Evil Kraig F (4/29/2012)
Jeff (and OP) if I read this problem correctly, he doesn't have an endpoint yet and is trying to find said endpoint. Also using that technique you could miss an airport near the center of the flightline if you're looking to locate emergency landing areas.You're basically just trying to find the closest point in the list from a selected point. This means you'll always have to calculate all of the rows against that point, sort the result, and pick the minimum. That's going to be a very heavy handed process. What Jeff's trying to point out is you need to restrict the list of what you're comparing so you can get a better query, and it needs to be precomputed or an existing attribute.
Try restricting it to airports in the same country, or those on the same side of the equator, or similar. You'll find the process will take a lot less time. In the meanwhile I'll see if I can come up with anything. Is that table you described the FULL table and the only attributes you have available, or was it simplified for the purposes of this example? If there's more attributes we may have some other ideas.
Actually, it's simpler than all of that for restrictions if you don't know what the end point is. You can't fly much farther than you have fuel for and, hopefully, you know how much fuel you have so could estimate the range left. Convert that range to degrees of Latitutde and use the same suggestion as before.
So far as any "mid point" error goes, how would there be an error unless you were off course? You can't be more than half way away the total distance from either end point unless you're fairly well off course.
--Jeff Moden
Change is inevitable... Change for the better is not.