http://www.bvipirate.com/ChartNav.html

This is an excellent page regarding Navigating with GPS, Charts and
Eyeballs. It really puts all these issues in prespective. It
provides sample charts and photo images. And totally related to
practical experience.

"A prudent navigator relies on charted latitude/longitude position
only on the macro-scale or meso-scale, but not on the micro-scale.
That is to say, a good chart can tell you approximately what is at a
given latitude/longitude, but not precisely. A good chart also shows
the position of smaller details, with the position being accurate
relative to other nearby features. To that end, almost all of the
current navigation charts, either paper or electronic, work very well.
For absolute position (latitude/longitude) they are almost all
accurate to within a few hundred yards (meso-scale). For smaller
distances (micro-scale), though, you must use relative position and
your eyeballs to locate things -- not the charted latitude/longitude."

"Rocks and coral and other small details, though, are typically no
more than a few feet or tens of feet in size. On a chart, this is
micro-detail, and its absolute position has the same inaccuracy as the
macro- and meso- data. Many charts simply are not accurate in absolute
position down to the scale of tens of feet. However, most charts do
have accurate representations of the micro-scale position of features
relative to each other and nearby meso-scale features."