Wouldn't it be great if we could find our way around anywhere and at any time using a small in our hand, pocket or bag? What a relief it would be to know that we couldn't get lost when arriving at a railway station in a foreign city...positioning would be an ongoing process, and the voice of our digital guide would pilot us through corridors, and up and down stairs and escalators, as easily as if we were checking emails on our smartphone. Alas, a universal positioning system that operates in all conditions and is unaffected by the type of land use does not exist. Even outdoor positioning by GNSS is faulty, since signals may get blocked or scattered when we move through an urban canyon or an avenue flanked by trees with dense foliage.
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