How does the Ground Proximity Warning System know the real distance to the terrain?

GPWS and TCAS are things which pilots are supposed to obey without thinking... wasn´t there a rule that a pilot must obey TCAS even if the ATC is ordering otherwise? Shouldn´t there be a direct connection between TCAS and GPWS with autopilot in that case? Subject to perhaps pilot alerts and possibility of manual overrule - what would you do if your TCAS is ordering climb and the other plane presumably is receiving orders to descend, but what the other pilot does not know is that you have a stall warning, too, against climbing?

Oh and by the way.. Why does so many non-pilot enthusiasts seem to think that pilots do almost nothing in their working days? as i said some discussions on forums make me a little upset because on some people it sounds like everybody can fly if they play flightsim..

I work very hard to make flying as my profession and its not that funny to get close to no respect for what i am doing

Actually UAVs don't take the pilot out of the system - just out of the cockpit - there still a need for a "pilot" on the ground - just doesn't need flying pay and perks!!

Well, it is not just pay.

In the cockpit, there is very restricted space and weight. You cannot have huge radars mounted on a commercial aircraft, nor radars in several distant positions. A pilot on the ground might easily afford such navigational aids, as well as other luxuries of not paying for space and weight, like having several pilots and flight engineers in a roomy tower, available to assist with a plane in critical phases of flight and otherwise engaging themselves with supervising planes which are, say, cruising or holding.

So, landing is certainly a stressful phase of flight. Would a pilot like to be able to share cockpit tasks on approach with pilot/s sitting in the airport tower, capable of getting the airplane attitude data from ground instruments as well as the plane-based ones, and giving pilot input to the plane controls?

I am glad that i´m not alive when these planes come to life because it sounds very very awkward to me that no pilot is up there, and i do not want to sit in a tower steering remote controlled A/C, how fun is that? i must say that if this did happen under my lifetime then my interest in aviation would die :{