Autotrackers on our interactive workstations have long been touted as automatic, implying that they get the tracking done quickly with little human intervention. The trade names given to autotrackers (Zap, Propagator, etc.) reinforce this image. But is this the correct image? Many interpreters have said to me, “The autotracker doesn't work in my data!” Given the above mindset, they are probably correct. Efficient, speedy, automatic autotracking requires good data with continuous horizons. The greatest value of machine autotrackers lies in their precision not in their automation. After the tracker has propagated onto the desired reflection on a given trace, the precision component locates the track exactly on the waveform crest (Figure 1). The precision with which the track is positioned is typically a quarter of a millisecond. Given that digital data is an array of sample values spaced by 2 or 4 milliseconds, this clearly involves interpolation. This interpolation is commonly performed on the postulate that the shape of the trace is locally parabolic.