Airborne GPS. Everyone is using it, right? Wrong. The theory behind airborne GPS has been proven, that is not in question. What is in question however, is the practicality of it for highway design purposes. Standards have not been set, specifications have not been written, and users have not truly communicated their results. Test projects have been done in what appear to be relatively flat, open areas and their results published. But what of hilly, forested areas? What really happens in urban areas? How many targets are needed? What GPS epoch length is needed or recommended for airborne collection? Can airborne GPS really be used for highway design, or only GIS Mapping? Being a typical Mainer, I am a bit skeptical of this technology. Don't get me wrong, if it's all it's cracked up to be I'd be the first to want to use it. I wanted answers to all of the questions posed above. So I did what any Mainer would, I decided to prove it to myself and ME DOT. I worked for the Maine DOT as the Photogrammetric and Control Engineer.

Two projects were selected for our Airborne GPS test trials. One is located in Portland, Maine and the other in Augusta, Maine. The Portland project is a location/alternative feasibility study along a coastal corridor and into an urban area. It involves a mix of railroads, interstate highways and bridges, city streets and the terrain is mildly undulating. Team leader Steve Landry wants to use the mapping for complete design. We chose to fly the project at a scale of 1:2500 to obtain mapping at 1:500. A dual flight line was set up with eleven pictures per line and a total of thirty-six targets set out. The Augusta project is a possible relocation project that is being mapped for environmental studies. Due to many constraints, the realignment of the project will be minimal, and the mapping will be used for environmental permitting.

Bradstreet Consultants, Inc. completed their contract well ahead of our time contraints and within budget. Their staff was exceptionally helpful in the research endeavor. His company used Erio Technologies’ Albany software as their adjustment program of choice. After many different calculations using four target coordinates and many constraint alternatives, we determined that a fifth coordinated target would be used to tighten the control to meet the 0.25 meter contour specifications. Final A-GPS constraints for the aircraft camera position decided upon by MDOT and Bradstreet Consultants are as follows: 6 cm for the XY and 4 cm for Z (elevation).

After Bradstreet Consultants submitted their initial mapping for final approval, the MDOT was pleased to announce the contract would be extended to include additional mapping toward Westbrook, again with airborne GPS. Project leaders have approved the continued use of Airborne GPS for future ME-DOT projects. The mapping from Bradstreet Consultants met, and in most areas exceeded, National Map Accuracy Standards using only 5 of the 36 coordinated targets. The remaining 31 on targets checked back to within 4-8 cm in both XY and Z, satisfying the most skeptical "Mainer’s".