Alex Rainert, writing for Medium, about using a Garmin Running Watch during a race:

During the race this past Sunday, being able to glance at my wrist for Current Pace/Average Pace/Distance whenever I wanted was fantastic (Note: I’ve found both the Garmin and the apps to have varying degrees of accuracy for distance covered). Seeing my live Heart Rate and Heart Rate Zones (coming via chest strap) also helped me know how hard I was exerting myself and when there might be a little more for me to give, even if I felt totally gassed. Finally, the fast & slow pace alerts were a total gamechanger as they provided actionable, contextual information in real-time, allowing me to make adjustments that saved seconds in the moment and crucial minutes over the course of the race.

This is a testament to one of the many health and fitness-related products you’ll be able to replace with an Apple Watch. Given that Apple Watch is a software and sensor platform, it can evolve to closely map to a user’s needs. The Garmin Running Watch is a stake in the sand, a product with a fixed design. What it does, it does very well, and will likely continue doing that job, and just that job, as long as its battery holds out.

The Apple Watch is limited only by the sensors it carries or connects to. Two things we know will be trackable with the combination of iPhone and Apple Watch are heart rate and distance traveled. Presumably, as the Apple Watch evolves, more sensors and, therefore, more health data will be added to the mix. The question is, who will control the access to that data?

From the quoted paragraph above:

I’ve found both the Garmin and the apps to have varying degrees of accuracy for distance covered

This is something I’ve heard from users of iPhone distance tracking apps, as well. Every app used (including Apple’s Health app) reports a widely varying measure of distance. Two different apps will give you two different measures of how many steps you took today, for example. This problem has a number of possible causes. Apps use different algorithms to calculate steps taken. Different sensors might differ in accuracy or, perhaps, require tuning to adjust to different stride lengths.

As reader Robert Davey points out, Apple bottlenecks this data via the CoreMotion framework. The question is, are developers not aware of the framework (I was not, as is evidenced by my original post)? Are they using it inconsistently? Incorrectly?

This is a quality control problem. As more sensors are added to the iPhone and Apple Watch, this problem will only get more complicated.