We can illustrate the need for a reference system via the following example:
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Alice and Bob are in an enclosed room and represent independent observers both seated in cushy seats and facing the way the arrows indicate.. Alice and Bob can't see each other becaue the room is very, very big.  A grid of lines is painted on the floor of the room.
Alice posses the ball of wisdom that Bob would really like to get his hands on.  Alice randomly throws the ball of wisdom in the enclosed room and it ends up at the position as shown in the figure below:
<P><center><img src = http://homework.uoregon.edu/pub/Grt/alicebob.JPG></center>
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Without an agreed upon coordinate system we can imagine this kind of conversation (remember A and B are independent observers).
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Bob to Alice (B to A): "Where's the ball?"
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A:  Uh, I don't know, its a bit to my left.
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B:  Huh, I don't see anything that is "a bit to my left."
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A: Try your other left ...
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B: Nothing there either
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A:  Maybe its behind you
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B: I can't even see you, A, so what does "behind me", mean?
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A:  I don't know I can't see you.
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B:  Where's the damn Ball?
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A:  I already told you, its a little in front of me on my left.
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B:  What does "in front of you" mean?
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A:  I don't know, just come over here and I will show you.
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B: I have no clue where you are Alice, how can I come over there if I don't know where "there" is.
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A:  Oh this is pointless, let me get out of my comfy chair and through the ball in your general direction.
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B:  But, Alice, you don't know where I am let alone what my general direction is.
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A:  Yes your right, it seems that you will never find the ball, hee, hee. It's really a wonderful ball.
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B: Wait a minute, there are grid lines on this floor - tell me the location of the ball using grid units.
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A:  What's a grid unit?
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B: One square on the floor is 1 grid unit.
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A:  How do I know that your squares are the same as mine?
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B: You don't - we must assume that your Universe and my Universe are the same.  if we don't then assume that then the ball of wisdom only exists in your Universe and not mine and that would be sad.
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A:  Okay then, the ball is  3 grids in front of me and 1 grid to my left.  That locates it precisely.
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B:  There you go, using that "in front of me" stuff again.  How does that help me?
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A:  Okay its in grid 17
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B:  What? - what the hell is a grid 17?   Where's grid zero?
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A:  I don't know where grid Zero is .. what is grid zero? - do we need one?
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B:  Gee, Alice, I don't know if we need one either.  This is impossible -- I am going to back to staring straight in front of me. Goodbye Alice.

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And so, Alice and Bob do not live happily ever after because they can never converge on locating the ball of wisdom.  Bob can only find the ball under two conditions:
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<ul> <li> Some agreed upon grid has to be (0,0)
<li> Bob has to know what grid comprises his current location (i.e. some one needs to tell Bob where he is at  or Bob needs to make some kind of external measurement to validate his position).
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Note, as an added bonus, if Alice tells Bob what grid she is in, Bob can find Alice (provided she didn't move grids and didn't inform Bob).
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So, without an agreed upon coordinate system, most measurments make no sense.  This essential point was not realized by the Aristotelians.  While Da Vinci thought about the need for a coordinate system, and Descartes will formalize this, it is really Nicholas of Cusa that emphasizes that all motion is relative to the coordinate system and hence position measurements of stars are indeed "far from precise".
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