Use of deviation on running fixes

[capt jgw]

We have the same type questions on the USCG license exams for a three bearing fix. You use the deviation for the ships heading. Same deviation for all the bearings. If you do it right, the bearings will cross at one point or very close to it. If you use the deviations for the individual bearings, the LOP's will make a big triangle, which should clue you that you goofed. A running fix wouldn't be that obvious.

[Soton]

Is this about magnetic deviation with respect to the difference between chart north and magnetic north or are we talking about the deviation card of the ships compass.

[nigel1]

Originally Posted by Soton

Is this about magnetic deviation with respect to the difference between chart north and magnetic north or are we talking about the deviation card of the ships compass.

The difference between true north and magnetic north is termed Variation.

[jackdale]

To plot the vessel's compass course on the chart apply both deviation and variation.

To plot the lines of position (bearings) from the hand bearing compass apply variation.

Below are some slides that I use for teaching.

http://www3.telus.net/jackdale/navlessons/Running.pdf

Note that my bearings are labeled in magnetic. A purist would label in true.

[Lodesman]

Originally Posted by Hoofsmit

if the fix is taken on that constant heading the same deviation is applied

That is a very good point. In a running fix, you don't necessarily have to maintain the same heading between LOPs, so long as you accurately depict the advance of the first LOP along your DRed track or tracks. And of course I should add, that deviation would then be applied from the new heading for the second LOP.

I don't really like the term "running fix" as it is not a "fix" per se; it is, more accurately, an EP as it is an improved DR.

[capt jgw]

Originally Posted by Soton

Is this about magnetic deviation with respect to the difference between chart north and magnetic north or are we talking about the deviation card of the ships compass.

It's about how to answer a test question. Very little correlation to the real world. And yes, the difference between true and magnetic north is Variation, and it's the same for all headings and all compasses in a particular location. Deviation is the effect of magnetic influences within the boat, and is different on every heading and different boat, and different for any additional compasses on a boat.

[Soton]

Originally Posted by nigel1

The difference between true north and magnetic north is termed Variation.

Thank you

[jackdale]

Originally Posted by capt jgw

It's about how to answer a test question. Very little correlation to the real world.

Like many of the concepts taught in sailing the running fix is a concept that is required at "higher" levels. A sun-run-sun fix in celestial navigation is a running fix.

When I am on long passages (long periods of tedium, punctuated by episodes of terror), celestial navigation provides a stimulus to my brain and provides a connection with nature and the universe that is wanting when using a chartplotter.

BTW - I do know how to use a chartplotter.

[Lodesman]

Originally Posted by capt jgw

It's about how to answer a test question. Very little correlation to the real world.

With respect, I have to disagree. While current technologies (GPS and e-charts) have made it somewhat unnecessary, not too long ago it was the way things were done in the real world. I assume the ASA wants to ensure a certain level of competence, in the event the electrics go down. Being able to navigate on paper charts with compass is still a valuable skill.

[buzzstar]

Originally Posted by Hoofsmit

Both
If it says the ships compass is used for the track and fixes, the deviation is applied on all points as stated on the ship deviation card
Better still - tell them to get it swung properly


Sent from my iPad.......i apologise for the auto corrects !!!

Completely off topic, even if on course, but no corrections are visible, auto or otherwise, at least from this reader's end.

[capt jgw]

Originally Posted by Lodesman

With respect, I have to disagree. While current technologies (GPS and e-charts) have made it somewhat unnecessary, not too long ago it was the way things were done in the real world. I assume the ASA wants to ensure a certain level of competence, in the event the electrics go down. Being able to navigate on paper charts with compass is still a valuable skill.

What I meant by little correlation to the real world is that's not how we do bearings. To take a bearing relative to the ships compass, you would need a peloris, and almost nobody carries one. In the real world you would use a hand bearing compass and ignore deviation as it can't be determined for a portable compass. I think that's the point of the questions being written that way, they're trying to trip you up.

[Lodesman]

Originally Posted by capt jgw

I think that's the point of the questions being written that way, they're trying to trip you up.

I get your point, but I don't think it's to trip you up. If you happened to be on a boat with a recently-swung pelorus compass, then the prudent mariner would use that over the HBC. Knowing the different corrections, one has to make for the different bits of kit one might have at one's disposal, is a reasonable expectation for a sailing certification.

[StuM]

Originally Posted by Soton

Is this about magnetic deviation with respect to the difference between chart north and magnetic north or are we talking about the deviation card of the ships compass.

You really need to learn the difference between deviation and variation

[Soton]

Originally Posted by StuM

You really need to learn the difference between deviation and variation

Just maybe that was what I was trying to establish.

[royalstar]

hello there.


You do use the deviation after taking bearing, then you have to convert compas bearing to true bearing to plot it on the chart.