Aerial Mapping in Paradise

Inclement and unpredictable weather greatly delays a much-needed aerial survey of Bermuda, but the practice proves worthwhile for the surveyors and the project.

By David A. Webb, CP

An aerial survey of Bermuda was flown in May 2012 to provide current geographical data of the island. The Department of Land Surveys and Registration, under the Ministry of Environment, Planning and Infrastructure Strategy, commissioned the survey, which was conducted in two parts by Aerial Surveys International, a Colorado-based firm specializing in large-format film and digital aerial imagery. 
 

Needed Data for Bermuda


Before the project started, Marc Bean, minister of environment, planning and infrastructure strategy for Bermuda, said: “The Bermuda Mapping Update Project is something that will be of great benefit to our island and is long overdue. Aerial photography and mapping were last updated in 2003, and since then over 1,200 new addresses have been created that are not mapped.

Significant infrastructure developments built since 2003 are missing from the maps, such as the National Stadium, Berkeley School, and large housing developments such as Whale Watch, Loughlands, and Harbour Side Village. Furthermore, many buildings and infrastructure that have long since been removed and no longer exist are still shown on the existing mapping, for example the Sonesta Beach Resort and the Club Med Hotel.”

The inclusion of non-existing infrastructure and the omission of key detail reduces the quality and value of mapping data from which very important decisions are made. Up-to-date, quality mapping is fundamental to Bermuda’s modern digital economy because the majority of government and private projects require spatial information to operate on a daily basis.

The result of this project will create a base structure of digital GIS mapping data/information that in turn will provide a more efficient, cost-effective, and accurate means of governance and decision-making.

Minister Bean said, “The money spent in commissioning this aerial survey is a worthy investment in bringing Bermuda’s mapping current and to the international standards. The finished products of this exercise are invaluable to the work of key government agencies such as the Ministry of Public Works, the Department of Planning, and the Department of Land Valuation, to mention a few.” 
 

Acquiring Images in Tropical Weather

The two-phase project, including aerial image acquisition and photogrammetric mapping, began with the ferry mobilization of the Aerial Surveys International‘s survey aircraft and crew on May 16. The first challenge of the day was trusting in modern aviation-guidance GPS technology to work flawlessly during a 690-nautical-mile flight from the United States shores to a small tropical island in the middle of the Atlantic Ocean. Failure to intersect with the GPS coordinates for the island airport destination was certainly not an option the flight crew wanted to consider.

The data acquisition required a specialty survey aircraft to fly at low altitudes over Bermuda, taking high-resolution aerial photography of the island, using a precision Intergraph DMC digital aerial mapping camera mounted in the floor of the fuselage. To complete the imagery, the airplane flew a series of 15 parallel, low-altitude (1,640 feet) flight line runs, taking a series of overlapping photographs of the ground; the Ministry advised the public in advance.

While flying this type of aerial survey mission seemed rather routine and simple, it was not without wind, rain, and cloud-cover challenges. Because each photo mission required a certain sun angle above the horizon and a sky free from overcast, rain, and/or clouds, many anticipated days were spent by the pilot and aerial photography crew, waiting for suitable weather conditions to cooperate. Days and sometimes weeks would pass before a few brief hours or, if we were lucky, a full day presented itself as clear enough to successfully acquire photographic images. 

While good weather has always been a key element of the aerial mapping industry, conversely, extended periods of unforeseen bad weather could eventually spell certain death for a projects’ profitability. For the Aerial Surveys International flight crew, tropical storms, monsoon-type weather patterns, daily rain and drizzle, and naturally low clouds (800’ AGL ‘white poppers’ as we call them) resulted in days of standby. However, these standby days were initially planned for, so, rather than sitting around in a hotel room, the flight crew chose the opportunity to tour the island and visit the local people. Marking time in paradise is not a bad job, if you can get it.

For any unpredictable and difficult offshore weather area, successful acquisition has always been a risky challenge. Because we could only plan for and not control the weather, this became the epitome of the nature and risk we experience in our business every day. With final completion of the last four partial flight lines in sight, tropical storm Beryl set in and delayed the completion of the project an additional two weeks. However, the fight crew was able to persevere through it all and rose above the challenges.   

The success of the image acquisition phase relied heavily on an on-board GPS dual-frequency survey system (separate from the aircraft GPS navigation system), which provided a fixed GPS satellite triangulated position every second to an accuracy of about 5cm. The camera’s position was constantly monitored by an inertial measurement system (IMU), which measured the camera’s pitch and roll, and was connected to the on-board GPS, which in turn provided centimeter-level positional accuracy.

This type of precision GPS system locates the camera’s position at the exact time the picture was taken, allowing every image to be accurately geo-referenced to its exact position on the ground. The camera system was also equipped with forward motion compensation, which reduced any camera image blur that would have been created by the speed of the aircraft’s forward motion in comparison to the ground.

In conjunction with the aerial survey GPS navigation and IMU positioning systems, the Survey Section marked the ground with large white targets that were surveyed to great accuracy and were used to orientate and scale the photographs to ultimately “connect” them to the ground.  Staff from the Land Surveys Department painted these targets to support the overall survey mission.

Although this first aerial photo acquisition stage began on May 16, it was completed over the weekend of May 26 and 27. Due to the unpredictable weather conditions, multiple flights and re-flights were required to ensure cloud-free conditions. On May 28, the aerial survey aircraft departed for its home base in Watkins, Colorado.

Flights were completed as the weather allowed; breaks during storms; overcast, light conditions; and periods of acceptable high, thin clouds and low, white, popper clouds provided flying times. All in all, it took three days of productive flying and 11 additional days of non-productive, aborted flight missions and standby days on the ground to finish the job. To put this in perspective, the Aerial Surveys International crew spent 112 hours to achieve the 2.5 hours of productive flight that it would have normally taken to acquire enough imagery to complete this type and size of project had the weather been ideal and cooperative.

Many challenges were offset by the great coordination of the airport control tower staff, as well as the constant support and encouragement by Minister Bean and the consummate, professional assistance of Peter Hopkin.

While the first acquisition stage of this project created some definite challenges, the second stage was routine because it took place inside and thus wasn’t weather-dependent.  Phase two entailed extracting horizontal and vertical positional information from the photographs by capturing the ground terrain elevation features as well as outlines of buildings and other infrastructure, to great accuracy. Thus we were able to create a digital map of Bermuda.

Upon inspection of the final imagery acquired, we decided fate had been on our side. The adverse weather we were forced to endure may have been the best thing about this project. The water depths around the island of Bermuda are about 60 feet. High, overcast weather conditions actually produced a slight- to no-water-glare effect, allowing the coral reefs in the imagery to be very well defined. The shadow definitions for land features were softer than normal, but the detail remained sharp across the images. The overcast conditions also assisted in minimizing the harsh sun’s reflection or “sun flair blow out” on Bermuda’s inherently white building roof tops.  Imagery detail in the light overcast shadow areas was significantly enhanced.

In the end, the results were worth it. After all, in the aerial mapping business, a picture (without clouds) truly is worth a thousand words, but a picture under the right overcast conditions may be worth a few words more.

David A. Webb is a certified photogrammetrist and vice president of business development for Aerial Surveys International. He has worked for 29 years in the aerial photo and geospatial mapping profession, planning and managing projects worldwide.

Permission to use text and pictures from bernews.com. is courtesy of Patricia Burchall of the Bermuda News and Minister Marc Bean of the Ministry of Environment, Planning and Infrastructure Strategy.
 
 
SIDEBAR
 
Why Hire Surveyors When There’s Google Earth?
 
Peter Hopkin, BA, FRICS, MSc (GIS), the temporary mapping officer at the Minestry of Environment, Planning and Infrastructure Strategy, said: “Following the announcement in March that this survey would take place, there were questions raised on various local social media sites regarding the usage of this mapping method over Google Earth—namely, ‘Google Earth is available on the internet, so why are we spending public money on something that is free to anyone?’”

There are key differences between this project and internet mapping solutions such as Google Earth. Google Earth is only available as an image and does not include map data. The map data is actually more important to the majority of users than the background imagery because it can be used by GIS software to calculate information. For example, if someone wanted to measure the total area of roofs in Bermuda (perhaps to measure our potential for solar energy or our water catchment), they would use the data and not the image.”

The map data will include a detailed digital terrain model, which will show the ground contours, and also a digital surface model, which will show the height of every building in Bermuda. From this it will be possible to create a 3D model of the whole country, which is certainly not possible from Google Earth.”
The Google Earth imagery is also low resolution. The largest objects visible on Google Earth are a few feet across, whereas the Land Surveys Section has purchased high-resolution imagery that will enable users to see objects a few inches across. For example, the Land Surveys Section are collecting the location of Bermuda’s mooring buoys with this project; the buoys are just not visible on Google.”
Google Earth, which uses publicly available imagery, has a valuable place in the GIS world; however, high-resolution imagery is necessary in projects needing detail for final government decision-making. Google Earth is not always detailed enough for all government uses.

Hopkin said, “We have also found that the Google imagery is not quite in the right place.  If you compare the coordinates of a point accurately positioned by our GPS equipment with coordinates obtained from Google Earth, there is a difference of about 10m or 30 feet.  This might not seem like much, but if someone wants to use our data to, for example, track their vehicles, this difference will count. Some years ago we tried to contact Google to correct their imagery, without success.”

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