This is because GPS positioning is *NOT* acurate with out a ground based signal to augment it !
It's much better than it used to be before they turned off selective availability.
The big difference is that GPS accuracy remains roughly constant during any specific approach, while navaid accuracy increases as you approach the navaid and altimeter accuracy increases as you approach the station elevation. For example, a GPS without WAAS might have an error of 150 ft at any altitude; a pressure altimeter (with the proper altimeter setting) might have an error of over 1000 ft at 8000 ft AGL, diminishing to less than 25 ft at ground level. Ten miles back at 3000 ft AGL, even the non-WAAS GPS is probably going to be more accurate than the ILS and altimeter; a half mile back at 200 ft, the ILS and altimeter win hands-down.
Obviously, the altimeter is the better choice for an ILS approach, but the GPS (even without WAAS) is a better choice for overflying a mountain range because of the altimeter's enormous errors at altitude. In practice, I have not noticed more than a 75 ft vertical vertical GPS error on the ground without WAAS (my current portable GPS does have WAAS support) -- that's not much worse than the 50 ft allowed altimeter error. Without WAAS and an approach-certified GPS, though, I wouldn't bet my life on it.
All the best,
David
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