Pace From A Footpod
The Best Running Watches can tell you how far you've run, but they're not very useful at telling you how fast you're running at that instant. This is because most watches have limited GPS Accuracy, which means they know roughly where you are (and where you've been.) This can be okay for distance measurement as these errors can be smoothed out and averaged to give a good approximation. However, the moment to moment errors in GPS Accuracy means that their idea of "current pace" can be wildly off. I generally see GPS based current pace display swing more than a minute/mile away from the true value. You can compensate for this a little on some watches by displaying the "lap average pace", which as the lap distance increases will tend to be more accurate. However, that's not much use if you want to know how fast you're running at any given moment. By comparison, a Footpod has errors of a different type. A well calibrated Footpod will give a reasonably accurate reading of current pace that will respond far more quickly as you speed up or slow down. With a Footpod, it's possible to tweak your pace to hit a specific target quite well. However, calibrating a Footpod accurately is a real pain, as you have to have a course with a known distance, and recalibrate for different shoes or even different placement on the same shoe. This means that typically a Footpod has an overall bias; it will read a few percent too long (fast) or too short (slow). This still gives a good indication of current pace, but the errors build up over distance. A bigger flaw of the Footpod is that it gives you no idea of where you are or where you've been. So the ideal solution is to have a watch that uses a Footpod for current pace, but uses the GPS for everything else. The good news is that some Garmin watches do just that.