NavList:

   

Reply

Doug writes:
-------------------
One gets the estimate of latitude at LAN from the reduced noon sight.One
now has an idea of ones' latitude.Are you going to trust a position line
40 hrs old to get an idea of the longitude?Yes,one can advance the
line.But,in reality,how accurate will the results be from this r-fix?I
was taught and practiced that the optimal time to advance or retard any
line was 3-6 hrs. So,one is in the position in this scenario not having
the "warm,fuzzy" feeling of confidence reguarding the ships'position. At
the least one can use the r-fix and the noon GHA of the DR or EP to get
a better idea of where  the position is or if some gross error exists in
one of the parts.
-------------------

I'm still confused.  Suppose the procedure you suggest does give
a reliable latitude (I have some questions about that, but don't
want to discuss them until I'm sure I understand the procedure.)
It looks to me as if you must then choose among some of the following
alternatives for a longitude:

1. The longitude of the EP or DR position.
2. The longitude at which the (estimated) course line crosses the
   latitude determined by LAN sight.
3. A longitude at which the (estimated) distance-made-good circle
   crosses the latitude determined by LAN sight.

#1 is equivalent to advancing your old fix to a new EP or DR and
then finding the nearest position that matches the latitude
determined by the LAN sight, since longitude lines are perpendicular
to latitude lines.

None of these methods gains any information about longitude from
the sight.  If there is some information that your procedure
provides, I've missed it.

In your example, what would have been the resulting fix if you
were actually 30 miles further east at the time of your sight?
Here's how I figure it, using Hc from the USNO web site:
EP 32*21'36"N,118*43'45"W (True position 31*19'55"N,118*13'47"W)
GMT 12:04:26 15 Jan 04 (I assume you make the sight at the same
time, based on your EP.)
Ho (from USNO web) 36*32.1  Latitude computes as 32*20.0"
As far as I can see, the longitude computations are exactly the
same as in your original post -- the fact that the true position
is off by 30 miles makes no changes to the longitude derived from
this observation.  If this is true, in what way is the sight
providing longitude information?

        -- Bill

   

Reply