Y'All,
This entire thread seems to be totally entranced with the possible conflict
of aircraft on a heading/course. Whereas, the most likely conflict is in
altitude between IFR and VFR supposedly flying with 500 feet of FAA
separation.

Some time ago I was told that ATC figures a + - error 300 feet.
The altimeter is likewise allowed a 75 foot + - error. Not knowing for
certain but assuming it is so. Look at the following senario.

If we have an IFR and a VFR fllying in opposite hemisphereic directions in
VFR conditions we have several possible extreme conditions. Take the first
aircraft indicating 6000 feet west bound. The second aircraft indicating
5500 feet east bound. If both the transponders and altimeters have errors
to the extreme in the opposite directions, they could still miss each other.

If the first aircraft is flying 250 lower than indicated due to accumulated
instrment error, while the other is actually flying 250 feet higher than
indicated we have only see and be seen to save the situation.

To me the probability of a midair is more likely to altitude error than
heading error. The odds of having two such aircraft with hemispheric
accumulative opposite errors in altitude sufficient to cause a midair is
unlikely but more likely than an opposite heading midair. I believe this
because the distances are matters of feet rather than miles. It takes both
to actually cause the midair.so the total emphasis on course/heading is only
a part of the equation.

I haven't even mentioned GPS altitude as a factor.
Mud wrestling anyone?

Gene