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ON-THE-FLY GPS
Take to the skies in this new approach to surveying
By Daniel R. Hadley, P.E., and Dean Pottle, P.L.S.
The Tongue River Railroad Company (TRRC) was formed in 1981 as a Montana limited partnership to provide railroad transportation to select coal reserves in southeastern Montana. After years of permitting and extensive environmental review, the Surface Transportation Board recently approved the TRRC Permit Application and authorized the company to begin construction of the proposed 127-mile Tongue River Railroad (TRR) from Miles City to Decker, Mont. At each end, the rail line will connect with the existing track of the Burlington Northern Santa Fe Railway (BNSF). TRRC and BNSF are working cooperatively to refine the alignment to help shorten and improve the route.
The $300-million railroad project will provide a shorter rail route to utility companies in the upper Midwest and Great Lakes region that must bring in raw materials. For example, this more direct route will help coal producers in southeastern Montana become more competitive in today's marketplace because they'll have a direct method of taking their goods to distant markets. The route offers a distance advantage and provides needed access to over four billion tons of "super compliant" coal reserves, creating an opportunity for coal production in new areas. These coal reserves are high in BTU's and low in sulfur, traits that help coal-fired power plants meet the air emissions standards of the Clean Air Act. Because of these traits, there is a growing demand for coal from Montana.
MAPPING REQUIREMENTS
With the permit application approved, the project then moved into the engineering phase. To complete the engineering design of the project, TRRC determined they would need sufficient survey information to produce two-foot contours to accurately model the existing terrain along the proposed corridor. Traditionally, photogrammetry fulfilled this requirement with 100-scale mapping. Accurate mapping at this scale, using conventional photogrammetric techniques, requires multiple photo identifiable control points every 10,000 feet along a project corridor. Because of the lack of planimetric features along this project corridor, controlling the project would require the placement and positioning of approximately 130 panel points, or targets, that could be identified from the photographs. Using airborne GPs to control the photographs could potentially reduce this number of required ground targets to between 20 and 80 on a project of this size. However, with either process, the targets would have to be distributed in a manner that would assure ample geometry for modeling the terrain regardless of access.
Due to the lengthy delivery time of photogrammetric mapping and the fact that only one-half of the 52 private landowners along the project corridor allowed access for surveys, the TRRC began investigating alternative methods of mapping in place of conventional aerial photogrammetry. Preferable methods of mapping would require neither an extensive ground-control survey, nor target density that would demand a great deal of access. The goal was to identify a method that would not require a great deal of access to private property and would allow a shorter period for data collection to final product delivery.
GPs MAPPING
Mission Engineering, Inc., of Billings, Mont., who was involved in the preliminary planning, permitting, and design of the TRR, became aware of airborne LiDAR (Light Detection and Ranging) surveying technology as a possible alternative. Integrated with differential kinematic GPs positioning, airborne scanning LiDAR is capable of providing sufficient information for two-foot contour mapping with minimal ground control and minimal time from data collection to product delivery. To model terrain, ranges are measured directly from an aircraft to the ground and three-dimensional (3-D) coordinates are calculated for each return. These coordinates can be directly input to most contouring software packages without the need for stereo modeling of photographs, saving valuable time.
Another advantage is that because kinematic GPs is utilized, there is minimal need for ground control to reference the laser points to a horizontal or vertical datum. Control can consist of existing monumentation with public access along a corridor. Alternatively, control can be established in easily accessed areas because geometry of the control network is not nearly as critical as in the photogrammetric processes. The unobtrusive nature of airborne laser mapping with kinematic GPs made it an attractive option for the data capture needed to design this project.
POSITIONING WITH "ON THE FLY" KINEMATIC GPs
"On the Fly" (OTF) ambiguity resolution has made kinematic GPs surveying practical for continuous or dynamic integrated data collection. The kinematic GPs surveying technique precisely measures the 3-D ellipsoidal offset, or vector, between two GPs antennas by comparing L-Band carrier phase measurements between satellites and receivers. In order to establish these differences, the integer number of cycles between the receivers and satellites must be established. With original kinematic techniques, this is accomplished by using a variation of a static baseline initialization. Because of this, kinematic is impractical for collecting 3-D offset data while in motion because of the risk of not maintaining continuous tracking with the two receivers. If lock is not maintained with both receivers, the integer count will have to be re-established with a static baseline initialization.
OTF makes it possible to establish the ambiguities while one or both antennas are moving. With this capability, the relative position of a "rover" that is in motion can be calculated from a "base" on a known point. Also, the orientation, including heading, pitch, or roll, can be calculated between two antennas that are continuously moving relative to each other without the need to return to an established baseline if lock is lost with the satellites.
Survey quality GPs receivers are capable of making multiple carrier phase measurements every second. This makes it possible to calculate multiple accurate 3-D position vectors between antennas every second. These measurements are very precisely timed relative to each other. This same referenced time can also be used to simultaneously "tag" measurements made with different sensors.
The ellipsoidal vector measurements can be rotated, scaled, and corrected to conform to any local survey datum. NAD83 is very closely correlated with the GPs datum, WGS84. Simply, this means that if the GPs measures a change in position between two antennas it can be added directly to the base station NAD83 coordinate to yield the NAD83 rover coordinate. In practice, vectors are measured from multiple control points to the rover antenna to yield a "best fit" antenna position. If a local datum is used that is not closely correlated with WGS84, such as NAD27, it may be necessary to perform some type of transformation of the position vector or rover position.
Typical vertical datums, NGVD27 or NAVD88, reference elevations of points above or below a physical "equi-potential" or geoidal surface that is normal to gravity. As this surface is irregular compared to the mathematical ellipsoidal surface, the change in ellipsoidal height that is measured with kinematic GPs between two points may not accurately estimate the change in elevation between the same two points. In order to accurately develop contour lines, it is necessary to correct vertical GPs measurements between points so that they more accurately represent a change in elevation. This is done by comparing GPs measurements between high-quality, vertical control points in the project area and developing corrections analytically. Or, this can be done by referring to geoid models made
AIRBORNE LASER MAPPING
During its investigation of airborne laser mapping, Mission Engineering contacted John E. Chance & Associates of Lafayette, La. Chance has operated its laser mapping system, FLI-MAP, since April 1995. FLI-MAP is a helicopter-based survey tool, incorporating precise positioning, platform attitude, optical imaging, and scanning LiDAR sensors. The basic concept of the system is that the helicopter flies over the corridor to be surveyed. The LiDAR sensor scans the terrain and objects directly below the helicopter at a rate of 8,000 points a second, with a scan width equal to the helicopter height above the ground. The terrain directly below and forward of the aircraft is imaged with high-resolution video cameras, and recorded with a digital time stamp. The output, including the video from the FLI-MAP system, includes XYZ positions of the laser returns.
The system currently uses four dual frequency GPs receivers and a vertical gyro to determine its position, heading, pitch, and roll. The data collected with the primary navigation receiver is post-processed with the data collected by the base station receivers, which are typically spaced every 10 to 15 miles along the project corridor. This produces an accurate 3-D position of the primary navigation antenna every one-half second or at a rate of two hertz (2 Hz.). Depending upon the length of the vectors between the base stations and the helicopter, as well as the geometry of the satellite constellation, the accuracy of the 3-D position is between five to seven centimeters relative to the control network. This GPs information is also processed with the data collected by the other three antennas to calculate vectors between the four antennas to produce system heading, pitch, and roll every one-half second.
The primary navigation receiver also provides a differentially corrected real-time position for following pre-determined flight lines. By using standard RTCM-104 correction information that is generated by an on-board OMNISTAR L-band satellite receiver, the GPs receiver provides sub-meter accuracy to a navigation system. By following the directions provided by LED light bars to indicate course deviation, both horizontally and vertically, the pilot flies the helicopter along the intended route.
The scanning laser is a custom-designed, eye-safe, reflectorless rangefinder capable of measuring first return ranges from 20 to 200 meters. Every scan has a width of 60 degrees and contains 200 range measurements. Each scan record contains timing, laser attitude, and data verification/error detection information. Operationally the laser scans at a rate of 40 times per second and has a coverage width that is approximately equal to the aircraft's altitude above ground level (AGL). By matching the GPs derived position, heading, pitch, and roll information with vertical gyro data, a position of the laser is determined for every scan. These laser positions, with the scanned ranges, produce the XYZ positions of the laser returns on the terrain along the flight path.
A color S-VHS video camera is mounted to the laser, and a second color S-VHS video camera is mounted in a forward-looking oblique position. S-VHS video recorders are used to record the down-looking and forward-looking video of the laser-scanned terrain below the aircraft. In addition to being used to show terrain conditions and identify objects, the S-VHS video can provide captured digital images that can be used by the data processor.
DATA ACQUISITION FOR THE TONGUE RIVER RAILROAD
Because of time constraints and access issues, the decision was made to collect the terrain and planimetric information with airborne laser mapping. Because of GPs, this technology is capable of producing the desired product that would have been difficult to obtain alternatively.
Mission Engineering then provided the project centerline (C/L) coordinates and the required corridor widths for planning. This information, along with accuracy requirements, was used to determine flight line coordinates to guide the system along the project corridor and collect enough data to assure full coverage. The accuracy requirements and the capability of the system determined the line spacing of 70 meters as the data would be collected at 90 meters AGL. The overlap information was used in assuring full coverage, as well as confirming the quality and repeatability of the data. Since the flight lines were flown in straight lines that paralleled the C/L and the corridor width as wide as 1000' in areas, 500 flight miles were planned for data collection.
The C/L information was also important in planning and implementing a suitable horizontal and vertical survey control network. In the vicinity of the project corridor, four High Accuracy Reference Network (HARN) monuments with published First Order Vertical NAVD88 values were recovered. An additional First Order vertical mark was also found. These monuments were used in establishing control points every 15 miles along the corridor in areas of easy access to provide accurate positioning for the FLI-MAP system and in modeling corrections for the vertical GPs ellipsoidal measurements.
The resulting value of adding the geoidal separation values provided by the National Geodetic Survey's (NGS) GEOID '96 Model to the ellipsoidal height differences between the control monuments agreed with the NAVD88 height differences to within five centimeters along the 127-mile corridor. This accuracy gave a high degree of confidence that GEOID '96 with the ellipsoidal height differences collected with FLI-MAP would provide reliable data for the project.
During each data collection flight, three base stations logging GPs data occupied control points close to that flight's area. Daily, the GPs data was downloaded from the base stations and the GPs receivers on the helicopter. Positions for the helicopter and vectors between the GPs antennas on the helicopter were calculated and matched with the gyro and laser data in Chance's FLIP7 software to calculate the NAD83 and NAVD88 coordinates of the laser returns. This data was viewed nightly in the field to assure full coverage and quality. If data gaps existed or there was a question in the quality, flights were planned and flown the next day to correct the problems.
Videos were copied and delivered within two weeks of data collection and final delivery of the planimetric information and the DTM data for the entire route were delivered 35 days after completion of data collection.
THE RIGHT TOOL
In the last few years, GPs technology has had a profound effect on the way topographic and planimetric information is collected. While the accuracy and repeatability of GPs have long been recognized, it has been the utility and reliability of GPs products in the last few years that has encouraged its use in solving problems often difficult to address with traditional mapping or survey practice.
In particular, the integration of GPs with other sensors, such as lasers and cameras, has made both the data collection and calculation of the positions of discrete points more efficient. This has affected not only the type of data that is collected, but also the way it is handled and analyzed.
In the case of the Tongue River Railroad, kinematic GPs mapping allowed the project to continue without seriously affecting a potentially sensitive situation. Overall, the cost of using airborne laser mapping was less expensive than traditional surveying because it saved time and staff expenses.
GPs may not necessarily be the right tool for every application, but its ease of use and flexibility provide options that can save time on engineering projects.
--Daniel R. Hadley, PE, is president of Mission Engineering, Inc., in Billings, Mont. --Dean Pottle, P.L.S., is operations manager of the Corridor Mapping Department at John E. Chance & Associates, Inc., headquartered in Lafayette, La. Reprinted from CE News, November 1998, with permission. ©1998 by Civil Engineering News, Inc. (770 664-2812) All rights reserved. Return to article listing
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