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Modeling Highways in the Sky
17 Oct, 2006 By: Kenneth WongYou can't see aviation corridors -- but you can design and analyze them using Autodesk Civil 3D.
There are highways in the sky. You can't see them, but they're there. At Landrum & Brown, a commercial aviation consultancy, they call them "imaginary surfaces." These surfaces define airspace used by planes for takeoffs and landings.
Frequently, proposed aerial highways intersect with something on the ground -- a freeway overpass, a cell phone tower or a skyscraper, for example. It's up to firms like Landrum & Brown to help airport planners arrange the imaginary surfaces in a sensible pattern that keeps planes away from obstructions while maintaining a highly efficient air traffic flow to airport runways.
What's the difference between surfaces on the ground and surfaces in the air? According to Bob Endres, manager of Landrum & Brown's CAD engineering department: Nothing. "Who cares if it's up in the air or on the ground?" he says. "Something that's above grade can be a mirror image of something below grade."
During a luncheon hosted by Autodesk, Endres watched a demonstration of Autodesk Civil 3D, a civil engineering software package for transportation, site, sewer, storm drain and subdivision projects. "We don't design roads on the ground, but we design roads in the air," observes Endres. Watching the way the software handled corridors, he thought, "I can model an exact flight path with that." That prompted Landrum & Brown to begin using Autodesk Civil 3D for its work.
Floating Field
If Endres is working on the final approach path of a flight, the analysis might encompass a space roughly 50,200 x 16,000ft. And that's only a fraction of the spatial analysis required to map out the invisible network of flight paths around an airport. For a typical airport, Endres says, "We'll use a digital terrain model [DTM] of about 40 square miles."
Clear for Landing
Usually a Landrum & Brown project begins with the planimetric data supplied by the airport, showing only the horizontal positions of ground characteristics. This serves as the project's base map. Often, Landrum & Brown is asked to do a systematic aerial survey of the area to collect data on obstructions. The method, Endres explains, "is flying [over the area] in a very controlled grid system to take aerial photography. In order for this data to be usable, the [data collection method] has to meet FAA standards for aerial photography accuracy and setup."
In 3D visualization terms, a plane's approach path is typically a sloping trapezoid (see figure 1). "They're enormous areas," says Endres. "The narrowest point is at the runway threshold. The further you go, the wider the profile gets. We have to evaluate every single vertical object obstructing that airspace" -- a tree or a utility pole, for instance. "In one of the project I worked on, there were over 8,200 objects involved."
The path of an approaching flight, shown here with the tree clusters in the area.
"Sometimes, a pilot might spot another aircraft on the runway, and might have to declare a 'missed approach,'" explains Endres. "When that happens, there's a set of FAA procedures the pilot must follow to safely climb out of the airspace." And that's why all the objects -- including the so-called obstructions that do not actually pose an imminent threat to the approaching aircraft -- must still be accounted for.
Suppose the airport administration has contracted a firm to cut down a tree obstructing the approach path. With Civil 3D's point-database system, Landrum & Brown would obtain the new elevation of the tree and update its latitude-longitude attributes in a single step. "I won't have to open several [AutoCAD] drawings and find out where that tree is on each," says Endres. "I just update the point database, and all my drawings dynamically update."
Dizzying Rotations
Previously, Endres wrote programs to analyze flight paths. "In [AutoCAD plan view], I drew a centerline to the runway's edge to show the approach path on a contour file," he recalls. "Then I'd have to write a program to digitize the z-axis value wherever a topographical feature intersects that centerline. Then I'd have to rotate that to get a profile view of it. Then I'd have to export it out of AutoCAD to get the UCS [user coordinate system] out of it. Then I'd have to bring it back in as a rotated object."
Building an airspace map using the old approach, Endres says, could take as long as two weeks. "On any given approach path, there can be as many as seven different surfaces that need to be evaluated," he explains. "With Autodesk Civil 3D's ability to process point databases, I can put all my analysis data into the point database. And, simply through the use of point-style settings, I can display what I want. In the past, we used our own proprietary software to do the analysis. Then we'd insert the output as an AutoCAD block with attributes. But to analyze a different surface, I'd have to rerun the analysis. So we'd have multiple drawings stored in the same object."
In cases where an airport anticipates that a certain runway would reach maximum capacity in 10 years, Landrum & Brown might be contracted to identify the best place to construct a new runway. Then, Endres and his team can easily test various what-if scenarios using Autodesk Civil 3D.
Shrinking Time
Conducting analysis on a flight path is a grueling process, and displaying the result is no easy task, either. "One of the big things Civil 3D has allowed me to do is to automate the creation of plan-and-profile drawings," explains Endres. "Before, it took me a while to create a drawing view that was acceptable to FAA. Now I have a set style that allows me to customize the exaggeration of the profiles, show the axes I want, display the text I want, show the ground in DTM and show all the airspace surfaces."
  A plan view and a perspective view of the surface of a runway. All colored objects showing through the bluish purple surfaces are in violation of the surface. The brown colored area represents the ground itself violating the approach surface. |
Before mapping technology, it took Landrum & Brown about nine days to analyze a single runway, Endres recalls. "With mapping software, we've reduced the time to about two days. Now, with Autodesk Civil 3D, we've reduced it to one day or less."
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