To put it into perspective. Imagine a dead-reckoning system with 1ppm error (ridiculously low) and no drift (again ridiculous). Say you use this for in-car navigation. After driving around for 10k miles, the error is now 53 feet, which is too great for navigation.
There's more to navigation than city streets. That accuracy would be awesome for shipboard use.
Grandparent was talking specifically about replacing GPS; I pointed out one example where even an absurdly accurate DR system wouldn't be sufficient to completely replace GPS.
TIL "Dead Reckoning" is more than just a badass movie and band name. That's really neat!
This term comes from "ded" (short for "deduced") -- deduced because you are advancing your last fix based on course and speed.