One of the several reasons that I only recommend using “published” GPS waypoints to “get you close”, is that many geographic features are not exactly where they are (paper) charted.
If you plot a Lat/Lon (from paper charts) for Bimini
(Bahamas), and compare that to an actual GPS Lat/Lon reading; you will find that the chart does NOT accurately locate the Island. It’s been a while (so I forget exactly); but I seem to recall
is actually about 2 miles south of it’s charted position. Perhaps one of our Florida
members can help us with exact data.
Using a published waypoint will get you close enough to Bimini to utilize your (visual) piloting skills.
The published waypoint will suffice to bring you to “within sight” of Bimini. Having eyeballed your way to (say) the first “near approach point” (the ‘Range’), you note an accurate GPS Lat/Lon, then work your way to the first “turning point “ (noting Lat/Lon) and etc. In this manner you will have developed a set of repeatable “Been There” waypoints, as accurate as your instrument, your descriptive notations, and the conditions extant.
Having developed these “Been There” Lat/Lon waypoints for entering Bimini, We’re now faced with several limiting factors (not including actual Lat/Lon accuracy):
How accurate is my descriptive note? Say I’ve noted the “First Turning Point” as:
“On the Range, heading 93 degrees True ,100 Yards off the beach”. I may have actually been 150 yards off, and being set by a tidal current (& Bearing 90 Degrees), and merely ALMOST on the range. *Important Note: I've not refered to my notes, and this example is NOT accurate.
Has the sand bar moved since I last plotted a safe entrance, so that my previous exact route
is no longer safe? What are the implications of the (unstated) exact state of tide?
My point is that you CANNOT "reference the numbers on the chart, and know exactly where you are", in any meaningful way.
Nor can you even reference your "Been There" numbers, and always know exactly where you WANT TO BE.
There are many other waypoints in my log, that I use with impunity & confidence; so I’m not suggesting that you cannot rely on (been there) GPS Lat/Lon positions. I’m merely trying to reinforce the obvious principle that any Data is only as accurate as it’s least accurate component.
So, Stede, CSYMan (etc); where (exactly) is Bimini?