Leveling with Four-Wheeled Chariots
To augment the vast economic benefits of the Global Positioning System (GPS) for surveying purposes, the Wisconsin Department of Transportation (WisDOT) has created the Wisconsin Height Modernization Program (WI–HMP). The WI–HMP is a statewide program that combines conventional differential leveling and GPS surveying methods to provide highly accurate geodetic control to 2-cm three-dimensional positional accuracy.
To perform the work for the WI-HMP, WisDOT looked to conventional operations and not so conventional ones. Sometimes a little creativity can aid in a project’s efficiency. In our case, creativity led us to strap some conventional equipment to four wheels—a four-wheeled ATV, that is.
The Height Modernization Program ConnectionIn 1998, Congress directed the National Geodetic Survey (NGS) to carry out the National Height Modernization Study to prove the need to modernize the vertical component of the National Spatial Reference System and to demonstrate how GPS can be used to accomplish the modernization effort with significant cost savings. As a result of the study, NGS is responsible for the Height Modernization Program in which they are working with several states, chosen for their progressive nature, access to funding and/or critical need, to refine geodetic surveying techniques for capturing and making the most effective use of elevation data. Based on the Congressional Study Report on Height Modernization, the state of Wisconsin has designed a project to further test and refine procedures and techniques for a statewide height modernization effort. The WI-HMP is a program that will (1) densify the horizontal geodetic network monumentation throughout the state to 6 to 8 km spacing; (2) provide geodetic level lines at 25 km spacing across the state; and (3) use GPS technology to transfer elevations from these level lines to the densified horizontal network. The result is a three-dimensional geodetic network with horizontal and vertical accuracies of 2 cm or better.
The basis for the horizontal positioning of the WI-HMP is the Wisconsin High Accuracy Reference Network (WI-HARN) established in 1991 on the North American Datum of 1983 (NAD 83). Likewise, the basis for vertical positioning of the WI-HMP is the existing NGS vertical framework based on the North American Vertical Datum of 1988 (NAVD 88). Unfortunately, for a variety of reasons, many of the existing NGS bench marks along the vertical framework are not adequate to support the development of modern geodetic networks using technologies such as GPS. First, approximately 50 percent of the bench marks have been destroyed through the removal of railroad lines, and from highway construction and other development activities. Second, many of the recovered bench marks have been determined to be unstable with substantial discrepancies between published elevations and current measurements. Third, many of the existing bench marks are not suitable for occupation with GPS receivers because of obstructions such as trees, buildings and overhead utility lines. As a result, substantial releveling of the state bench marks is required to satisfy the needs to implement the WI-HMP.
“Leap-frogging” Through Conventional LevelingWe at WisDOT have used conventional leveling to establish the basic framework of level lines at the 25 km spacing. Bench marks are established at an average spacing of 2 km with a maximum spacing of 3 km. Second-Order, Class 1 leveling according to specifications of NGS’s Federal Geodetic Control Subcommittee (FGCS) uses two invar rods and one geodetic quality digital level. General leveling procedures require using three people during the observation period. Procedures require an even number of instrument setups and that foresight and backsight distances are equal within 2 m. The basic procedure is when one rod is placed on the monument and sighted with the digital level, the second rod is placed an equal distance in the opposite direction and sighted. The whole process is repeated by “leap-frogging” the first rod and digital level past the second rod. This “leap-frogging” is repeated until arriving at another bench mark. The maximum spacing distance between the rod and the level is 60 m (approximately 200 ft).
Through extensive research, WisDOT determined that conventional geodetic leveling would be more efficient with a motorized leveling operation. Several vehicles were considered including lightweight utility vehicles, golf carts and small pickup trucks. Small pickup trucks did not satisfy our economic needs nor were the pickups an efficient fit for leveling operations. After consulting with the WisDOT Office of General Counsel, it was determined that the only legally approved vehicle to be used within highway right of way for surveying purposes was an All Terrain Vehicle (ATV). Wisconsin Statute 340.01 (2g) defines an ATV as an engine-driven device, which has a net weight of 900 pounds or less, has a width of 48 inches or less, is equipped with a seat designed to be straddled by the operator and is designed to travel on three or more low-pressure tires. WisDOT decided to use Polaris Magnum 500 ATVs (Polaris Industries Inc., Medina, Minn.) to motorize the leveling operation and selected a contractor to develop a custom-built racking system to transport and protect the precise leveling equipment.
“Racking Up” the Leveling SystemsWisDOT contacted Archetec LLC of Fox Lake, Wis., to assist in the development of custom-built racking systems to facilitate the transport of the precise leveling equipment. The ATV model selected needed an automatic transmission and liquid-cooled engine because the running time would include a large amount of idling time compared to driving time. The particular ATV selected determined the design criteria for mounting the transport fixtures.
The original concept created by WisDOT and Archetec included one ATV to carry a Trimble DiNi 12 digital level (Trimble, Sunnyvale, Calif.) mounted to a tripod and two ATVs to carry one Trimble Invar Bar-Code level rod each. Along with the need to transport the leveling equipment, there was a need for safety and thus a high-visibility safety strobe lighting system was added. In addition, a Calc-An-Acre distance measuring instrument (DMI) (Micro-Trak Systems Inc., Eagle Lake, Minn.) was selected for the operator to use to easily balance the backsight and foresight distances for each setup.
After careful review of the operation, it was determined that a uni-directional level rod transport fixture would offer the level rod operator the ability to exit on either side of the ATV. This required the development of a universal level-rod carrying system that could be easily moved from one side of the ATV to the opposite side. The digital level/tripod fixture was designed to allow the operator to simply fold the legs and securely stow the level/tripod for travel, thus preventing the need to re-adjust at each setup.
The fixture frame design centers around the existing rack mounting points on the ATV. The frame needed to be tall enough to allow the installation of the high visibility safety strobe lighting above the operators, so it was necessary to locate four mounting points to stabilize the structure in all directions. Extruded anodized aluminum frame members offered high strength, low weight and corrosion resistance. These slotted extrusions provide adjustable mounting points for the components, and also serve as wireways for the electrical cables. Each ATV is equipped with one rear frame and one forward frame. The rear frame carries the rear level rod tray, tray axle and the safety strobe light. The forward frame carries the forward level-rod tray, tray axle, DMI and the electrical switches. Ample space is left available on each end of the ATV racks for auxiliary equipment and supplies.
The level rod trays include a stainless steel tray lined with high-density UV resistant foam, mar-free edge trim, and hook and loop fastening straps. The level rod fits securely in the trays while allowing easy stowage and retrieval. A heavy-duty vibration dampening concentric pivot allows the tray to level itself when being loaded and creates the uni-directional feature requested. The operator simply removes a retaining pin, slides the tray off the axle and places it on the opposite side of the tray axle, then replaces the retaining pin. After both front and rear trays are moved, the ATV can be driven to accommodate the existing traveled conditions.
The digital level/tripod fixture is mounted to the main structure at the rear center of one ATV and includes a lower tray lined with an ultra high-density self-healing foam rubber pad and adjustable foam padded yokes for trapping the tripod legs and the digital level during transport.
The DMI selection required the positioning of alternating magnetic poles on a rotating surface. A rear wheel rim provided a consistent rotation and a protected location for both the magnets and the proximity sensor. Precise placement of the magnets around the rim circumference resulted in distance measurements accurate to 10 percent. This accuracy satisfies the FGCS balance backsight/foresight specification for Second-Order, Class I leveling. The DMI is located directly in front of the operator console and positioned to allow easy access to the controls. The illuminated nonglare LCD screen display is easy to read while the ATV is in motion.
The combination of the design criteria and features provides a functional, user-friendly and reliable motorized leveling system. The expected increase in efficiency was exceeded and the change from walking and carrying the equipment to driving between turning points was accepted quickly.
Additional features available for the motorized leveling transport system include yellow anodized frame members for safety, auxiliary marine battery supply for the DMI and safety lighting, universal mounting pads for alternative ATV manufacturers, and tilting light mounts to allow transport of the ATVs in standard enclosed trailers.
High Performance ATVsThe ATVs are now used as the main method for WisDOT geodetic conventional leveling for WI-HMP. We, of course, walk when ATVs are prohibited for use (e.g., on interstate highways), are not practical or are not safe to use along the transportation corridor. For example, we prefer to walk in congested urban areas, on high-volume transportation corridors and on highway corridors with narrow shoulders. During the early stages of reconnaissance, motorized leveling is weighed heavily in the selection of bench mark locations and leveling routes. Good reconnaissance of leveling routes has resulted in nearly 85 percent of the leveling performed using the ATVs.
The use of the ATVs has proved to be very successful in accelerating the conventional leveling needed on the WI-HMP. Early results have indicated that motorized leveling has increased production by as much as 41 percent over walking. Approximately 786 km of leveling has been analyzed and has yielded an average rate of 1.69 km/hr motorized versus 1.20 km/hr walking. Clearly, motorizing the leveling operation has not only reduced leveling costs, but has also decreased the time to complete the WI-HMP.
Motorized leveling has increased daily production time by reducing the physical strain on the leveling crew and by providing rapid transit between the survey vehicle and the work area. Motorized leveling also increases access to resources (e.g., food, water, emergency kit, weather gear, etc.) while away from the survey vehicles, reduces down time, improves safety and provides better adjustment to the variable Wisconsin weather conditions.