Helicopters Provide the Most Accurate Surveys

by Larry Christenson, Woolpert LLP

If you’re planning to perform the initial corridor study for a city bypass route, aerial photography from a fixed-wing aircraft is probably the ideal starting point.

What if your goal is to design the reconstruction of intersecting interstates in downtown Detroit or another major city? Sending surveyors into the roadway for detailed elevations can be life-threatening for the surveyors. And it’s a nuisance to motorists when a traffic lane is closed for the surveyors to work in.

The direct cost of blocking traffic lanes runs into the tens of thousands of dollars. Direct costs can run even higher when a department of transportation is forced to restrict the hours when surveyors can work — usually in high traffic urban areas — to no-peak traffic volume times, like Sunday morning. To reduce the impact on the public, the restrictions can add premium shift time to the cost of surveying.

As high as the direct costs run, they can be dwarfed by the indirect costs — that is, the cost to individuals and businesses who are delayed by the project.

Obtaining data

Good survey information is a basic necessity. You can’t start the project design without it. The solution can come with rotating blades of a helicopter, equipped with the same GPS flight navigation, camera controls, and high-quality, large-format, aerial mapping camera used in fixed-wing aircraft. The aircraft and the camera are synchronized with GPS for navigation and photo flight efficiency.

Fixed-wing aircraft are generally forbidden to fly at less than 1,000 feet above ground features in urban populated areas. A helicopter can pass over the same area at 300 to 500 feet of elevation — close enough for the camera to capture small pavement fractures and vaulting; close enough to allow the photogrammetric capture of vertical data that’s accurate enough to support engineering design for major reconstruction projects. Taking the photograph closer to the ground provides higher picture resolution and enhanced detail of the ground features, which allows for more accurate point-to-point measurements. The altitude, scale, and resolution of the original photographs are directly proportional to the accuracy that can be obtained.

How accurate is it? Mapping firms generally fly helicopter photography at scales from 1 inch = 50 feet to 1 inch = 83 feet to achieve the vertical accuracy required to meet the engineering specifications for the design. Typically, the rule-of-thumb vertical potential is one part in 10,000 of the flying height. Therefore, if we fly at 500 feet above terrain for a photo scale of 1 inch = 83 feet, the accuracy exceeds or equals +/-0.05 feet. Achieving these accuracies photogrammetrically depends on establishing a survey control network that is high enough in precision to reach near geodetic quality. It also depends on establishing and adhering to a set of rigorous survey standards developed especially for this purpose.

What about costs?

Those photogrammetric accuracies are the key to evaluating the price tag for helicopter mapping. A helicopter is probably four to five times more expensive to fly and maintain than a fixed-wing plane. But it isn’t appropriate to compare the cost to the typical aerial flight because vertical accuracies are much greater than from 1,000-feet altitude 1 inch = 167-foot scale photography. Instead, you have to compare the cost for helicopter mapping not to the cost of a fixed-wing aircraft, but to the cost of surveying in the field. Aerial photography saves time and money over ground survey for large projects; a helicopter costs more than fixed-wing, but produces a level of quality two to three times more accurate than conventional altitude photography and is comparable to manual field survey. And, it’s far less expensive than a manual field survey.

Since helicopter-based photography is most useful where traffic is heavy, it was first used around the major cities of the East Coast. There simply weren’t any helicopters dedicated to aerial photography available in the Midwest — and the cost of transporting the helicopter was a major factor in overall project cost.

When Midwestern cities learned about the possibilities — Michigan DOT learned about it in a professional association presentation by the Virginia DOT, for example — they had to bring in the helicopter from the Eastern states, where there are perhaps a half dozen firms with helicopter capability. Now, only a handful of mapping firms supply the service locally to the Midwestern states.

When to use it

Deciding whether to use helicopter aerial photography is a function of the detail needed. In most state departments of transportation, the engineering group is a separate section or unit from the survey/mapping group. The engineer knows what’s needed to accomplish the design and requests mapping and surveying with a particular level of detail and accuracy.

If the engineer requests 50-scale mapping, the survey/mapping group is likely to ask more questions regarding the type of project: reconstruction vs. rehabilitation, feature details and pavement elevation accuracies needed. For urban areas, 20- or 50-scale maps with 0.25- to 0.5-foot contours are reasonable requests for reconstruction projects where it’s critical to meet and match elevations and grades at bridges and drainage structures. Once the need is established, the survey/ mapping group would begin to develop a low-altitude flight plan to provide the detail the engineer needs. And 20- to 50-scale mapping would probably be a call for a helicopter — especially if it involves high-order elevation data in the city.

Usually, helicopter photography is used for major reconstruction projects where detailed feature collection and drainage issues are critical. It is typically applied to projects that require matching and meeting existing surfaces and grades as opposed to a mill-and-resurface project that begins with milling off an inch of pavement and replacing it with a new surface. In essence, it’s the difference between reconstruction and rehabilitation projects.

Drainage is often a key factor in the decision because it requires such vertical precision. And the detail available with a helicopter-mounted camera can be useful for bridge deck evaluations and maintenance issues. Of course, the camera can’t see under the bridge or below the drainage grate in the curb, so supplemental field work is needed as well. But, the detailed surface mapping is a good start for these projects, too.

From the helicopter’s altitude, the photography shows pavement cracks on roads, airport runways and bridge decks. When the photography is overlaid with planimetric mapping and contours created from the high accuracy digital terrain model, it’s an excellent tool for engineering design.

As good as the end product looks, how can the engineer be sure of its accuracy? Most of the firms build field checks into their projects. Typically, they perform and provide field checks to prove to the clients that it’s as good as they say it is.

But it is essential to perform the field checks in non-dangerous areas. It’s no help to use a helicopter to prevent traffic delays and keep surveyors safe — then move surveyors into the same areas to check the data accuracy. So field checks are set up out of harm’s way. And those checks verify the map accuracies — +/-16 mm with photography flown at 500 feet of altitude.

In protecting surveyors, in avoiding delays for motorists, and in saving the public’s money, helicopter mapping has proven to be a safe, economical, and accurate solution to highway reconstruction needs. 

Reprinted from Better Roads Magazine
October 2002

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