A challenging photogrammetry mission through The Grand Canyon.

Mapping the Grand Canyon, more particularly the floor of the Grand Canyon, raised a few eyebrows around the office when the call came in. Few projects generate the excitement or present the challenge that we faced this past Memorial Day weekend during the photo acquisition for the mapping effort. For most professionals, a project like this comes along only once in a lifetime.

The Grand Canyon Monitoring and Research Center (GCMRC, part of the United States Geological Survey) in Flagstaff, Ariz., needed precise topographic mapping of the Colorado River and the floor of the canyon in 11 different locations. Accurate information would be required for the analysis of erosion and sedimentation along the riverbed. A 25 cm (0.8 ft) contour interval was selected.

A flight elevation 2,000 ft above the river was chosen to achieve the required accuracy. Most photo missions are carried out at altitudes greater than what was selected for this project. Why was a low altitude selected for such a challenging project environment? An analogy could be made to the eye chart at your doctor’s office—the closer you are to the chart, the more detail you can see. Low-altitude flights are required for high-precision, detailed maps. Higher-altitude flights are used when the project requirements are less demanding.

Timing of the mapping would be critical. The flow of water from the Glen Canyon dam, which controls the level of the Colorado River in the Grand Canyon, was restricted over the Memorial Day weekend so more of the riverbed would be exposed than under normal conditions. The aerial flyovers had to take place during this low flow period. The flights could not begin until Sunday, May 26 and had to be completed by Monday, June 3. Photo acquisition would begin in the upper reaches of the canyon and progress downstream.

A Complicating Factor

Placing targets along the river’s edge in the canyon is a significant task. Establishing the precise position of these targets is even more challenging. Both tasks for this project were accomplished by the GCMRC. Access to the sites was by raft, floating the river.

The GCMRC was very interested in any new technologies for controlling photo projects due to the substantial effort required to establish conventional control targets on the ground. Airborne GPS (ABGPS) can provide the accurate three-dimensional position of the aircraft at the time of each photo exposure. And an inertial measurement unit (IMU) can be used to measure the three-dimensional rotation of the camera at the time of the exposure. The combination of the two can eliminate the need for ground control for the right project.

While we at Photo Science Inc. (Lexington, Ky.) have been using both technologies for years, several major issues were of concern for this project. First of all, there are limitations on the accuracies gained from using a combination of ABGPS and IMU technology. Under ideal project conditions, the horizontal position of the aircraft can be determined within 8 to 10 centimeters. However, as with other kinematic GPS applications, the elevation determination is less accurate, providing accuracies in the range of 15 centimeters.

Secondly, the Grand Canyon doesn’t present the ideal environment for carrying out an ABGPS mission. Its rugged beauty creates a very challenging environment for the application of this technology. Flying below the rim of the canyon blocks access to some of the lower elevation satellites while the canyon walls provide an environment for multipath.

Moreover, ABGPS photo missions place significant constraints on a flight crew. It is critical to maintain lock on at least five satellites during the entire photo mission. This requires flat turns and gentle climbs or descents so the GPS antenna mounted on the aircraft’s fuselage maintains its ability to receive the GPS signals from the satellites. Steep banks or climbs tilt the antenna away from some satellites causing cycle slips. Obviously, these additional requirements would further complicate an already difficult assignment. The decision was made to test the combination of ABGPS and IMU technology on four sites within the canyon.

Planning the Mission

The Grand Canyon stretches some 277 miles from the Glen Canyon Dam in northern Arizona near the border with Utah to the Hoover Dam in western Arizona near Las Vegas. At its deepest, the river is some 6,000 feet below the rim of the canyon. The canyon is very narrow near the Glen Canyon Dam and stretches some 18 miles from rim to rim at its widest. The average width is around 10 miles.

Capturing low-altitude photography for this project required the flight crew to work 3,000 feet below the rim in the lower part of the canyon. The walls of the canyon are very irregular, which further complicated flying in the canyon. A great deal of planning was required to carry out the mission at the required altitude in a safe and careful manner. Each of the flight lines was digitized on existing digital topographic maps of the canyon. A flight elevation was established for each flight line based on the elevation of the river at that point in the canyon. The surrounding contours for the canyon walls located in the vicinity of each flight line were then carefully reviewed.

The flight crew had to find a way to safely fly each line. How would they get to the lines? Should the lines be flown to the east or the west? Where could turns be accomplished in order to get to the next flight line? How would they get in, and out of, the canyon? While questions like these are straightforward for most aerial missions, this project required some creative thinking.

The crew met with GCMRC representatives in Flagstaff on Friday before the holiday weekend to be briefed on the critical safety issues. The canyon is home to a strong tourist industry. There can be as many as 160 tourist flights an hour over the canyon. While the tourist flights would not pose a problem while flying in the canyon (they are restricted to elevations above the rim), they were a major concern into and out of the canyon.

A pilot and a camera operator make up the crew for typical photo missions. But this was far from the typical photo mission. A co-pilot was added for this project to serve primarily as an observer for both the canyon walls and the tourist flights.

After the safety briefing, the crew flew reconnaissance missions over the project sites on Saturday, May 25 to test their mission planning. As you might imagine, no amount of planning on paper can truly prepare you for the challenges faced during the actual flight. The reconnaissance flights were invaluable in preparing for the real deal. Some of the flight lines had to be adjusted as a result of new obstacles found during the reconnaissance.

The flow of water from the Glen Canyon Dam was minimized to expose more of the Colorado riverbed for the mapping effort.

A Challenging Mission

We awoke on Sunday morning to a high, thin overcast. This would actually work in our favor, as the overcast would minimize shadows in the canyon. Most projects require a minimum sun angle of 30 degrees above the horizon, but the steep canyon walls resulted in heavy shadows during much of the day. Flying was restricted to the middle part of the day when the sun was at least 60 degrees above the horizon. At the canyon, the sun reached our minimum around 10:30 a.m. Mountain Standard Time.

Everything was a go for the day. I headed out to control point L 404, an A-order monument located near Page, Ariz. I would be manning a Trimble 4000 SSI GPS unit on the ground, collecting data at a 0.5 second epoch to correspond with the collection in the aircraft. The plane flew over the site just before 11:00 a.m. to begin the initialization of the GPS session. From that point on, the limitation of flat turns and shallow climbs was in place. After completing the photo acquisition, our plane flew back over the ground station to complete the day’s work.

The ground station monitored for ABGPS flights has to be within close proximity to the actual flights. Sunday’s flights took place around 5 km from me, well within the prescribed range. But the work in the upper part of the canyon was completed on Sunday, so I moved from Page to Tusayan, located just south of the entrance to the Grand Canyon National Park near the center of the canyon. Monday’s flights would be close by. The three-hour drive from Page to Tusayan took me through some of the most beautiful country I have ever seen. If you haven’t made the drive along the south rim of the canyon let me suggest one more item for your “must-see” list.

The photo acquisition for the second day would prove to be much more challenging for the flight crew. The flight lines near Page were actually above the rim of the canyon since the canyon is relatively shallow in the upper reaches. But the flights for the second day were taking place almost 3,000 ft below the rim of the canyon. GPS reception onboard the aircraft would be restricted to virtual elevation masks of 26 and 30 degrees for the two ABGPS sites on this day, thanks to the canyon walls that loomed above the flight lines. A quick check of the GPS constellation for an elevation mask of 30 degrees revealed two short periods of time with the six satellites and maximum positional dilution of precision (PDOP) of four needed to ensure accurate data acquisition. Luckily, these periods fell within the time the sun was above 60 degrees.

The location of the ground station was much more interesting for the second day. My monument was located just 5 ft from the rim of the canyon. I was cautious setting up the tripod since there were no rails or barriers between the tripod and the canyon wall. This location would prove to be a great vantage point to watch the crew carry out their mission below me.

Again, just before 11:00 a.m., our crew flew over the ground point to initialize the kinematic session and headed down into the canyon. It was amazing to watch the crew fly along the river capturing the required

photography, then maneuver the aircraft among the canyon walls to the next site. Just like the day before, the crew flew back over the ground station after completing the ABGPS flight lines. I knew the flight crew had earned their stripes on this day.

The third day did not include any ABGPS or IMU collection, freeing the flight crew from the strict limitations that go along with that collection. While the sites remained challenging, the photo acquisition for this day went well and that part of the project was complete. Several weeks of careful planning had resulted in a safe and successful mission.

Promising Technology

The Grand Canyon project was one of the most challenging and exciting in Photo Science’s history. The initial results from the project look very promising. The aerial photography of the canyon floor is stunning. The initial GPS and inertial data look good. Careful checking and analysis will be enlightening and provide important information regarding the use of new technologies and applications in challenging project environments.

The USGS and the GCMRC have been great to work with on this project. They are obviously interested in the evaluation of the ABGPS and IMU test sites. The next few months will bring answers to the applicability of new technology for future projects.