Originally Posted by
Smythe
It took a long time to get a position from the Omega system and 20m depth is basically on the surface for a sub.
Omega was indeed used by US Navy submarines, as
this (caution: large document) makes clear. There was no need for the submarine to rise to 20m, a long-wire antenna (also used for communications) could be floated up towards the surface. Ironically Omega was made redundant by higher-accuracy IRS's in the 70's and 80's and several Omega stations were converted for communications use.
Yes, there are hyper accurate maps that are used in navigation, it just requires the sub to ping with SONAR, giving away its position.
....which is why many navies operate "geophysical research vessels" to produce detailed gravimetric and magnetometric maps, not as accurate as bathymetry perhaps but completely passive. Of course a lot of important scientific results were also generated.
The gear required to measure the acceleration due to gravity to a fraction of a percent in a surface vessel subject to wave motion has to be seen to be believed.