Instant pace on a running watch is a smoothed guess and it teaches bad habits

The number updating every second is the output of a filter, and chasing it produces exactly the surging most runners are trying to eliminate.

Look down at the watch. It says four fifty two. You lift the effort slightly. Ten seconds later it says four forty one, so you back off, and twenty seconds after that it says five oh one and you push again. You have just spent a minute accelerating and decelerating in response to a number that was never describing the moment you were in.

Satellite positioning gives a sequence of position estimates with error in each. To turn that into a pace, the watch differentiates position over time, which amplifies the error badly, then smooths the result heavily to make it readable. Some models blend in accelerometer data from the wrist. The output is a filtered estimate that lags reality by several seconds and cannot resolve small genuine changes in speed at all.

Under trees, beside buildings, on a track with a fence, or on a day with poor satellite geometry, the error gets worse. The classic version is the four hundred metre track, where the watch will insist you ran four hundred and twelve metres and hand you a pace that is wrong in a direction you can prove.

None of that is a fault of the manufacturer. It is what you get when you estimate velocity from noisy position data in real time. The fault is in how it gets used.

The behaviour it produces is the thing I care about. Chasing a lagging number generates a control loop with a delay in it, and delayed control loops oscillate. You end up surging, which is more expensive than running evenly, and worse, you never build the internal sense of pace that a runner needs when the watch is wrong or the course is hilly or the race is the one that matters.

The fix is not to abandon the watch. It is to stop looking at instant pace and look at lap pace over a fixed distance, which is a real measurement of something that already happened rather than an estimate of now. Set the autolap. Check once per lap. Adjust by feel between checks.

On the track, take the watch off entirely and use the clock and the numbers painted on the ground, which are the only pacing tools in the sport that are actually accurate.

The watch is a good logbook and a poor coach.