wrote in message ...
and are not assessed with the precision that are accorded IFR
altitudes.

I'm not sure what you mean by "precision" here. The parameters are
clearly
specified: 1000' above any obstacle in the designated sector. (The MSA
doesn't assure navaid reception, though, so that has to be assessed
separately.)

The folks who design approach procedures at the FAA use very precise
topographical information to design the published segments of an
instrument
approach procedure. For MSAs, though, they simply use sectionals, which
may not
provide the required obstacle clearance at all times, simply because
sectionals
do not have the accuracy that USGS 1:24,000 topos have.

Plus, when the FAA assesses the published segments they add 200 feet of
assumed
adverse obstacle ("AAO") pad, because folks can construct towers, etc, up
to 200
feet high without notifying the FAA, unless the towers are within certain
distances of an airport. There is no AAO assessment made for MSAs,
though.
Also, spot elevations on sectionals can be off by a fair abount, without
adversly affecting their stated purpose; i.e., VFR navigation charts.
And,
contours on sectionals are very coarse, although that isn't usually an
issue in
non-mountainous areas.

Hm, so you're saying that the MSA doesn't necessarily provide the obstacle
clearance that it's advertised to provide. I hadn't considered that
possibility. Are you familiar with any example of an obstacle that's less
than 1000' below a current MSA?

Thanks,
Gary