Point of Beginning

The Fine Points of Scanning

April 24, 2003
The value of laser scanning technology is proven in two east coast projects.



The advancement of technology in the surveying and mapping industry can be exciting. Electronic instruments, Light Detection and Ranging (LiDAR), reflectorless technology and numerous other tools have come onto the scene in recent years, offering surveyors and others ways to complete projects much faster and with more efficiency. 3D laser scanning is another technique being used more frequently by surveyors. Medina Consultants P.C., of Hackettstown, N.J., is one company that has been utilizing this technology for almost three years, and the workers at Medina give it glowing reviews. What follows is a synopsis of two projects in which Medina workers used laser scanning. They saved money and time, and provided better quality deliverables—three of the most valuable aspects of business.

Scanning at the World Trade Center

We at Medina Consultants had the opportunity to use 3D laser scanning technology at the World Trade Center (WTC) Ground Zero site shortly after reconstruction had begun following the attacks of September 11th. The project supported re-establishing a subway station in the WTC “bathtub,” the 3' thick x 70' high slurry wall constructed many years ago to provide support for the proposed construction of the site.

Before being destroyed in the Twin Towers collapse, the subway station had served thousands of WTC and nearby Wall Street commuters. Designers were concerned with the possibility of obstructions interfering with the path of the new subway. The greatest impact would be from slurry wall tie-backs (large metal pipes that house a structural cable) that project from the face of the wall.

The Port Authority of New York needed accurate locations of the tiebacks nearest to the proposed track alignments and was also interested in estimating the work needed to resurface the walls that were badly cratered from the collapse. A lot of debris from the explosion, including license plates, bottles and cans, etc. was still embedded in the wall surfaces. Other than these sobering reminders, the site itself was very much a typical construction site. We had considered using a reflectorless total station for all of the work, but we would not have had confidence in the exact horizontal tieback projections.

The work at the “bathtub” site needed to be done as soon as possible. Port Authority Surveyor Rich Danko, PLS, had learned about 3D laser scanning from prior demonstrations we at Medina had given with the Cyra Cyrax 2500 (Cyra Technologies, San Ramon, Calif.), and wanted to take advantage of the technology’s speed and detail for this project. After some evaluation, the Port Authority awarded the work to be done on the east wall to a crew at Medina Consultants.

Quickly and Accurately

Our specific goals for the project were to map the locations of the furthest horizontal projections of 75 tieback structures and to provide vertical cross sections of the wall at the tieback “columns.” A secondary goal was to create a 3D contour map of the wall.

To meet our accuracy goal of 1⁄4" (a Medina set goal in conjunction with the client’s design needs), we set the scanner 100' back from the wall, capturing 125' to 150' lengths of wall per setup within 165' (the manufacturer’s spec for 1⁄4" accuracy) of the furthest tieback for each scan. We conducted eight major scans with a 10' overlap and a “coarse” scan density of 1" x 1" at 150'. This density allowed us to locate geo-referencing targets and tiebacks for subsequent localized fine scans. These fine scans enabled us to precisely geo-reference all scans and to accurately model the tiebacks into pipes. We used a combination of Cyrax’s flat targets and hemispherical targets with a minimum of four targets within each coarse scan. Cyclone’s EDIT SCRIPT tool instructed the scanner to automatically move from the coarse scans to the localized, fine scans. For fine scans of the tiebacks, we used a 4' x 4' area with a 1⁄4" x 1⁄4" scan density. Geo-referencing targets were also surveyed with a Leica TCRA 1101 reflectorless total station (Leica Geosystems, Atlanta, Ga.). To place the scanned data in a “real world” coordinate system, the targets used to register the scans were surveyed in from control stations the client had provided. This allowed the merged scan data to be in the same coordinate system as the client’s base mapping. Cyrax’s ability to focus on a small area for fine scans (what Cyra calls “addressability”) is a huge plus; otherwise we would have had to fine-scan the entire scene, which would have taken much longer and resulted in large, unmanageable datasets for office processing.

Our total time in the field was only nine hours. While scanning, we did spot-accuracy checks, inversing between targets with both our Cyrax and the Leica total station. Our worst-case check was 0.015'.

Despite some logistical challenges at the World Trade Center site, total field time was only two days with the Cyrax scanner.

From Point Clouds to Deliverables

We registered all scans and tied them to the WTC survey network before establishing tieback locations and creating sections. Control points were downloaded from the total station into the scanner’s laptop with a PCMCIA card. We did some registration in the field as QA and finished the registration on the ride back to our office.

Cyclone-MODEL software (v3.2) was used to “best fit” the dense scans of the pipes to create 10" cylinders for all 75 tiebacks. We then used Cyclone’s Virtual Surveyor feature to click on each tieback point furthest from the wall. This created an x,y,z coordinate and ID of each tieback for ASCII export. The Autodesk (San Rafael, Calif.) Land Development Desktop AutoCAD Station & Offset function was used to create station and offset data for each furthest horizontal projection to the proposed track alignment. Coordinates, IDs, and station and offset information were exported to a spreadsheet.

Sections were created by first cleaning up the raw scans (e.g., removing extraneous points captured when a person walked in front of the scanner’s beam) within 1' from the wall, then meshing the point clouds in Cyclone-MODEL. We then placed cut-planes at tieback column locations through the mesh. The resulting sections were exported to AutoCAD. A planimetric map showing tiebacks with respect to the proposed track alignment was also created. Total office time was 48 hours with three days of Cyclone processing and three days for final drawing preparation. We turned around the project in just nine calendar days. A week later we were awarded the east wall project.

Contractors were preparing to erect steel near the wall, so they needed us to complete the fieldwork quickly. We ran into some site logistical problems with the east wall, including the destruction of one of our control points. The east rim of the bathtub had limited access for us to place targets, so we used a lift vehicle to aid us. Excavation work in the area and a 6' high construction wall in front of the east wall provided obstructions. To scan over the 6' wall, we raised the scanner atop a CST/ Berger (Watseka, Ill.) elevated tripod.

Despite the logistical challenges, total field time was still only two days. Office work took nine days, including generating final contour maps for both walls.

Originally, we’d been asked to supply vertical sections at tieback locations. Although not a part of initial deliverables, we generated detailed contours of the walls at 0.1' contour levels. We found >1' depth variation in the surface of the walls. Point clouds were exported via Cyclone-MODEL into AutoCAD LDD where the final DTMs and contours were created. If we need to provide geometry of more tiebacks or additional detail for the wall surface in the future, we can find it all there in the scan data without returning to the site.

To achieve maximum scanning range at the desired scan density, we elevated the scan head on a heavy-duty CST/Berger tripod.

Big Savings and Satisfaction

If we had tried to do this project completely conventionally, we likely would have used a man-lift and a technician holding a peanut prism at the edge of each tieback and at intermediate points along the wall at each column of tiebacks. Our estimates: a two-person crew in the field for 21 days to get interpolated cross-sections and five days in the office developing AutoCAD DTMs and creating final drawings (including interpolation time to get good sections). A reflectorless total station would not have provided us with exact horizontal tieback projections with confidence. With our Cyrax equipment and software we were able to capture complete, accurate field data in just three days compared to four weeks conventionally. Overall, 3D laser scanning and data processing saved us 55 percent of total personnel days, field and office combined, compared to traditional methods.

A few weeks after the project, a TV program was showing a WTC remembrance ceremony with hundreds of officials and luminaries at the WTC site. The TV footage showed some of the square blue laser scanning targets we had left in place on the walls of the bathtub. Seeing those officials at the ceremony and seeing the evidence of our own survey on that very site really struck home where we’d been.

Saving Money At Collingswood Circle

A month before the WTC project, we used Cyrax’s ability to accurately survey busy roadways without having to close traffic lanes, which showcased laser scanning’s advantages.

Professional engineering and consulting firm Dewberry-Goodkind Inc. of Rutherford, N.J., awarded Medina Consultants a survey project to support the replacement of the Collingswood, N.J., traffic circle, the intersection of Routes 30/130, with straight roadway. We were tasked to provide control for aerial mapping, conduct a utilities study, provide surface elevations along proposed alignments, and perform right of way and wetlands surveys. The surface elevations survey included a railroad bridge/overpass and 6,500' of four-lane and six-lane roadway, both northbound and southbound. Road cross sections were to be created from the surface elevation data. Original deliverables called for surface elevations at 50' intervals for the right of way and adjoining roads. Accuracy requirements were per NJDOT specifications (.01' for hardscape and 0.1' for terrain).

We had originally won the project, including the roadway and bridge survey, based on an estimate using traditional survey methods. The roadway portion of the original proposal included procuring 20 days of lane closure services at a cost of $1,200 per day, which was typical for the area. We determined that the costs and logistical issues of using low altitude photogrammetry to set targets and obtain the necessary coordinate values involved in this project were excessive so we had decided against using an aerial survey for the roadway. As the project approached kick-off, we reconsidered our proposed approach based on our recent successes with laser scanning. We concluded that we could potentially eliminate $24,000 in service fees for road closures by using our Cyrax laser scanner. Also, as safety is a primary consideration at Medina, we felt that we could capture the roadway geometry faster and avoid occupying the busy roadway, thus reducing the risk.

Field Methodology

The Collingswood Circle project was our biggest scanning project of this type. To achieve maximum scanning range at the desired scan density, we elevated the scan head on a 12' heavy-duty CST/Berger tripod. Even with the scan head elevated, we could still control the scanner and view the scans in real-time on our laptop, which was connected to the scanner via a lightweight cable.

We set the scanner at 150' intervals to ensure 1⁄4" accuracy. Barrier walls in the center of the divided road led us to occupy both sides of the roadway with the scanner. Cyrax’s 6" hemispherical targets, which tied all of the scans to control, were placed every 50' in a staggered pattern, a minimum of four targets per scan. The large white spherical side is easy to see and the reverse flat side is easy to survey with our reflectorless total station. The target rotates so you can use either side as needed and either side can be fine-scanned to extract an exact target center. We also used the targets as independent QA with total station measurements.

The entire survey was done from the side of the road without having to retain road closure services or occupy the road. Scan density was approximately 0.2' point-to-point spacing at 150' range. Total field time to scan 6,500' of roadway and one bridge was five days, 46 main scans. This was four times faster than surveying the road using traditional methods. Moving the tall tripod with the scan head attached was somewhat cumbersome, though; we are evaluating alternate ways to improve this part of our field procedures. Logistics of the divided roadway and median barrier necessitated a crew of four. We had estimated 20 days to survey the road conventionally with a two-person crew. Overall we cut our field personnel days in half, eliminated road closure costs and avoided extra per diem costs by doing the work in just five days instead of 20. The bridge survey took three hours compared to the eight that we’d originally budgeted considering traditional practices.

Office Processing

As always, the first data processing step with any laser scanning project is to tie all scans together and to control. We did some of this in the field each night for QA and completed the rest in the office later. After we had registered all scans, we showed them to our client to review the desired final deliverables. After meeting with the client, we settled on breaklines, the top and bottom of curbs, lane lines, edge of driveway and sidewalks, spot surface elevations, and an ASCII list of points collected along 25' sections.

We used two different methods to generate 3D lines. Cyra had recently released its Cyclone CloudWorx software that lets users create deliverables right in AutoCAD or MicroStation directly from the scan data. Everyone who does scanning will most likely try to export point cloud data directly into his or her CAD application, but the file sizes are generally far too large for that. Some try to break up the point cloud into smaller chunks for export and eventually this will work, but it’s very cumbersome and time-consuming. CloudWorx is a big step forward. We processed some of the data using Cyclone-MODEL v3.2 to generate breaklines and the balance using Cyclone CloudWorx for MicroStation v1.0. We wanted to compare these two methods in terms of accuracy and office processing time. To compare the accuracy of these two methods, we used Cyclone-MODEL’s cutplane tool to create cross sections through the meshed point cloud and we compared this with our results from manually digitizing cross sections directly in MicroStation via CloudWorx. We found exact agreement between the two methods. We also found that the office time was virtually the same for both processes. We then “collected” several hundred intermediate x,y,z points approximately every 7' to support the full digital elevation model (DEM). We did this exclusively in CloudWorx for MicroStation. The entire office processing time was 15 days. We estimated this to be about the same as processing conventional data into final deliverables.

Laser scanning eliminates the high cost of road closure services and improves field safety.

Detailing the Benefits

One of the key aspects of laser scanning is the added detail that the scans collect. This detail plays out in several ways on projects. On the Collingswood Circle project, we were able to generate roadway sections at 25' intervals. If we had done the project conventionally, it probably would have been at 50' intervals. The added detail also provides users with extra visual confidence in the results delivered to the client. Users can literally see how well linework matches up with the raw, dense 3D scan data, providing built-in, added insurance against both errors and omissions. Finally, when we showed our client the point clouds, he was excited about the potential use of the scan data for future needs, such as wire heights, pole locations, sizes of poles, clearances on the bridge, etc., without having to go back to the field. The data could be used, for example, for potential right of way issues that might arise later or could be used to settle disputes during construction.

The detail of laser scanning is also what makes it possible to survey a busy road, as proven in this project. The scanner quickly captures so much data that passing vehicles are captured as just a few vertical “streaks.” These streaks are easily deleted from the scan data and don’t affect the quality of the deliverable. With single-point surveying, a passing car can ruin a shot, but with scanning it’s not a factor.

Medina’s use of laser scanning technology proved beneficial in completing both projects accurately, within an aggressive schedule, with more detail and with better accuracy than traditional methods. Overall, we’ve had much success in deploying the technology with significant benefits to both our clients and our own organization (within the first 16 months of owning a system, we were approaching a million dollars in bookings). That being said, we believe that we can further reduce our field and office costs as we further develop our procedures, and as suppliers continue to enhance their products. We are excited about the future of this technology and the opportunities it provides.