This mapper function returns the variance of all the values found in each sliding window. The location and elevation returned are those associated with the most recent value in the sliding window.
This is similar to
mapper.var except this version implements Welford's method which is 10% slower but is much less subject to numerical instability.
If the number of values in the sliding window is greater than one, Bessel’s correction is applied.
mapper.var.welford function can only be applied to values of type LONG or DOUBLE, when applied to STRING or BOOLEAN it does not return any value.