Digital Edition Exclusive: A Digital Desert

October 8, 2009
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Chad Shields, PLS, checks the status of the GPS unit during an OPUS observation session.


The U.S.-Mexico continental boundary is a 2,000 mile stretch that follows the Rio Grande from the Gulf of Mexico to El Paso, Texas, continues westward over the harsh deserts along the New Mexico and Arizona borders, takes a short jog north along the Colorado River and, finally, traces the southern edge California out to the Pacific Ocean.

The Secure Fence Act of 2006, enacted in October of that year, required the Department of Homeland Security (DHS) to construct--in the most expeditious manner possible--the infrastructure necessary to deter and prevent illegal entry along the southwest border. The goal was to have approximately 670 miles of pedestrian and vehicle fence, as well as the necessary roads and infrastructure, installed at various locations in Texas, Arizona, New Mexico and California. In general, pedestrian fences are used in urban areas while vehicle fences are used in rural or remote regions.

As the primary project facilitator for the U.S. Army Corps of Engineers, the Engineering & Construction Support Office (ECSO) was tasked with overall program management responsibility. ECSO, in turn, called in Michael Baker Corp., headquartered in Moon Township, Pa., as part of an indefinite delivery/indefinite quantity (IDIQ) contract to provide program management, engineering and surveying support.

With less than two years to complete the project, the Baker team had to survey, design and build almost simultaneously. The team had to best-guess fence alignments, geographic logistical complexities and road access requirements. Baker’s surveyors were tasked with performing GPS mapping control surveys along the border as well as numerous corridor survey tasks and technical reviews of the required boundary survey plats, legal descriptions and CADD files within the proposed 2,000 mile stretch of fence corridor involving numerous U.S. Border Patrol sectors.

Working with Keystone Aerial Surveys Inc. out of Philadelphia, the Baker team used aerial photogrammetric methods to acquire orthorectified imagery and topographic maps in some areas so that they could perform quick site assessments. Gaining right-of-entry to place the target panels necessary for the aerial mission proved challenging in certain areas, so the team used airborne GPS to reduce the number of land-based panels required. In situations where property rights-of-way were granted, Baker assigned surveyors to locate and mark aerial panels on the ground.

Launching the Expedition

Extending from Texas to Arizona and down into Mexico, the Chihuahuan Desert is the largest desert in North America. As its landscape varies from bolsons to medium-sized mountain ranges, elevations reach from 1,970 to more than 5,500 feet. The desert’s southwestern New Mexico region near Antelope Valley is largely barren. It was in this remote, unpopulated section of the desert that the Baker survey crew would set up their base camp.

Shields put together a survey crew that included Rick Huston, Scott Landsberger, Larry Lehmier and Rick Probert. Each individual brought a specific skill or level of expertise to this challenging project. Huston was charged with keeping in contact with Border Patrol agents and the Baker survey team as they traversed the desert landscape. He also helped adjust the predetermined GPS schedule to ensure safety and efficiency. Landsberger, the team’s expert quad driver, was tasked with maintaining the four-wheel-drive quads that the team would use to navigate through the desert. Lehmier was responsible for maintaining the cargo trailer and operation of the Magellan Crossover GPS units. And Probert was tasked with navigating the 10,000 mile journey from Pittsburgh, Pa., to McAllen, Texas, to the unincorporated community of Antelope Wells, N.M.

Together, this crew gathered the vehicles and equipment necessary to support a remote surveying and camping expedition. “It was our goal to be as self-sufficient as possible in order to complete the GPS mission within the time schedule we were provided,” Shields says. “We needed a backup of our backup so that we could minimize any downtime or miscommunication with the aerial missions.”

With assistance from C.J. Valero, owner of Valero Century RV in Delmont, Pa., and Millennium Cycle in Beaver Falls, Pa., the crew obtained the trailers, towing equipment, four-wheel-drive ATVs, ATV accessories, generators, additional propane bottles, 100-gallon steel gas tank, 125-gallon potable/black-water tanks and pumps, tools, spare tires for all the trailers and ATVs, wheel bearings for all the trailers, and the rest of the items needed to support the extended camping expedition. Baker supplied two ¾-ton Chevy Suburbans and a ½-ton Dodge pickup to tow the trailers. “Ironically, the main travel trailer’s model name was ‘Surveyor,’ so we felt we were off to a good start!” Shields says.

Before the adventure began, Shields tasked Mobil Satellite Technologies to install the Mobilsat SkyEdge Broadband 256, a satellite-based Internet service system, on the roof of the main travel trailer. “We knew digital communication between the field expedition and the outside world would be critical not only for safety but for effective coordination internally and externally,” Shields explains. “Cell-phone coverage is not an option in Antelope Wells.”

In addition to logistical support, the crew ensured that all survey equipment was in top-notch condition and that the laptop computers and backup external hard drives were in working order. Armed with five Leica System 530 GPS dual-frequency receivers and two Leica TPS 1103 total stations, the crew was ready to head out on their mission.

In mid-March, the surveyors arrived in Lordsburg, N.M., where they met the U.S. Border Patrol team that would accompany them to the staging area located on local ranch property in Antelope Wells, about 200 yards from the U.S.-Mexico border. The closest gas station was two hours away and the nearest city, Deming, about two and a half hours away.

With little more than antelope, jackrabbit, buffalo, coyotes and an array of reptiles, birds and bugs to keep them company, the surveyors put together a reasonably comfortable temporary home from which to launch their surveying expedition.

A Leica GPS 530 unit during a static GPS observation session. In the background is the existing U.S.-Mexico barbed-wire border fence and border monument.

Working in a Digital Desert

The crew started a typical day before sunrise by organizing the days’ activities based on a security update from the border agent. “We conducted daily planning sessions that covered our intended operating areas, access roads to travel, and the logistics of getting the ATVs and GPS equipment on-station,” Shields says.

The Magellan Crossover GPS units were installed on the ATVs and loaded with the approximate coordinates of the panel point. Once all the GPS units, paneling material, lunches, water and extra batteries were loaded up, the team would head out to perform the day’s mission. The crew members traveled by ATV 30 to 60 miles each day across rugged terrain, up steep hills, and over large rocks to position the aerial panels. “Most days, the crew worked a 12- to 14-hour shift,” Shields says.

At times, the survey crew packed their gear and ATVs in the cargo trailer and headed north on Highway 81 for an hour-long drive to an access road. From there, the crew drove the ATVs for another hour or more to reach the most-remote and inaccessible areas.

After the surveyors returned to the main camp, Shields downloaded the GPS data to an onsite computer, where they were processed and checked for the required accuracies using Leica GeoOffice 4.0.

The project was based on the NAD 83 adjustment of 2007, also known as NAD 83 (NSRS 2007), projected to the New Mexico West Zone state plane coordinate system. The vertical datum used was NAVD 88. The surveyors were only able to recover two NGS monuments during their travels--12Mile and Fireplace. However, these two NGS monuments were 10- to 30-miles north and west of the border surveyed. To supplement horizontal and vertical control along the border, the team established 12 Online Positioning User Service (OPUS) points. Occupations were between six and eight hours with several OPUS points being observed on different days to provide an averaged coordinate value for each.

The GPS network survey typically averaged 10- to 15-minute sessions. “The sessions were often extended a few minutes just to ensure that we would get good baseline results and didn’t have to return to those remote points,” Shields explains. “Overall, we had 1,100 baselines, two of which would only provide float solutions and were rejected.”

Loop closures were typically in excess of 1:100,000 with the lowest being 1:34,000 (short baseline). A free adjustment of the network was performed and produced satisfactory error ellipses of less than 2.0 centimeters. The final constrained adjustment produced similar and satisfactory error ellipses of less than 2.5 centimeters. Once the data were checked, they were exported to an ASCII file of the control points and e-mailed back to Baker’s Beaver, Pa., office to enable the border mapping process.

Over the three-month tour of Antelope Wells, the survey crew set 440 aerial targets, performed the GPS network survey, established 12 OPUS control points and rechecked panels 24 hours before the aerial missions.

Rick Probert performs a static GPS observation session with the Big Hatchet Mountains in the background.

Locating Waterlines and Monuments

As the project neared completion, the Baker crew was tasked with locating 22 border monuments and approximately 2 miles of waterline. Using Leica TPS1103 total stations, the surveyors occupied and backsighted the aerial targets and turned a position to the border monument. They also took photos of the monuments as part of the deliverables.

The waterline, exposed and laying on the ground, supplied a watering hole for the cattle roaming across the desert ranch. For this task, the surveyors used Leica GPS-RTK System 530 base stations and rovers. Each base station was set up on the nearest aerial target with the adjusted coordinate value. The rover unit received the real-time corrections from the base station’s Pacific Crest PDL radio modem.

The Forest River Surveyor travel trailer served as both home and field office for three of the five-person survey crew during the project.

Accomplishing the Mission

The survey team finished setting the network of aerial panels in less than three months and was back on the road to Pennsylvania by mid-June. “We had a truly dedicated team of surveyors who made sacrifices to make this adventurous expedition a success,” Shields says. “As a team, we were particularly pleased with the unique combination of solutions that came together in support of our efforts. From ATV-mounted GPS to the satellite Internet system, we found a way to employ advanced capabilities in an unfriendly, remote environment.”


View this article in the digital edition here, www.nxtbook.com/nxtbooks/bnp/pob_200910/#/34.

Sidebar: Mapping the Border

Before Baker’s engineers could begin the design phase, the entire project was mapped using softcopy stereo compilation. Keystone Aerial Surveys Inc. acquired the color aerial film-based imagery at 1"=400' using its Piper twin-engine aircraft and Leica PTW30 aerial mapping camera. Upon acceptable photo acquisition, Baker performed analytical aerotriangulation to support 1"=50' planimetric and topographic mapping and digital orthophoto production. Stereo map compilation was done using Baker’s ImageStation softcopy mapping system. During compilation, a DTM was created that would support the generation of 1-foot contours. The DTM was also used in the rectification of color digital orthophotos with a 0.5-foot ground-pixel resolution.

The deliverables included both Microstation V8 and AutoCAD digital files, orthorectified aerial imagery and a digital terrain model. The mapping and digital orthophotos were also used to review overall fence alignment, prepare property acquisition plats and legal descriptions, and measure construction progress.

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