- SPECIAL REPORTS
- THE MAGAZINE
"Predictions are hard to make, especially about the future," once quipped famed Yankee Manager Yogi Berra. Because laser scanning-or high-definition surveying-has dramatically improved the way we solve transportation survey challenges, we're predicting the future is here now.
For more than 25 years, American Surveying Consultants (ASC) in Chicago has specialized in meeting the surveying challenges of the transportation industry in the Midwest. ASC has earned a reputation as a leader in its field. A key reason for this has been the company's success in deploying new technologies to meet the needs of clients, including its latest addition-deployment of design-grade laser scanning. Once a specialized technology for structural as-builts, ASC has turned this technology into a cost-effective, everyday tool for transportation surveys.
As more transportation clients realize they can trust digital terrain models (DTMs) that include mapped topographic features surveyed using design-grade laser scanners, more surveying and engineering firms are turning to this technology. It offers more efficiency over traditional methods, is cost-effective and greatly enhances safety in the field.
Initial Transportation Industry UsesMost rural and urban highways are composed of long corridors of relatively flat planes-a challenge for those performing surveying. Some of the challenges of surveying in the transportation environments, regardless of data collection technology, include pavement accuracies within 0.02 of a foot, poor inherent geometry with subtle elevation changes, interstate logistics, required nighttime lane closures, and crew and motorist safety.
One of ASC's first challenges with laser scanning technology was to apply the proven success of the technology in plant applications to the complexities of transportation surveys. Potential issues included lacking "industrial plant" type features to tie scans together, maximizing the scan length and scanning enough area yet not too much. ASC has succeeded in addressing these challenges by developing efficient field and office workflows.
Field Workflow: Our Innovative ApproachUrban interstates became our early focus because of their logistical challenges, safety considerations, requirement for nighttime work and the motorist frustration that comes from lane closures. We realize that good field results start with good measurements. The accuracy levels achieved by our Leica Geosystems HDS (San Ramon, Calif.) 3000 scanner have been outstanding. They result from the scanner's high single-point accuracy and the amount of good data gathered. Results fall well within the design grade of 1 cm tolerances. Scanning has even helped us catch errors in conventional control networks.
To survey transportation corridors, we brought our commitment to high-definition surveying (the term Leica Geosystems uses for laser scanning) along with old time surveyors' common sense, aerial mapping techniques-and some innovations in field accessories. We focused on geo-referenced control points, scanner site selection and scanner height, and adjusting data point density.
Geo-Referenced Control Points
Due to the lack of multiple planes in an elevation view of a transportation corridor, it is more difficult for scanning software to automatically register or "tie" overlapping scans together. Therefore, we had to develop a way to "button-down" each scan with control already geo-referenced. For each scan, similar to the practice used for aerial surveys, we had two or three overlapping control points near the end limits of the scan. We tied these control points together from primary control points using a robotic total station, achieving excellent coordinates (x, y, z) with subcentimeter accuracy.
One methodology suggested launching from known control, conducting numerous scans and then tying back to known control. We found this approach insufficient because it led to unacceptable undulations in the resulting DTMs. Instead, we were able to produce the design grade accuracies needed for civil engineering design by orienting each scan to highly accurate geo-referenced overlapping control.
Scanner Site and Scanner Height
Site selection is important for any line-of-sight tool, including a laser scanner. A scanner, however, does not have to be set up over a control point (its location can be determined as a resection from geo-referenced control); therefore, we had greater freedom in the scanner's placement to maximize the data collected from each setup.
We also learned that increasing the tripod heights to about 10-15 ft allowed the scanner to easily pick up 300 ft of pavement on either side of the instrument. This led to fewer setups with comfortable overlaps while maintaining 500 ft distance between setups. Additional scanner height also helped achieve a better angle of incidence, increasing the accuracy of our hard surface shots. Overall, our field efficiency increased by more than 40 percent.
Target Acquisition and Checking into the Backsight
Target acquisition occurs during the first phase of the scanning, and is needed for control purposes and to accurately tie scans to each other. To assist in finding targets, we used the internally mounted, bore-sighted high resolution digital camera in our Leica Geosystems HDS3000. The device provides a 360Â° x 270Â° panoramic photographic image that is automatically accurately aligned to the point cloud data. Targets can be scanned by "fencing off" in the photo, which is a way of selecting targets so the scanner can automatically locate and scan them at target density (i.e., very fine scan).
We always check back into our backsight by changing the target point number to a check shot series and comparing the resulting vertices. This practice makes us feel secure that the scanner was not affected by some sort of blunder such as leg settling or drifting of targets.
"Throw Away" Targets and a One-person Scanning Crew
Our initial efforts on urban interstates included using standard vendor-supplied targets. We diligently placed them on control points and spent hours retrieving and moving them up the corridor. To avoid such time-consuming workflows in interstate environments, we migrated to inexpensive orbital targets. Mark Wood, our vice president of operations (Chicago), developed these from hardware store materials for an almost disposable price. On each scanning day, our one field person would make one pass placing these targets (often 20 or more) on control points along the length of the corridor to be scanned that day. The one-person scanning crew then would scan segment-by-segment for the day, leaving the backsight targets in place.
Scripting over a Full Field-of-View (FOV)
Other workflow innovations noticeably improved our field efficiency. One was moving the scanner along the median (setting up over the barrier wall) and always orienting the scanner along the centerline toward our last setup. For safety, our one-person scanning crew would remain in the van, keeping an eye on the scan while also beginning the office workflow of data cleanup and cloud registration.
A second innovation was creating a standard repeating scanning script that took advantage of the scanner's ability to selectively address a full FOV from each setup. Scripting dictates which horizontal and vertical swathes need to be scanned at which densities. We wanted dense data collection on the hard surface pavement but needed less density for mapping topographic features off the pavement. The process is similar to setting up a 360Â° water sprinkler head so coverage is consistent and effective with the surface but does not over-or under-water. Effective scripting reduced our net field scanning time by 50 percent.
We incorporated quality assurance procedures in our workflow not only for our own benefit, but also to help agencies considering scanning technology. While establishing control, we also acquired edge of pavement shots at approximately 100-ft intervals. The additional time it took to capture this data was negligible, but certainly helped us and our client compare the incredible accuracy of hard surface elevations.
ASC's Office Workflow: "Virtual Surveying" Offers a Familiar ProcessWhile laser scanning is changing the way surveyors and engineers think and act, at this stage it is important to give clients deliverables like those created conventionally so that the design process can proceed normally.
For the project described in Table 1, we captured 67 million data points. The general approach is to first register all scanned data to each other and geo-reference it to our control. Next, the data is cleaned up and checked to eliminate any possible errant points. After this, a "virtual surveying" approach is used in which we select and code-in the office-data from the scans to create the final deliverables. Due to the high density of the data and design survey grade accuracy of each scan point, we can easily create a more comprehensive deliverable than if we had done the project conventionally.
In the office, we used Leica Geosystems' companion scan processing software, Cyclone 5.2, to take the field data into GEOPAK (Bentley Systems Inc., Exton, Pa.). Registration, the process of linking all the point clouds to each other, is the first office procedure. Some of this was done in the field. "Home ScanWorlds" are the place in Cyclone software where point clouds, also known as "ScanWorlds," are tied to each other and to control.
Our first step is to link the ScanWorlds to each other and then to the Home ScanWorlds where the geo-referenced control points are used to "button up" each individual ScanWorld. The final data set is created by linking all the ScanWorlds to the Home ScanWorld.
We went through the Cyclone's diagnostics report and chose our own constraints. A time-saving office procedure we use is "unification" of the point clouds after each operation, which makes the performance of the point clouds more efficient and keeps our data set clean and manageable.
Office QA with Check Shots At this time, we import our edge of pavement check shots as a quality assurance check. We import the points into the fully registered scanworld and compare them with nearby scan data for elevation accuracy. After this, we have fully registered and geo-referenced point clouds that we can process into final deliverables.
Cleaning Noise with "Region Grow" Tool
Cyclone 5.2 features an enhanced ability to quickly and automatically clean noise (people, vehicles, etc.) data from scanworlds and thus separate pavement data from non-pavement data. Taking a small amount of time to remove noise and define the pavement not only helps us in subsequent office procedures, but also allows sharing point cloud images with our clients while we are in the process of creating the final deliverable.
Topographic Feature Mapping
With Cyclone's "Virtual Surveyor" tool, we can apply standard (client-defined in most cases) point and line feature codes to the data. These point and line codes are then exported to an ASCII file and into the CAD package the client specifies. Most CAD packages allow automatic mapping of these point feature codes to correct symbology and connecting them into appropriate linear features (edge of pavement, crown line, etc.).
Creating Cross-Sections for Export to CAD
One of the fundamental challenges of creating deliverables from a high-definition survey is to reduce the data size to a manageable group of points. While software tools exist that "decimate" the entire point cloud, we prefer to reduce file size by creating cross sections. This process dramatically reduces file size (approximately 9 GB for a typical highway project) by as much as 90 percent without compromising the density and integrity of the hard surface pavement data where it is most relevant.
Creating cross-sections allows us to:
- Import/create alignment
- Specify section width and interval
- Virtually collect points on each cross-section band
- Isolate cross-section bands and export to Cyclone's "ModelSpace" for final processing prior to export
- Create DTMs (Digital Terrain Models) or TINs (Triangulated Irregular Networks)
After we create contours, we use Virtual Surveyor to pick relevant break lines, such as crown lines, and code them appropriately to export as an ASCII file. Virtual Surveyor allows us to selectively pick individual scan points, assign point and line codes, and export points in ASCII.
From the 67 million data points we captured on the sample project, we successfully mined the essential data necessary to create a design grade DTM. After exporting to the client's CAD package of choice, the drafting was done automatically from the ASCII point and line coding. Design can then begin with a DTM that is indistinguishable from one created conventionally-although this DTM is backed by an accurate and measurable representation of reality.
The Future Looks Extremely BrightUntil the advent of laser scanning, it was cost-prohibitive to survey any more pre-design data than necessary for completion of the design. Now, we can capture almost an endless supply of rich data described by accurate coordinates, and mine that data for the accurate measurements. At ASC, we envision the use of laser scanning for roadway surveys of all types-interstates, state highways, county roads and municipal streets.
The DTM and contours created by Cyclone software from Leica HDS3000 scan data are a more accurate representation of pavement realities than the exported ASCII points. The road really does have all those minor undulations. As the civil engineering design community grows in its understanding of the power of mining rich laser scan data for design, many surveyors in the transportation industry will own a design grade laser scanner. Transportation designers will no doubt find ways to improve their designs based on the richness of data captured by laser scanners.
We foresee that in a very short time, virtually surveying a 3D representation of reality and virtually designing in that 3D representation of reality will become the norm. The construction industry is already using 3D designs on machine-controlled equipment.
The transportation industry will take the lead in the acceptance of design grade high-definition laser scanning. Because it is safer, more efficient and more effective, best practices are quickly shifting to incorporate laser scanning into everyday surveying. In the future, we will also see survey, design and construction workflows take further advantage of this technology as it automatically enables 3D representations of reality to better reflect the world around us.