only provide information about surfaces, but you can get information about volume changes by taking more than one scan over time. You also can scan a site, import the image into computer assisted drawing (CAD) software, and do design work. Then by comparing the actual image to the design geometry, you can look for deviations. So for example, you could determine how many cubic yards of dirt will have to be moved on the site.
In many ways 3-D laser scanners look like total stations or robotic total stations. When set up on tripods, they scan at a low angle to the subject. This doesn't make any real difference if the object is vertical, but for horizontal surfaces, low-angle scans are limited to as little as 250 lineal feet. “You reach a point when the laser doesn't return to the instrument anymore,” says Jacobs. To get more accurate information, the instrument can be placed higher off the ground or even on an aircraft.
In terms of the laser portion of a scan, the amount of ambient light makes no difference. Scans can be completed in total darkness, althouth the photo part of an image will be poor in low light situations.