Dear hlt,

I produce the freely redistributable airspace files for the US upon which at least some of the Uvalde files are based, and I can provide a little insight. I didn't produce any of the contest files directly, but my work in transcoding the FAA data for the entire US has been used in part by the contest organizers.

The issues you are seeing are artifacts of the conversion of the special use airspace data from the text-based raster descriptions published by the FAA in the unlicensed NASR data to the Tim Newport-Peace and OpenAir formats. For Uvalde, only Special Use airspace such as Alerts and MOAs (as opposed to ATC-controlled airspace such as Class B, C, and D) are affected in this way.

The problem arises because the unlicensed digital versions of the FAA data for Alerts and MOAs do not currently contain arc descriptions at all -- just a raster list of some points along the edges. I have written software that does a regression analysis of those points and does a best fit arc through them. This might be a little hard to understand, but (for an arc that isn't a full circle) even though the arc radius and center point may not be perfectly computed, the arc itself can very closely approximate the published position, with only very slight differences in curvature. The parameters of my fitting algorithm are a tolerance of .05 nm, or about 100m. This issue is described in the header of the file. The arc consistency issues you mention originate directly in the FAA's own digital versions of the data. The endpoints of the arcs are not part of the fit.

Normally, MOAs and Alerts aren't used in contest scoring, but I understand in Uvalde they will be. I'm not on the contest committee, but I understand that the organizers have established scoring versions of the airspace files that set the boundaries of the off-limits airspace slightly outside of the boundary as set forth in the FAA data. As long as pilots all use the same description of the airspace as published by the contest committee, things will be OK.

This whole issue of arc fitting will be behind us as early as October, when the FAA will use a new format AIXM5, for publishing unlicensed versions of the airspace data. Of course the contest will be over by then. Other vector-based versions of the FAA data do exist, but they bear license restrictions that prevent their free redistribution.

Lynn Alley
"2KA"