Interpolated retrieval - how it works

The following illustration shows how the values for an analog tag that is configured for linear interpolation are returned when using interpolated retrieval.

Data is retrieved in interpolated mode with a start time of TC0 and an end time of TC2. The resolution has been set in such a way that the historian returns data for three cycle boundaries at TC0, TC1, and TC2. P1 to P12 represent actual data points stored on the historian. Of these points, eleven represent normal analog values, and one, P7, represents a NULL value due to an I/O Server disconnect, which causes a gap in the data between P7 and P8.

The green points (P2, PC1, PC2) are returned. The yellow points (P7, P11, P12) are used to interpolate the returned value for each cycle. The red points are considered, but not used in calculating the points to return.

Because P2 is located exactly at the query start time, it is returned at that time without the need for any interpolation. At the following cycle boundary, point PC1 is returned, which is the NULL value represented by P7 shifted forward to time TC1. At the last cycle boundary, point PC2 is returned, which has been interpolated using points P11 and P12.