Stumbled upon this article “Measuring GNSS Accuracy on Android Devices” by a GPS app developer, it makes for a nice read (also applies to iOS, as far as I know).
https://barbeau.medium.com/measuring-gnss-accuracy-on-android-devices-6824492a1389
TL;DR:
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- do not trust the ‘accuracy’ values casually reported by smartphone apps or GPS receivers, they are really guesstimates;
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- a robust, lasting accuracy measurement has to make use of an external ‘ground truth’ reference (a geodetic survey marker, or a secondary fixed gps station, and so on);
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- do not assume aerial imagery (‘satellite view’ on various websites) to be a reliable ground truth reference;
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- at the very least, try and assess how really (in)accurate the ‘accuracy’ reported by a device or app is.