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HIGHWAY STUDY BENEFITS FROM HELICOPTER LASER SURVEY by James W. Langston, P.E. and Tina L. Walker, PE

The use of laser surveying from a helicopter flying just 230 ft. above ground over densely forested terrain gave engineers the data required to create a digital terrain model (DTM) of a 42-mi long, 1,500-ft wide highway corridor. The innovative approach saved $1.5 million and at least nine months' time.
The technology was used as part of a comprehensive corridor study of a 42-mile section of U.S. 59 in East Texas. The long-term solution identified in this study will be designated as Interstate 69. The corridor study encompasses Angelina and Nacogdoches counties and the cities of Lufkin and Nacogdoches. Its northern and southern limits are the smaller cities of Garrison and Diboll. The terrain in the study areas is flat to rolling. Timber is a predominant industry, and much of the landscape is covered with dense pine forest.

A MASTER PLAN
The US 59 Corridor Master Plan was envisioned by the Lufkin District staff of the Texas Department of Transportation (TxDOT) in 1996 in response to the transportation issues facing Angelina and Nacogdoches counties, which have a combined population over 130,000. US 59 is a major transportation thoroughfare for local, regional, and interstate traffic. The area already was experiencing significant traffic problems and historically, traffic volumes double every 20 years on US 59.
In addition, it was anticipated that the US 59 corridor might be the future corridor for an extension of I-69 - also known as corridor 18 - from Indianapolis, Indiana to the Rio Grande Valley in Texas. And the existing US 59 is not designed to interstate standards. Recognizing the regional and interstate significance of US 59, TxDOT decided to study the entire route through the master plan process to look at short-term safety and mobility needs as well as long-term mobility needs.
A design team led by Parsons Brinckerhoff (PB) was selected in late summer 1997 to conduct the master planning study. The team included engineering sub-consultants HNTB Corporation (Kansas City, Missouri); BarWin Consultants (Nacogdoches, Texas); Goodwin-Lasiter (Lufkin, Texas); Aerial Data Services (Tulsa, Oklahoma); and real estate appraisers William J. Lyon Associates (Lufkin).
The study comprises six phases:

• project initiation, including accident, service level, and deficiency analyses, formation of technical and advisory committees; and initiation of public involvement;
• travel demand modeling, i.e., creation of a TRANPLAN model with about 210 traffic analysis zones, socioeconomic data, and land-use trends;
• development of near-and long-term alternatives to meeting traffic needs;
• detailed evaluation of long-term improvement corridors, including environmental evaluations,preliminary alignments, and cost estimates;
• development of priorities for near-term projects, including development of high-priority projects for schematic development and environmental assessment; and
• conceptual or schematic design of the long-term corridor solution.

From the onset, the project had an aggressive schedule of 23 months from initiation to conceptual or schematic design, depending upon the funding availability. TxDOT district staff felt that the emphasis would be on developing the near-term improvements: projects that could be completed within five years. The long-term solution was considered to be 20 years in the future and would be developed to a conceptual level, not a full schematic.

TEA 21 DESIGNATION
The corridor screening analysis for the long-term reasonable and feasible alternatives was in progress when President Clinton signed the Transportation Efficiency Act for the 21st Century, which included language that designated Interstate 69. With designation of I-69, the ultimate outcome of the master plan shifted from conceptual design to full schematic design for the 50-mi long corridor.
At that point, only 17 months remained in the original schedule and the preferred corridor would not be announced to the public for another five months. The surveyors on the team were asked for cost and time estimates to perform a survey with ground crews. Their estimates were $2 million and a minimum of ten months to complete the work. Because the survey work could not start until the preferred corridor was selected and shown to the public, only 12 months would remain for survey and design of some 50 miles of new location highway. A major challenge loomed over the study team during summer and fall 1998: how to get some 50 miles of 1,500-ft wide corridor surveyed to produce a schematic design in 12 months.
PB project engineers became aware of Light Detecting and Ranging (LiDAR) technology that had been used in laser surveying from an extremely low-flying helicopter in other disciplines, such as power transmission lines, pipelines, and railroads. LiDAR technology had clear potential for this application: Lasers can penetrate through vegetation to the ground, much in the same way that sunlight filters through the trees. Where the sunlight can penetrate to the ground, the laser can penetrate and obtain a measurement.
John Chance Land Surveys, Inc. (Lafayette, Louisiana) was selected to prepare the DTM for the I-69 project. The firm had developed a unique system, called FLI-MAP(r), which incorporates video replay with a CADD file of data collected using a low-power (eye-safe) laser interacting with airborne GPS control.
FLI-MAP technology has its origin in military applications. Mounted on the bottom of a helicopter, the laser scans the ground and collects 8,000 points per second. This point density converts to approximately one point per square foot. The helicopter is equipped with four GPs receivers on the front, back, and each side. Additional mobile GPs stations are positioned on the ground. As the pilot flies the corridor, gyroscopic sensors take pitch and roll readings, which are later used to correct the data.
Each data point collected has a time stamp that corresponds to the time on the video. This allows the data to be seen simultaneously with the video data, and aids in the processing of the DTM. The stated accuracy of the data was within six in. vertically, and 4 in. horizontally. Chance agreed to perform a demonstration project that would be ground surveyed for confirmation of the stated tolerances.

PUTTING IT TO THE TEST
In December, a test site within the preferred corridor in Nacogdoches was selected. This site, about one-half mile long by 1,500 ft wide had dense forest one side, and on the other an open area with heavy grass or other vegetation. For this project, the helicopter flew at 230 ft to collect data. Data were also obtained by ground survey using a total station.
Both sets of data were transmitted in an ASCII x,y,z format for evaluation. Using Bentley GEOPAK(r) software, a DTM was created from each set of data. Then a grid of 36,418 points (10 foot on center) was draped across both DTMs to obtain common x,y coordinates so that the z values could be compared. These data were imported into Microsoft(r) EXCEL, sorted by x,y values, and compared. About 76 percent of the points were within the six-in. requirement; 94 percent of the points were plus or minus one foot.
The areas where the differences greater that one foot occurred were localized in nature and small in area. The team investigated these areas in the field, determining that in some cases the LiDAR was correct, and in some cases the ground survey was correct. Much of the data that were between one to three feet off were along small, narrow creeks. Lasers cannot measure data through water because the water absorbs the energy and does not allow a return.
Ground crews will be required to perform channel surveying of the 13 major streams that are crossed by the preferred corridor. These areas will provide an opportunity to perform "quality control" on the LiDAR work throughout the length of the project. Overall, the team decided that the LiDAR data were acceptable for use in the schematic design work and determination of right of way.

BENEFITS OF FLIGHT
Due to the uncertainties of the alignment location north of this project's study area, the team decided to terminate the schematic development at US 259 north of Nacogdoches. The LiDAR survey was performed on the 42-mile stretch of US 59 from Diboll to US 259 in mid-February 1999. Right-of-entry was not required to perform the surveying, instantaneously shaving off months of waiting for the forms to be returned. Data in areas where no rights-of-entry were given were still possible to obtain. Nevertheless, sensitive to potential community concerns about low-flying helicopters, TxDOT notified local media, police, and sheriffs' offices in advance of the flights to inform the community about the process and its purpose.
A benefit was the reduced processing time to generate the DTM. Twenty miles of the survey data for the DTM were delivered two weeks after the flight was completed. The remaining data followed in two more weeks. This turnaround time saved at least nine months, and enabled the designers to begin the schematic design in March with all of the data for the DTM.
Using LiDAR technology to obtain the existing ground data had other benefits. It saved approximately $1.5 million on survey costs. Since the data come with a video that is time linked to the point data, the corridor can be "flown" in the office to investigate for constraints. The low elevation flight uses two cameras, one focused forward at 45 degrees, the other down at 90 degrees. Buildings, cemeteries, ponds, and other features can be seen for alignment work as well as environmental assessment work. In addition, a flight survey of rugged areas has the potential to reduce the chance of injury to survey crews.
In this situation - new highway alignment through thick vegetation - the technology provided a great benefit. As the technology increases, and as more firms expand the capabilities of LiDAR, the applications of and the accuracy of aerial lasers may increase. Meanwhile, the US 59 master plan moved ahead on schedule and that is what's really important to TxDOT and the communities today.

--Mr. Langston is Deputy Project Manager, Parsons Brinckeroff, Dallas, Texas.
--Mrs. Walker is TxDOT Project Manager, Texas Department of Transportation, Lufkin, Texas.

This editorial originally appeared in the March issue of Public Works® published by Public Works Journal Corporation, 200 South Broad Street, Ridgewood, NJ 07450 ©2000 Public Works Journal Corporation. All Rights Reserved. Reprinted with Permission.

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