Deep valleys, such as Manhattan offers ;) In other words, this is one of the most often encountered scenarios, and for momentary dropouts, even consumer-level n

Deep valleys, such as Manhattan offers ;) In other words, this is one of the most often encountered scenarios, and for momentary dropouts, even consumer-level navigation apps have dead reckoning algorithms (I was surprised by this when my smartphone kept the "you are here" dot moving inside a tunnel, fairly precisely)