| my account | login-logout | resources | support | catalog | home | get webcard |

Online Classroom
Topic Closed  Topic Closed


Post New Topic  
Topic Closed  Topic Closed
search | help desk | commons

This topic has been moved to Cel Nav Student Discussion Forum.    
next oldest topic   next newest topic
» Online Classroom   » Celestial Navigation   » Public Discussion of Cel Nav   » dr Latitude and Longitude in T or M

   
Author Topic: dr Latitude and Longitude in T or M
navstar


 - posted June 26, 2013 02:28 PM      Profile for navstar           Edit/Delete Post 
The dr lat/long in small boat sailing is recorded in magnetic. I just want to verify that for celestial navigation they always have to be converted to True for the calculation process in sight reductions and plotting.

In addition, I assume all positions resulting from the sight reduction process in celestial nav is also in True and then would have to be converted to magnetic for charting purposes as in coastal nav.

David Burch


 - posted June 26, 2013 02:42 PM      Profile for David Burch           Edit/Delete Post 
Are you signed up in our online cel nav course?

if so, please post your questions in the Student Discussion forum. thanks.

If not, please confirm. thanks.

From: Starpath, Seattle, WA
David Burch


 - posted June 26, 2013 03:02 PM      Profile for David Burch           Edit/Delete Post 
on a quick check it looks like you were signed up for the course in 2009 and started then, but did not finish. That course is still open to you to complete.

You just need a valid webcard to carry on with the course. these are available at

http://www.starpath.com/resources2/get_webcard.htm

then it is important to register an extension of the new card with your same user name and password you used for the course, then you are back in the course and can address some of these basic matters, and finish the full course if you like.

in the meant time, yes. cel nav is always true. Plotting on the other hand is always true as well for all navigation, coastal or celestial. in coastal you can use the magnetic compass rose, but you are plotting true directions on a chart in all cases.

From: Starpath, Seattle, WA
navstar


 - posted June 30, 2013 11:38 AM      Profile for navstar           Edit/Delete Post 
Where is the student forum? My current web card is supposed to cover the questions regarding cel nav until it expires and then I would have to renew.

Just as a point that in small boat coastal navigation the chart plotting is often done in M and not true.

David Burch


 - posted June 30, 2013 12:08 PM      Profile for David Burch           Edit/Delete Post 
it is inside of the cel nav course:

http://www.starpath.com/cgi-bin/ubb/ultimatebb.cgi?category=2

which you can also get to directly when you are logged in at:

http://www.starpath.com/cgi-bin/ubb/ultimatebb.cgi?ubb=forum;f=4

---------

This is a matter of semantics. Plotting is neither true or magnetic. once a line is on the chart it has a specific orientation, which you can then refer to a true rose or a magnetic rose, unless, for example, you live in Alaska, where the charts do not have magnetic roses.

The phrase plot magnetic, i assume means you are laying down your lines using the magnetic chart rose taken from magnetic bearings. that is fair enough.

But referring back to earlier discussion, there would never be any occasion to "convert back to magnetic" for plotting.... ie there are no charts made that have magnetic roses and not true roses.

As a rule we recommend doing all inland or coastal navigation and plotting with magnetic bearings (unless you live in Alaska!) as that is what most vessels work with inland, and so the main answer to your question is, once you go offshore, this would change, and you would do all of your navigation in true bearings, and only ever think of magnetic at the very last step when you are telling the helmsman what course to steer.

Throughout all of cel nav, we have no reason nor justification to even care that the earth has a magnetic field.

Sorry if i muddled up that answer.

From: Starpath, Seattle, WA
David Burch


 - posted June 30, 2013 12:11 PM      Profile for David Burch           Edit/Delete Post 
A follow up note for clarification. AK charts do not use magnetic roses because the variation is changing so fast up north, that it would be inconvenient and indeed misleading to have the roses be different on different parts of the chart.
From: Starpath, Seattle, WA
navstar


 - posted July 01, 2013 11:53 AM      Profile for navstar           Edit/Delete Post 
Confusing answer. I think things are getting mixed between plotting your course on a navigation chart vs that of a plotting sheet in cel nav.

An example of where it is confusing in doing the plots on a plotting sheet in cel nav is Fig 6.3.1 (pg 77) where it labels the course in magnetic (even though the line is drawn "true"). While in Figure 6.4.1 (pg 78), every thing is in true. I assume everything on the plotting sheet in cel nav should be in true (and it would be even better to label the course in true - not as it is done in Fig 6.3.1). Being consistent helps reduce the confusion. Then if your are using magnetic on your navigation charts for the course lines you would change back at that time.

Thanks

David Burch


 - posted July 01, 2013 12:41 PM      Profile for David Burch           Edit/Delete Post 
Please post your question in the student discussion forum and we can continue there.

You can just cut and paste what you have here.

Thanks.

From: Starpath, Seattle, WA
David Burch


 - posted July 01, 2013 12:44 PM      Profile for David Burch           Edit/Delete Post 
In the meantime, let me add this and any further discussion should take place in the classroom student discussion forum.

-----------

I think that this might appear more confusing than it really is. There is nothing special going on.

We plot lines on a chart or a plotting sheet by some means (either a magnetic rose or true rose, it does not matter) and we label them to best meet our needs (we can label a course true or magnetic, depending on how we are navigating).

When plotting use the compass rose that is most convenient to you. As noted, on inland and coastal waters you may be doing all navigation in terms of compass courses, without any need for true directions. In this case you would plot your courses on a chart using the magnetic compass rose if a magnetic rose is available.

And in these cases, it would typically be most convenient to label your plotted courses with magnetic headings.

When in the ocean, you will be doing cel nav and all LOPs that you get will be given to you in true bearings, so when you plot these you use the true rose, which is the only one provided on the plot sheets.

But as soon as you choose to plot an actual course you are steering or striving for on the plotting sheet, you will typically be keeping your logbook records in magnetic (recommended), so it is logical to label the charted courses with the magnetic heading that actually match the log book. this is shown in several examples in the text.

This process will always require you to correct the magnetic heading to true heading before plotting on a plot sheet because they do not have magnetic roses.

On the other hand....

We are simply recommending this as the best practice, meaning easiest to do and least likely to cause confusion. Every navigator can adopt whatever convention best suits their individual preferences.

In other words, if there were a test question saying, What is the right way to label a course steered on a universal plotting sheet, True or Magnetic? The right answer would be: Whatever works best for you.

On the other hand, the convention of labeling such a course line with C 075 M or C 075 T on top and S 7.0 kt on the bottom would have a stronger recommendation for use, because this is an international standard that all navigators would understand, and if you deviate from that you risk losing this clarity.

From: Starpath, Seattle, WA


All times are Pacific  
Post New Topic  
Topic Closed  Topic Closed
Open Topic    Move Topic    Delete Topic next oldest topic   next newest topic
Hop To:

Starpath School of Navigation

Copyright, 2003-2021, Starpath Corporation

Powered by Infopop Corporation
UBB.classicTM 6.3.1.1