NavList:

   

Reply

Peter Fogg wrote (clipped)

"Let's start at the beginning. What is a position line? It is the position
expressed as a line (for our purposes - really an arc that, extended,
completes a circle around the SSP). It is the position that has been derived
via a mathematical process. It is a rational artefact. It is the ONLY
position that can be rationally derived from a specific body at a specific
altitude at a specific moment.

To say that this is not an expression of the position, that the position
lies at an indeterminate point that could be located anywhere at all in the
whole wide universe, except along the position line, is a bit of a furphy.

I do understand and appreciate the statistical 25% / 75% argument. However,
statistics are most useful navigationally, it seems, when employed to deal
with superfluous data. An example of this is the derivation of a fix from
within encompassing LOPs, where the sum of least squares can be used to
derive the centre of the shape.

Another way to put this is that statistics can be a good servant but a bad
master."

It seems to me more useful to eliminate random error as much as possible at
source, rather than worrying later about the resulting indeterminate nature
of a fix derived from random sights.

For practical nav in a small boat or for any moving target great analysis is
probably not worth the effort, but as many contributors to this list work
from a known position on land and are mainly interested in what accuracy
they can achieve, some appreciation of these methods could be useful. While
we're on that theme, the traditional triangle formed from 3 LOPs is less
than ideal. Four or more position lines is better.
-------------------
Robert comments:

Peter,

thankyou, thankyou, thankyou and thankyou del la puissance treize (to the
power of 13)  It is ironic that with a last name like Fogg, you have done
more to clear up the miasma of confusing arguments on this topic than has
any previous poster (with no insult intended towards the other
contributors).

Robert





--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To unsubscribe, send email to NavList-unsubscribe@fer3.com
-~----------~----~----~----~------~----~------~--~---

   

Reply