Understand interpolation type

Analog tag values can be retrieved using stairstep or linear interpolation methods to help calculate a value if none is stored for a given point in time.

• Stairstep: The value at the cycle boundary is assumed to be the same value as the last stored value before the cycle boundary.The last known point is returned with the given cycle time. If no valid value can be found, NULL is returned.

  

• Linear: The historian calculates a new value at the cycle boundary by interpolating between the last stored value before the boundary and the first stored value after the boundary. If either of these values is NULL, it returns the last stored value before the boundary.

  Expressed as a formula, Vc is calculated as:

  Vc = V1 + ((V2 - V1) * ((Tc - T1) / (T2 - T1)))