Accuracy also depends on an antenna, and even on a ground plate.
The 2.5m accuracy stated in a chip's docs usually presumes a good (active) antenna, good ground plate and good weather and sky conditions.
The antenna in the watch is very little, passive, and there is no ground plate.
So, we should not expect, the accuracy will always be high.
I am working on speedometer app, it uses Doppler velocity from GPS fixes (rather then calculates speed from coordinates). Though it can detect correct speed (compared to car speed), very often time lag can be up to minute. Like, I am driving already 40 km/h, but the watch still shows 10 km/h, then after ~40 sec, it finally shows 40 km/h.
This is kind of frustrating, but I understand why it cannot be precise enough.
I am thinking about to use a separate GPS unit with bigger antenna, and use the watch as a display only, sending data via Bluetooth.
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