An ArcGIS GeoProcessing Model for the Obstruction Analysis of Military Airfields

By Ryan Warne

One of the major encroachment issues facing military operations are airfield obstructions.  Aside from natural objects such as hills, mountains, and trees, human construction such as towers, buildings, and wind turbines are continually planned for sites that have a potential to impact safety associated with military operation on airfields.  While a wind turbine several miles away from an airport may seem perfectly legitimate, there are specific height restrictions that must be maintained to protect incoming or departing aircraft.  The Federal Aviation Administration (FAA) provides these requirements that all new or altered constructions near or on an airfield be subject to an Obstruction Evaluation.

As part of the Research and Development program at GISi, I took the opportunity to develop an ArcGIS tool that could be used to provide acceptable locations for new or altered structures around the larger types of military airfields, Class B.

Image 1. A 3D model of a Class B Imaginary Surface

My R&D project was a big success. I was able to create a tool that automated the creation of obstruction zones around Military airfields based on specific user inputs.  It took a while to develop and there are certainly areas of improvement, but this was a proof of concept that a model could be created using out-of-the box tools from ArcGIS 9.3.1. Since the model uses Navy data, and the intention was to empower installation planners to perform site analysis, 9.3.1 was used over 10.0 due to the Navy’s current software availability.  Version 10.0 provides several improvements that would increase the efficiency of the model, but it can be redesigned to that version fairly quickly for non-DoD users.

Image 2. A Class B Imaginary Surface based on NAS Jacksonville runway 10/28.

The tool operates by analyzing the topography of the landscape and comparing this with the FAA Part 77 imaginary surfaces surrounding the military airfield while compensating for user inputs of airfield elevation and the obstruction height – all used to produce an Obstruction Zone (lower right corner of Image 3).  The trick was to create a model using data in the format that it is typically available in for the Navy (e.g., contour lines for elevation) so that this could be usable at other installations.

Image 3: Raster topography and TIN imaginary surfaces combine to create an Obstruction Zone

The complexity of the model is portrayed in Image 4. Recognizing that the reader can’t see the details of the model, here is an overview of the tool’s processes:

  • Create 4 TINs based on the outer transitional slope, inner transitional slope, approach departure surfaces, and the horizontal surfaces of the imaginary surfaces
  • Convert the topography to TINs
  • Modify the topography to subtract the runway elevations and add the Obstruction Heights
  • Compare the adjusted topography and Imaginary Surface elevations
  • Create raster grids of “above” or “below” Imaginary Surfaces
  • Create a subset of the values “above” the Imaginary Surfaces
  • Convert the “above” raster values to vector

Image 4: Airfield Obstruction Model zoomed into the Raster Minus comparisons and Raster Mosaic

The end-result is a polygon that shows areas where the user input value of “Obstruction Height” would penetrate the imaginary surface at that given location (based on topography).  These would be the areas that should be ruled out for new construction or to monitor existing features at or above the “Obstruction Height” entered by the user.

For the test location of Naval Air Station Jacksonville, the model took about 16 minutes to process, mostly due to the size and resolution of the elevation model. The processing time could be improved greatly through a few modifications.  For example, it would be much faster and effective to have specific topologies already loaded and processed into their raster format.  This would save the local computer from having to perform the transformation from vector to raster.  It would also be an improvement if the Model Parameters allowed the user to specify the raster resolution as well as the contour details (subset every 1 ft, 2 ft, 5 ft, etc.).

Another benefit would be to add options to allow compatibility with different airfield types, such as Class A, Outlying Fields, or civilian runways.  While Class B runways are the largest and typical for the bigger Navy installations, adding each of the other airfield classes to the options would complete the toolset for creating obstruction zones.

Right now the model only works if the imaginary surfaces area already created and attributed correctly.  Additional functions so that the user only needs to specify the runway classification as well as two points, one for each end of the runway, would greatly improve the usability of the tool.  Despite all of these potential improvements, the model proves that it possible to create a tool in ArcGIS 9.3.1 to determine obstruction zones for airfields.