Out with the old, in with the new
Pennsylvania Department of Transportation (PennDOT) has a problem. It has hundreds of bridges and overpasses that need to be inspected every two years, some even more frequently. PennDOT uses both in-house personnel and consultants to conduct inspections. They would pull the existing drawings of the bridge prior to heading out to the field for the inspection.
But there is a problem—most bridges in the state do not have adequate as-built drawings.
Capturing as-built information of existing large-scale projects such as buildings or bridges is a daunting and time-consuming undertaking. Traditionally, surveyors capture key points of the object one point at a time with the help of lasers and reflective targets. Points captured this way can be used by engineers to create an "as-built" drawing of the project. This method is not only very slow and expensive, it also is prone to errors and creates safety issues because the theodolite reflectors need to be manually placed at tall or hard to reach objects, thus exposing surveyors to hazardous conditions.
PennDOT called on a new technology, 3D laser scanning, to solve the problem and help the department to generate as-built drawings in the shortest span of time. The scanner, provided by CYRA Technologies of Oakland, Calif., has the range to scan very large objects, such as bridges, buildings or even process plants.
Known as the Cyrax 2400 system, it consists of hardware components including 3D laser scanner, power unit and PC, and several software modules such as Cyrax Scan, Model and View. Additionally, the 3D laser scanner can capture up to 800 measurement points per second over a 40û x 40û field-of-view, with 6mm positional accuracy at a range of over 50m.
Via the software, the resulting point cloud of the object can be queried for accurate measurements to get instant visualization or to convert the points into walls, railings, steel beams and other construction elements. Converted models can be exported into popular CAD programs such as AutoCAD or MicroStation.
Put to the test
A trial scan of the bridge over Spring Creek in Lamont, Pa., was performed with the following goals in mind:
- Capture the physical characteristics of the bridge;
- Create as-built drawings of the bridge and adjacent areas; and
- Draw comparison between existing data acquisition methods and Cyrax.
The test provided an interesting comparison between traditional methods and Cyrax. In terms of scanning time, Cyrax needed only 31/2 hours to finish 13 scans using five locations. With set-up time included, this translates to a total of 10 man hours compared to the 100-130 man hours estimated for traditional methods.
However, the saving of time also is evident when the post processing times are compared. Because the Cyrax software uses millions of individual measuring points to semi-automatically convert scans to components like steel beams, roadways, railings and pylons, the whole post- processing only took an additional 28 man hours to finish.
While PennDOT cold not accurately estimate the magnitude of timesaving between the traditional method and Cyrax, it did estimate the actual overall saving at 500+ man hours for the project. In addition, it was found that the Cyrax generated as-built was more accurate than the traditional method.
Based on the tests, PennDOT budgeted for two of the Cyrax 2400 scanners, a $400,000 purchase for fiscal year 2000, with delivery having taken place in late spring. In addition to bridge inspection, the system will be used in geodetic surface modeling, construction monitoring, planimetric mapping, roadway and overpass modeling, rock-wall surveying, bridge and road design work, forensic data recording and GIS information.
Brad Foltz, chief of the photography and survey division at PennDOT, commented on their recent purchase.
"What we have been seeing is that Cyrax can gather thousands of points in one day which would take us weeks using traditional methods. Another compelling issue for us is safety. When we used it the previous day to scan a rock face on Highway 232, we could just point the scanner up to the rock and get detailed data of the shape and volume of the rock face. We used this data to calculate the amount of rock we planned to remove and compare our calculations with estimates from our contractors."