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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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  #41  
Old 07-20-2007
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No, what your doing now IS compass correction. You're referring to compass adjusting, which while referred to in the problem in "swinging ship", has no real bearing on the problem. One of the niceties of the CG exam questions is that they may, and do, give you information that may be extraneous to the answering of the question. Oh joy!
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  #42  
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Quote:
Originally Posted by sailaway21 View Post
No, what your doing now IS compass correction. You're referring to compass adjusting, which while referred to in the problem in "swinging ship", has no real bearing on the problem. One of the niceties of the CG exam questions is that they may, and do, give you information that may be extraneous to the answering of the question. Oh joy!
Hmmm... Okay. I did get 3 & 5 correct. I was wrong about 4. I'll have to give it some more thought to figure out why.

Jim
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  #43  
Old 07-20-2007
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Variation, deviation what's up...........

This post brings up a great question. I attended a USCG boating course several years ago shortly after I purchased my sailboat. Everything was fine until we got to the two classes on navigation. I was really confused with the presentation (everybody was), and that bothers me a lot. Somebody straighten me out here please. Am I missing something here?
Nautical charts are made with true north referenced at the top of the chart. When we draw a line connecting two points that becomes a “true course” correct (actually great circle route)? Prior to GPS and microprocessors we had no instruments available to us that would allow us to follow a true course directly; as we only had the magnetic compass. The magnetic compass points to magnetic north which is in a different place on the planet than the actual North Pole which is “True North”.
On most places on the planet magnetic north and true north are not lined up. The difference will either be to the left or to the right. This “error” is called variation and is either easterly or westerly and changes with latitude as well as longitude (it also changes over time). You are either going to subtract easterly variation or add westerly variation. For example if your true course is 040 degrees and the area you are sailing/boating/flying in has 10 degree easterly variation you would subtract 10 from 40 and come up with a magnetic course of 030. You would sail a magnetic course of 030 to actually sail a true course of 040 that you drew on the chart connecting the two points. But we are not quite done yet.
The next error is induced into the vessel’s compass by the interference of the structural metal and electrical equipment installed in the vessel itself has on the compass. This error is called “deviation” and each compass must be individually corrected and compensated for. You would consult the error card for a heading of 030. Let’s say that you had a +3 error on that heading. Then you would subtract 3 degrees to compensate for the deviation error resulting in a heading of 027 degrees.
Given my example I would want to sight a magnetic course off the compass of 027 and sail that to actually sail a true course of 040.
I thought I had a good handle on this subject but after the "course" I was really confused. Am I missing something?
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Old 07-20-2007
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Northbeach..
Nautical charts are made with true north referenced at the top of the chart. When we draw a line connecting two points that becomes a “true course” correct?

Any line you draw on a chart is simply a course. Only when you able it with a TRUE heading does it become a true course. If you label it with a magnetic heading...it is a magnetic course.
Example...if you pick one of those pre-drawn lines on the chart going east to west (latitude lines!).....and draw a course line over that your course can be labeled 270 degrees TRUE since latitude lines are referenced to true north.
But if you have 7 degrees of W variation...you can also label it 277M.

Other than that nit-picky observation...you've got it right...and since you have a GPS...who cares!! (G)
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Northbeach said,
"Nautical charts are made with true north referenced at the top of the chart. When we draw a line connecting two points that becomes a “true course” correct (actually great circle route)?"


A straight line on a chart is only a great circle if it is drawn on a Gnomonic chart or is the equator or line of longitude when drawn on a Mercator chart. On a Mercator chart a straight line called a rhumb line and is usually a spiral when plotted on the globe unless it is the equator or a line of longitude.
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Robert Gainer
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Quote:
Originally Posted by camaraderie View Post
...and since you have a GPS...who cares!! (G)
Too obvious by far, cam

Jim
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Semi...OK...here's more subtle...(G)

The Republican Navigator

A woman in a hot air balloon realized she was lost. She lowered her altitude and spotted a man in a sailboat below. She shouted to him,
"Excuse me, can you help me? I promised a friend I would meet him;
an hour ago, but I don't know where I am."


The man consulted his portable GPS and replied, "You're in a hot airballoon, approximately 30 feet above a ground elevation of 2,346 feet above sea level. You are at 31 degrees, 14.97 minutes north latitude and 100 degrees, 49.09 minutes west longitude."

She rolled her eyes and said, "You must be a Republican."

"I am," replied the man. "How did you know?"

"Well," answered the balloonist, "everything you told me is technically correct, but I have no idea what to do with your information, and I'm still lost. Frankly, you've not been much help to me."

The man smiled and; responded, "You must be a Democrat."

"I am," replied the balloonist. "How did you know?"
"Well," said the man, "you don't know where you are or where you're going. You've risen to where you are, due to a large quantity of hot air. You made a promise that you have no idea how to keep, and you expect me to solve your problem. You're in exactly the same position you were in before we met, but, somehow, now it's my fault."




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  #48  
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Quote:
Originally Posted by sailaway21 View Post
4. Enroute from Rio de Janiero to Montevideo, the ship is "swung" to take a round of bearings from the standard compass. With Ilha Rasa about ten miles distant, the swing is made and the following data noted:

Ship's Head pSC...........................pSC bearing on Ilha Rasa
000...........................................036. 5
030...........................................037. 0
060...........................................037. 5
090...........................................038. 5
120...........................................039. 0
150...........................................040. 0
180...........................................040. 0
210...........................................039. 0
240...........................................038. 5
270...........................................037. 5
300...........................................037. 0
330...........................................036. 5

After the swing has been made, the ship resumes the voyage on a course of 210 pSC. Which of the following is the correct deviation when the ship is on course 210 pSC?

A. 2 West
B. 1 East
C. 1 West
D. 2 East

Note: the above question is a deliberately tricky USCG question designed to determine whether you know your theory of compass correction. It is not a "practical" question. (anotherwords, lots of people get it wrong-never figuring out the point to the question)
Okay, I think I've figured it out.

So as not to ruin it for anybody else, I'll give my analysis in the AHOC thread.

Jim
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  #49  
Old 07-21-2007
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Northbeach,Perhaps one of the things that is confusing you relates to terminology. For example, we never actually sail a "magnetic" course. Everything you said is correct except you refer to sailing a "magnetic" course when you should be referring to a "compass" course. The only time we can sail a "magnetic" course is when deviation is zero. Otherwise, we have no instrument that actually tells us our "magnetic" course, it's only a mathematical computation along the way. Feel free to disregard the following if it sheds no light.

When discussing compass correction we must be rigorous in the usage of our terminology. In this case we have three co-ordinate systems that we can relate to each other.

The first is the well known terrestial system. This consists of parallels of latitude and meridians. All meridians, at all times, point towards Polar North, or True North. All parallels point, at all times, east and west. This is true on all chart projections, with the most common being the Mercator. The confusing part arises when, if the chart scale is small enough, that is it covers a large area, the parallels and meridians will not intersect at a 90 degree angle. This can best be explained by the fact that the most accurate representation of the earth, is a globe. But a globe is rather inconvenient to navigate upon. So, in one way or another, we slice the surface off the globe and flatten it out so we can go to work. In the case of the Mercator chart, we end up with meridians that do not converge towards the poles, but always point north, and parallels that always run east and west. This results in the distortion of landmass that makes Greenland appear larger than the North American continent. But it works admirably for navigating at sea.

So our terrestial system gives us True, or Polar North. We often work out these problems using a "spider" diagram, which resembles a clock face when sketched out. We label True North as "Pn" for Polar North on it, and it is always pointing straight up on our diagram. Any angular measurements made from it, in a clockwise direction, are considered True Courses, and are labeled with the degrees clockwise from Pn and the letter "T".

Our next co-ordinate system is the Magnetic Co-ordinate system. We use the same global sphere for it, but our reference points are different. In this case our north is Magnetic North, or "Mn", and it is the magnetic north pole. The magnetic meridians we use from it are similar to terrestial meridians but subject to variations due to the effects of land masses and other inequalities in the earth's composition. Our various compasses are attempting to point towards Magnetic North, "Mn". In reality, our compasses will rarely point towards Mn accurately, unless perhaps we are in a liferaft with no metal objects about us. Our compasses will have the strongest pull or tendency to point towards Mn at the magnetic equator. This directive force will become weaker the closer we get to the magnetic pole. Specially balanced compasses are necessary for navigation in high latitudes as a result. The difference between True North and Magnetic North is called Variation. Due to the anomolies described above, variation is not a consistent number or ratio. Local charts will list variation, while sailing or oceanic charts will show lines of equal variation, known as isomagnetic lines.

Our third co-ordinate system is our compass system. The compass points towards Compass North, or "Cn". Compass North is where our individual compass thinks Magnetic North is located. We commonly label the courses derived for this by the name of the compass used, the most common being the standard or steering compass, hence the abbreviation "pSC" for per Steering Compass. The difference between Mn and Cn is Deviation. Deviation has many components, but for purposes of simplicity we will say that it is the error induced on our compass by the magnetic field(s) of our boat. Deviation varies with the heading of the boat, hence the need for a deviation card. Deviation also varies with the loading condition of the boat. If you rig a SSB antenna on a halyard you will change the deviation of the boat from the deviation it had un-rigged. If you modify the boat, running battery cables forward to a new windlass, you will change the deviation. Lastly, deviation varies with the magnetic heading of the vessel and not the true heading; the forces being operative are the isomagnetic lines of force-not the boats true heading. (and if that ain't confusin', nothin' is!)

True Virgins Make Dull Companions
You can work this formula forwards or backwards. On our spider diagram the line pointing up is Pn and is the same as the meridian on our chart, it points to True North. All of our labellings on the chart will be "true" courses, although we may add sub-labellings for magnetic or compass courses, with only the latter really being used. For example: we would label a DR track showing a course of 045 True as 045T on the top of the DR track drawn, and, say 038pSC under the DR track drawn. This then gives us our true course intended, which equates in an angularly true fashion with our meridians and parallels on the chart. The pSC for the same course is merely the conversion of 045T to that which we wish to steer at the helm. It should be endeavored to never label the course laid down with only compass or magnetic headings, always including the true heading or course. The compass, and most certainly the magnetic, course has no relevance whatsoever to what we are plotting on the chart, UNLESS. "Unless" will be explained below. Let's ignore it for now.

It is best to only consider variation in east or west terms. In my experience, labelling it plus or minus ends up confusing in the end. If we have 10 West variation we can draw another radial line on our spider diagram 10 degrees counterclockwise from Pn, and label it Mn. If we now look at the diagram we can, by inspection alone, determine that our 045T course would be 055M, or magnetic. But this does us no good as we do not have a "magnetic compass" we only have our Steering Compass which is subject to Deviation as well as Variation. Let's say our deviation card says that, on a course of 055M, our deviation is 2 East. We now go back to our spider diagram, and can draw in another radial line, this one two degrees clockwise from our Mn line, and label it "Cn" for Compass North. We commonly scribe arcs with the angular measuements between all of these on our diagram. It ends up looking somewhat like a portion of a spider web, hence the name. Note that, while doing this, our Pn line and our TC line, 045T, never change. The variables are variation and deviation, and the diagram allows us to relate them to one another and our true course so we can derive what to steer.

Now the reason I said not to assign pluses or minuses is that if we envision this spider diagram we will be able to properly place Mn to either the west or east of Pn, and then Cn to either the west or east of Mn.

Without the spider diagram, we can do this mathematically, and then we will use a plus or minus, remembering to ALWAYS apply the formula in the order written. You can work it backwards, but must then reverse the corrections. For example: 045T, 10 West Variation, 2 East Deviation Using TVMDC and "West is Best", "East is Least". We take our desired 045 True Course and add (best) 10 to it for a 055 Magnetic Course (NOT COMPASS!) and then subtract (least) 2 from it for our 053pSC course.

Conversely: We're steering 053pSC. What is our True Course? Working TVMDC backwards, with the rules reversed, we proceed; 053 plus 2 gives us 055M. 055M minus 10 gives us 045T.

It is best to learn the TVMDC, west-best, east-least, very well and then rely on your ability to transpose for working backwards.


The "unless" comes in with the use of the compass rose on the chart. The inner circle shows us "magnetic north". If our deviation is zero, or insignificant, we can use that compass circle to convert our true course to magnetic, which will be the same as compass.

The procedure would involve laying down a course unkown between two points. We then either walk our parallel rule or triangles over to the compass rose until centered on the rose. The outer ring, where we cross it, yields our true course, the inner ring our magnetic. I find it a hassle and, using triangles, I walk the triangle over to the nearest meridian and read my true course off of it. I then just apply the variation intuitively for the magnetic course. The advantage of triangles or the aviation parallel rule with protractor is that you do not have to slowly traverse the entire chart over to the compass rose, just the nearest meridian.

Now if that does not muddy the waters.... I gotta find a bored twelve year old to show me how to scan and post sketches.
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Old 07-21-2007
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Sailaway-

Just a take a digital photo of the sketch. On a PC, just use Picasa and upload them to a Picasa web album. On a Mac, use iPhoto and a Flickr.com plugin. Much easier and faster than trying to scan them IMHO. You do have a digital camera, don't you???
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