If you're off-airways, either flying GPS or on vectors from ATC, the lowest safe altitude is the off-route obstruction clearance altitude (OROCA) for the area in which you're flying (see the chart excerpts on page 14).
The MSA is the minimum obstacle clearance altitude surrounding identified fixes (and not always the airport or the final approach fix, so watch out) and is further defined by radials or bearings and distances from that fix.
A GPS MEA may be higher than, equivalent to, but not lower than a Minimum Obstruction Clearance Altitude
(MOCA) associated with a given route segment.
Yeah, that GPS navigator listening to all those satellites may know precisely where you are but unless it jumps up and slaps you with some form of terrain-proximity alerting function, you need to check the MEA and clearance altitude
against what you plan to fly.
These charts depict numerous minimum altitudes, including the Off Route Obstruction Clearance Altitude
(OROCA), which provides obstruction clearance with a 1000-foot buffer in non-mountainous terrain and a 2000-foot buffer in designated mountainous areas.
Even then, you can't go below the Minimum Obstacle Clearance Altitude
TERPSs require a greater terrain clearance altitude
for CAT E.
Their MEAs are often lower than the sector Off-Route Obstruction Clearance Altitude
(OROCA) too, so they can buy you a little more cushion between your wings and the freezing level certain times of the year.
The TSB is now asking Transport Canada (TC) to require that approach charts used by pilots to depict the optimal path to be flown rather than the line joining the obstacle clearance altitudes
which is currently shown and that the stabilised constant descent angle approach technique be used by Canadian operators that conduct non-precision instrument approach procedures.
Midwest, 3000 feet is common and some minimum obstruction clearance altitudes
(MOCAs) are lower still.
I also have an Icarus NavAlert above the EHSI that normally is used to record clearance altitudes
but can also display a selection of other data including a Mode C altitude and CDI display.
Off-Route Obstruction Clearance Altitudes
(OROCAs)--those big numbers in each quadrant of the en route chart--provide this level of obstacle clearance in reference to the highest obstacle in that geographic area plus four miles beyond.