Airborne GPS is perhaps the greatest technical advancement in mapping in the last 30 years. Never-the-less, use of this technology is not without its own technical and logistical challenges, particularly in remote areas of the world. AeroMap has been involved with testing airborne GPS for the Remote Sensing/GIS Center for the Cold Regions Research and Engineering Laboratory, U.S. Army Engineers since early 1994. In addition, we have successfully completed independent airborne GPS operations in urban and remote areas of Alaska.
      Our most recent project was the control and acquisition of aerial photography for creation of digital orthophotos at Cape Yakataga for the University of Alaska, Office of Land Management. AeroMap was contracted by Rick Rogers, senior forester, at the university to produce 1"= 1000' digital orthophotos with 50' contours for timber management and planning for a 125 square mile area. We were also to acquire 1"= 1320' color resource photography.
      In analyzing the project, we determined that the ortho imagery could be acquired as black and white photography, flown at 20,000 feet above mean terrain to minimize control requirements and reduce the number of mapping models. The resource photography could be acquired independently as uncontrolled imagery.
      Cape Yakataga presented logistical challenges for project photo control in that the area is inaccessible by road, the terrain is difficult and rugged, rising from sea level to 5000 feet with glacier covered mountains. The project is 380 miles east of Anchorage by air along the north coast of the Gulf of Alaska. Access to Cape Yakataga was limited to a 3800 foot gravel airstrip.
      After laying out flight lines for the 1:40,000 black and white photography, we determined that traditional ground control would require approximately 42 horizontal and vertical points. Layout and planning for the control was made more difficult due to glacial conditions in the inland high country.
      As an alternative, AeroMap recommended airborne GPS photogrammetry. Airborne GPS relies upon a GPS-camera interface to determine exact horizontal and vertical camera exposure stations during the flight. Using this technology required only six ground control stations plus one additional station for occupation by a base GPS receiver during the photo acquisition flight. We felt that six stations gave us adequate ground registration plus a small buffer for redundancy.
      The university contracted separately for survey of the six control stations and the base receiver observation station. The surveyors used differential GPS for the ground control based upon existing National Geodetic Survey control stations near Cape Yakataga. For purposes of resource mapping, we decided that GPS vertical control adjusted to the Alaska 1994 Geoid Model would be adequate.
      The surveyors opted for post-marking rather than pre-marking the control. As they reconnoitered the project with a helicopter to select control stations, they laid photo panels over each station. They then used the helicopter to hover above the stations at 1000 feet to acquire several vertical photographs of each panel, taken with one of AeroMap's 70 mm handheld cameras. The panel was then immediately retrieved to eliminate the need to return at a later time.
      Once the photography had been acquired and processed, AeroMap used a control transfer device to accurately mark the controlled stations on the 1:40,000 scale photography from the 70 mm photographs.
      When the control work was complete, the surveyors notified AeroMap so that we could begin a weather watch for the project. In early June, a high pressure system began moving through the interior of Alaska, causing a dry northeasterly flow of air over Cape Yakataga. Our flight crew and GPS operations manager readied their operational plans. Early in the morning on June 10, Andy Alexander departed Merrill field with the GPS base receiver in his Cessna, bound for Cape Yakataga. He was on sight by 10 a.m., where he recovered the base station control point located near one end of the airstrip. The photo ship departed Merrill at about 9:30, allowing time enough for the sun to rise above 35 degrees while enroute.
      AeroMap uses "On-the-fly" (OTF) GPS post processing techniques which means the flight crew did not have to land at Yakataga to initialize. They initialized while airborne within about 15 miles of the base receiver. Andy had set up his base station and was monitoring a VHF radio so that he would know when to begin logging data. At the appropriate signal, both the ground station and the airborne receiver were activated.
      The AeroMap crew flew the required four parallel flight lines plus three cross-flights over the project using standard 60 percent endlap and 30 percent sidelap with a Zeiss TOP camera. Though at least one cycle slip occurred during a turn, the receivers reinitialized properly and the mission continued successfully. When all lines had been flown to the satisfaction of the flight crew, the GPS receivers were shut down. Color resource photography was flown independently.
      Upon return to Anchorage, the film was downloaded, processed, edited, and annotated. The GPS data was also downloaded for post processing to determine camera exposure stations. The exact camera station positions were input into the aerotriangulation process which was completed using specially modified software for airborne GPS applications. The aerotriangulation included 37 exposures from the four primary flights, plus 28 cross flight exposures for a total of 65 exposures in the simultaneous block adjustment.
      One of the six control stations was located on a mountain top in a small opening between snow fields. The point was difficult to read by the plotter operator during the aerotriangulation. We therefore decided to eliminate this station from the control array rather than bias the entire project.
      We are pleased with the results of this project. Using five control stations, plus six additional check points provided by the surveyors throughout the project, AeroMap was able to obtain residuals with root mean squares of 2.4 feet in X, 2.2 feet in Y, and 2.4 feet in Z. The collection of digital terrain data for contour generation, digital elevation modeling, and digital orthophoto production followed.
       With the logistical challenges of Cape Yakataga behind us, AeroMap is now preparing for two linear projects. One is the 85-mile Tyee Lake to Swan Lake Intertie for Alaska Energy Authority in Southeast Alaska. The second is a 71-mile road project from Savage River to Kantishna for National Park Service in Denali National Park. Both projects will be completed with airborne GPS relying upon OTF post processing. Due to the lengths of these projects, multiple base stations will be required along each route, using helicopter support for deployment. The weather conditions, deployment of ground stations, simultaneous activation and deactivation of ground stations and airborne GPS receivers are all critical to the success of these projects. As importantly, in remote areas where control points and nearby airfields are not available for initialization, on-the-fly GPS post processing is essential.
      As can be seen from the above discussion, technical knowledge is extremely important, but of equal importance is the logistical experience, coordination, and planning that must go into a remote project. A simple coordination oversight can be as critical as a technical error. Either mistake may cause loss of valuable time and resources. In remote locations, there is no substitute for detailed planning.

About the Author:
Bob Schweitzer has been surveying and mapping Alaska and other western states for over 30 years. He is president of the American Society for Photogrammetry and Remote Sensing, Alaska Region and a past president of Alaska Society of Professional Land Surveyors. He is the business development manager for AeroMap U.S.

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