# How would you go about describing a precise position?

1. Jan 9, 2009

### OAQfirst

Say you have a point on a surface, such as somewhere on the tip of your fingernail, and you needed to express that position to within an atom. How would you do it? I was wondering if anyone even does this sort of thing.

I understand that some position would need to be chosen relative to the point, but beyond that I'm stumped.

2. Jan 9, 2009

### mgb_phys

You need to choose a set of axis and some way to fix them and a zero position to measure from. In all you need 6 fixed values.

You could measure the position of an individual atom on a finger tip with an AFM (but you might need to remove it first)

3. Jan 10, 2009

### Staff: Mentor

Can you elaborate what you mean by "6 fixed values"? Especially why 6, as 4 points seems to be enough. Could be I am missing something.

4. Jan 10, 2009

### mgb_phys

An x,y,z of the origin and roll,pitch,yaw angles for the axis.
Alternatively two xyz points.
To lock down 6 degrees of freedom you need 6 values.

5. Jan 10, 2009