Enter the measured x and y values and the true x and y values into the calculator to determine the total variance from the true position.

True Position Formula

The following true position formula is used to calculate the total variance from the true position.

TPV = 2 * SQRT ( (mX-tX)^2 + (mY-tY)^2) )

  • Where TPV is the total variance from true position (this should be less than the total tolerance.)
  • mX and mY are the measured x and y coordinates.
  • tX and tY are the true x and y coordinates.

To calculate true position variance, subtract the true coordinates from the measured coordinates, square the results of each, add them together, then take the square root of that result and multiply by 2.

True Position Definitoin

A true position is a type of geometric tolerance used to describe the true position of a feature with respect to 1 or more datums. For example, a hole position along the x-y plane.

Can a true position be negative?

A true position cannot be negative. A true position is a measure of the absolute value of the variance of the position of a feature and such can only be equal to 0 at the very least.

Does true position need a datum?

A true position should use a datum when used properly. The datum is usually referenced with x and y coordinates as the basic dimensions.

Is a true position a radius of diameter?

A true position is most often described as a circle around a point with a certain diameter. For example, The true position with a tolerance of .010 would be a circle around the point with a diameter of .010.

Does true position control perpendicularity?

When a true position is called out with datums on the face and sides of a part, the perpendicularity is also controlled by the true position. So in short, yes, the true position does imply perpendicularity when applied properly.

How to calculate true position?

  1. First, measure the values of the x and y coordinates of the actual part.
  2. Next, determine the true values of the x and y coordinates outlined in the true position tolerance.
  3. Finally, calculate the variance from a true position using the formula above.