Originally Posted by
double_barrel
So I should ignore the information I get from those who do announce their position? [...]
Let me turn the argument the other way around. Suppose, for a minute, that all VFR aircraft do position reports, that all these reports are dead accurate, that there is a common set of reference points (VORs, for arguments sake) that everybody knows and uses, and that they do this position reporting often enough so that the information stays relevant and accurate until they do the next position report.
You mention that you fly in the neighbourhood of 10.000 feet. At that altitude, the horizon is a little over 100 miles away, and you can hear transmissions of aircraft at 10.000 feet at double that distance - almost 250 miles away. In other words, you are going to hear all the position reports of all aircraft in an area that's about 200.000 nautical miles squared or well over 600.000 km^2 - approximately an area equal to the whole of France. Any idea how many VFR aircraft are out and about in France on a good day?
(For fairness sake, I'm assuming that this area is served by a single FIS/Unicom/whatever frequency, which is probably not the case. But that's hardly relevant. I'm just trying to get the point across that there's a vast area within line of sight, and thus within VHF range, at 10.000 feet.)
How many aircraft would be flying around such an area? Your post suggests that there's a fair bit of traffic about, and in that statement I'm assuming you're talking about the relatively close area. For arguments sake, let's be pessimistic and assume that there's 100 aircraft operating inside this 600.000 km^2 area. For position reports to be useful for other traffic, my gut feeling is that they should not be more than 5 minutes apart - your average GA spamcan covers at least 10 NM in that timeframe. So you now have to process 100 position reports every 5 minutes, or one every THREE seconds.
Can you do a position report yourself in less than three seconds? Is the frequency you're operating on really this busy with position reports? Can you interpret a position report within three seconds?
Heck, even if there's only ten aircraft on your particular frequency, then that still means a position report every 30 seconds. And each and every one of those needs to be interpreted, in addition to all the other mental tasks that you've got to perform as a pilot.
So for all practical purposes, position reports simply don't work, especially not for aircraft going from A to B at 100+ knots. So the vast majority of pilots don't make them in the first place, or at least not often enough to be practically useful. So you've got to assume that 99% of the traffic that you might encounter, has not made a position report at all. For the other 1%, is it worth spending an immense amount of mental energy interpreting their position reports? Or should your time be spent doing other things, like looking outside?
Position reports don't keep you safe. What keeps you safe is looking outside, TCAS/PCAS/Flarm or something similar, some form of radar/traffic information service, and the fact that the sky is really, really big and aircraft are comparatively very, very small.