T&T: GPS and chart accuracy

[capteric36]

 There is one more item that needs to be
considered when trying to get the maximum accuracy
out of any chart plotting method.
 The GPS system uses the WGS84 DATUM as it's base
point for referencing the calculations to all maps
and charts.
 Since not all (very few) charts are based on that
DATUM it is necessary to enter into the GPS which
DATUM your chart is based on.
All Trimble GPS receivers allow the DATUM of your
chart to be entered, I know nothing of modern
marine GPS receivers but I sure hope they do too.
This is true if you plot your own positions on a
paper chart, or if you are using a chart plotter,
or if you are using a software product like
Coastal Navigator or Nobeltech.
 You can find the DATUM of the chart you are
looking at in the title block. It is sometimes
hard to find, but if you persevere you can find
it.
In some cases the wrong DATUM can create quite a
significant apparent GPS error that would be
greatly reduced if you entered the DATUM of the
chart you are using.

 Of course that will not help if the chart was
surveyed in the 1800s. Only a more recent survey
would help those chart errors.

 I hope this helps too.

Eric Thompson
S/V Procrastinator
South San Francisco
capteric36 at sbcglobal.net