# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Improving accuracy of fix by shooting 4 stars
From: Gary LaPook
Date: 2007 Nov 03, 23:47 -0700

```A while ago Geoffrey Kolbe posted that one can determine index error
and get more accurate fixes by shooting 4 stars 90� apart. (See his
post below)

The ION cd has an article that discusses this also, it is item 158.

http://www.ion.org/publications/toc/celnavTOC.pdf

gl

Alex, the index error of a bubble sextant can be determined from an
unknown
position.

By taking sightings on bodies in opposition, the separation of the
resultant position lines will, on average, be twice the index error.

In my posting #2531, I advocated taking four sightings, a North-South
pair
and an East-West pair, on bodies at the Cardinal points. The North-
South
pair effectively give you your latitude and the East-West pair your
longitude. The position lines from the four sightings should form a
square
box if index error is the dominant error. If the box is not
substantially
square, there is a non-systematic error somewhere which is of the same
order or greater than the index error.

See http://www.pisces-press.com/desert/14thMarch.jpg for an example of
what
I mean. In this example, the box is substantially square, giving us
confidence that the dominant error is indeed index error. The size of
the
box is about 10' on a side, from which we deduce that the uncorrected
index
error was 5'. The position lines are all 'away from' the body in
question,
so the sextant altitudes are 5' too small. I need to add 5' to my
altitudes
to correct the index error.

A subsequent round of sights at a different location a few days later
show
the results when this 5' is factored in. See
http://www.pisces-press.com/desert/19thMarch.jpg

Geoffrey Kolbe

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