You really need 2 markers to determine what you are looking at. Just remember
"RRR" applies when the numbers are getting higher. If the numbers are getting
smaller you are "leaving", not "returning".
When you get inshore you really need a chart to figure out what you are looking
at. Even then "privately maintained" markers may not be charted but they will
look exactly like the USCG markers and can be in very close proximity to a
channel with similar numbering. The area around Big Carlos Pass in SW Florida
is notorious for conflicting channel markings. We have 2 channels running next
to each other, winding their way through the mangroves and it is very easy to
jump from one to the other by accident, ending up in the wrong river.