Interpolation with Expressions

When plotting data with expressions, the system determines the interpolation type for the expression based on the interpolation type of the individual tags in the expression.

If the interpolation type in the tag metadata is linear then linear interpolation is used, otherwise stair-step interpolation is used. For more information, see Interpolated Retrieval in the AVEVA Historian Retrieval Guide.

Special rules apply when using some expression functions:

1. An expression using the SIGN function uses stair-step interpolation.

   For example, Pump101.Flow is an analog tag representing the flow rate for a pump. While the tag uses linear interpolation, the expression SIGN(Pump101.Flow) uses stair-step interpolation.

2. An expression using the ROUND function uses stair-step interpolation.

   For example, Tank100.Level is an analog tag representing the level of fluid in a tank. While the tag uses linear interpolation, the expression ROUND(Tank100.Level) uses stair-step interpolation.