"Zeroing" around a base value (SnapTo)

This analog filter lets you force values in a well-defined range around one or more base values to "snap to" that base value. For example, you can use this filter when a tank is known to be empty, but the tag that stores the tank level returns a "noisy" value close to zero.

The filter can be used with all retrieval modes, but its main benefits are in the aggregate retrieval modes: average, integral, minimum, and maximum.

To zero values around the base value, specify the SnapTo() filter in the wwFilter column of the query.

The syntax for this filter is:

SnapTo([tolerance[,base_value_1[, base_value_2]…]])

This filter has two parameters:

The following are supported syntaxes.

• SnapTo() – Same as SnapTo(0.01, 0.0)

• SnapTo(3.7) – Same as SnapTo(3.7, 0.0)

• SnapTo(3,) – Syntax Error

• SnapTo(,0) – Syntax error

• SnapTo(,) – Syntax error

• SnapTo(3, 4, -5) – Tolerance=3, Base Values 4 and -5.

When the Snap to filter is specified, point values falling inside any of the ranges [Base value – Tolerance, Base value + Tolerance] will be forced to the base value before the point goes into further retrieval processing. The result is undefined if the base value +/- tolerance exceeds the range of the double data type. The range is calculated using this expression:

If (x <= Base value + Tolerance AND x >= Base value – Tolerance)

x = Base value

where x is the value of the point then if ranges overlap, the first matching base value will be used.

A query example from the History table looks like this:

SELECT DateTime, Value, wwFilter

FROM History

WHERE TagName = ('TankLevel')

AND DateTime >= '2008-01-15 15:00:00'

AND DateTime <= '2008-01-15 17:00:00'

AND wwRetrievalMode = 'Average'

AND wwResolution = 3600000

AND wwFilter = 'SnapTo(0.01, 0, 1000)'

The following rows might be returned:

When a value is snapped, the QualityDetail bit flag 0x2000 is set.

If the filter syntax is not correct, a syntax error is returned and no rows are returned.