  The extraction of the gravitational wave signal, within the context of a
characteristic numerical evolution is revisited. A formula for the
gravitational wave strain is developed and tested, and is made publicly
available as part of the PITT code within the Einstein Toolkit. Using the new
strain formula, we show that artificial non-linear drifts inherent in time
integrated waveforms can be reduced for the case of a binary black hole merger
configuration. For the test case of a rapidly spinning stellar core collapse
model, however, we find that the drift must have different roots.
