Beginning
in October 1992 the horizontal geodetic referencing system used in all
charts and chart products published by the National Oceanic and
Atmospheric Administration (NOAA)/ National Ocean Service (NOS) changed
from the North American Datum of 1927 (NAD27) to the North American
Datum of 1983 (NAD83). This
change was actually mandated by Congress and actually changed the
latitude and longitude coordinates change for most place on the earth.
The old system used a series of control
points with a reference point a point in Kansas, the new system uses the
center of the earth as the control point.
The new third edition of course uses the newer NAD83.
This in part explains why many of the older charted positions of latitude
and longitudes simply do not agree with what you find on your GPS
instruments.
GPS
users in most cases should use on sight readings
(such as are provided in our guide) for best reproducibility.
Where on sight readings are not available and the mariner must
resort to extracting GPS waypoint from a chart, care should be taken to
use a
chart using the new data.
