:why the hell did i spend the extra on my differential GPS?

Perhaps you wanted to be an early innovator?

The differential add-on uses, I believe, LORAN signals and calculates a position from that, as well as from the GPs. The combination of the two should be more accurate than either alone. Should the government ever turn that GPS "selective availability" error back on -- say in case of war -- you'll be better than most of us.

My GPS displays a position down to hundredths of a minute. (degrees, minutes, and 00 to 99/100ths of a minute) Assuming the signals and the calculatons in my receiver are accurate, that should get me somewhere within a 60 foot circle of my actual position. As I see it, I would need three decimal places (thousandths of a minute) to get *a display* of accuracy within 6 feet.

Now this just applies to the actual indicated position. I suspect that when my GPS calculates my my position it really knows the answer to more decimal places than are displayed on the screen -- and it uses these more precise figures to calculate such things as velocity. Otherwise, how would it know I had moved if I was going at only 2 miles an hour? Hopefully the change will give me more accurate speed readings, but until I get a GPS that displays another decimal place, I can't see how I will be able to determine my position any more accurately than before. Maybe I'll just be more certain that what I am seeing is really right.

PGJ