What does an Update do against existing data?

An update in the context of the AVEVA Historian Server typically involves modifying existing data by writing a new data layer as a constant value for a specified time period. This means that instead of simply adding new records or removing old data, an update retains the original entry but alters the time series by applying a new constant value across the defined duration. This capability is particularly useful for correcting errors or making adjustments to historical records where it's necessary to maintain a continuous and accurate representation of the data.

The choice of writing a new data layer allows for flexibility in data management, enabling users to change the interpretation of historical data without losing the original context. This helps in situations where data integrity is critical, as well as when tracking changes and managing revisions is necessary for compliance or analytical purposes. The original data remains intact, but the updated layer provides a modified view that reflects the changes made.