NavList:

Fred,

If you are comparing a celestial line of position of Polaris or Jupiter to a GPS position you are doing well to be within 2 nautical miles. Celestial is used when there is plenty of sea room. When in sight of land there is a switch to piloting where positioning will be more precise using bearings, horizontal angles, soundings, and ranges. Bowditch will explain how sight variables will produce uncertainties which in some cases will be over 3 nautical miles even with a perfect observation and no mistakes in sight reduction. Some of the uncertainty variables are time, height of eye, temperature, pressure, index error, and the perpendicularity of the sextant to the horizon.

Greg Rudzinski

[NavList] Position discepancy
From: fredstevens---com
Date: 13 Dec 2011 10:43

I have a known location... well, "known" as far as Google
Earth goes. I am assuming that it is a correct position
since it is verified by GPS. When I go to either the NA or
online almanacs, like USNO Almanac or Henning Umland's to
verify the coordinates using Jupiter and Polaris, there is
always a significant discrepancy between them and those from
GPS/GoogleEarth, a few nautical miles at the very least.
Enough to be worrisome if at sea.

Any idea why so much difference?

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