When creating a transform graph, it is important that the data conform to exact intervals to allow data from different history sources to properly compare and coalesce into larger data sets. To do this, a process of “quantization” is used to fit the timestamp value of a record to an exact interval.

In the case of histories, the timestamp of each history includes not only the hour, minute, and second that the history was created, but also the millisecond value. When attempting to align different histories for comparison, this difference in milliseconds prevents an exact comparison. The History Source dialog box provides fields that allow you to manually set intervals to predefined increments or choose predefined increments using the Interval option list.

The interval can also be set to Automatic so that the current interval of the history collection is used. For example, if the history is already set to collect at 15 minute intervals, the automatic quantization interval of the history source node is also set to 15 minutes.

It is important to note that all values are quantized based on a starting time of 0:0:0.000. This means that an interval of 15 minutes will quantize a record whose timestamp reflects the value of 11:13:52.093 to 11:00:00.000.

Often it is desirable to use only a segment of a History rather than the entire history for a point value. Use the Date Range property to select the starting and ending date and times for the desired history data. All history records between these date values, including those histories which fall exactly on those date values, will be included in the data set at the time the graph is resolved.