# GD&T Interpreting Datums in Two Single Segmented Position Tolerances

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#### vinmech32

I have a position tolerance requirement just like this (refer attachment). Here datum F is defined by 3 points and C by 2 and G by 1. Now this is perfect. But i have a second segment position tolerance which calls C as primary datum. Can i use the same 2 point to construct a line and measure it or do i need to create a plane in C by 3 points.

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#### True Position

I have a position tolerance requirement just like this (refer attachment). Here datum F is defined by 3 points and C by 2 and G by 1. Now this is perfect. But i have a second segment position tolerance which calls C as primary datum. Can i use the same 2 point to construct a line and measure it or do i need to create a plane in C by 3 points.

Why are you using the minimum number of points possible? You open yourself up to massive errors.

#### Proud Liberal

##### Quite Involved in Discussions
What exactly are the datums? Planes? Circles? ....

#### Michael_M

Trusted Information Resource
Using the reference frame that you are showing (without the print to see) I would interpret this that you have to hold true position of .9 using -F- -C- and -G-. The second frame is saying that datum -C- alone must be no more than .4 of the .9. Note, I am ignoring MAX for my description.

I have typically seen the second call out to mean "to itself". in other words, you have two holes in line separated by a gap or empty 'space'. However since datum -C- is called out in both frames, I don't see this being the case.

#### Proud Liberal

##### Quite Involved in Discussions
If datum -C- is a plane, then the last callout is the same as perpendicularity to -C-.

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#### True Position

I have a position tolerance requirement just like this (refer attachment). Here datum F is defined by 3 points and C by 2 and G by 1. Now this is perfect. But i have a second segment position tolerance which calls C as primary datum. Can i use the same 2 point to construct a line and measure it or do i need to create a plane in C by 3 points.

You should be using more points anyway.

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