Back bearings with compass

[JohnHuth]

Hi -

This is mainly for people who took the navigation class on Sunday (thanks for your enthusiasm by the way).

So, here is an illustration of how to use the compass as an angular measuring device. Recall the exercise yesterday where you saw the Boston Lighthouse with an extended finger next to it. The lighthouse was at a bearing of 30 degrees magnetic and from the finger size relative to the lighthouse, you were supposed to get a position using a line-of-position and range.

This is illustrated in the two attached figures.

Without anything fancy, here's how you can use the compass to solve the angle problem - note that I went around and showed people this.

First, you need the chart prepared as we did with the variation lines drawn in.

Disregard the compass needle, you'll only need the dial and baseplate for this.

First, rotate the dial by the angle of the bearing (30 degrees in this case). Flip the compass 180 degees to get the back bearing. Align the orientation lines on the dial with the variation lines, and then move the edge of the baseplate so that it touches the object sighted - in this case the lighthouse.

That line (dotted in the Step 2 illustration) - will give you your line of position. No calculations- just one rotation and alignment of the compass.

Note - I had some e-mail exchange with John Carmody, and I'd like to pass along some notes. He prefers to do the addition and subtraction for variation. He's seen students who do not have the concept of variation and this is dangerous if, for example, you don't have a map with variation lines drawn in. This is one reason I'm always thinking in terms of both true and magnetic.

Secondly, he fears that drawing lines on the map may cover up features like rocks.

So, these caveats are good to keep in mind. I find that different people find different ways of doing things that's to their preference, so I don't know if anything is 100% foolproof - like the poor guy who got east and west declination confused and got lost in Nova Scotia!

[JohnHuth]

Rather than waste bandwidth with more topics, I'm attaching here a little chart of typical angles associated with hand widths. I'll give a formula for figuring range in just a bit.

[JohnHuth]

I posted my slides at this URL. It should be public, but I don't know if this works. If you cannot (or can) access them, please let me know.

https://drive.google.com/folderview?id=0B35Z4TtP-ZZhN2ZNY0RpUWRQOFk&usp=sharing

Note that the files are kind of large, so you you'll probably have to download them from the site.

Edited January 29, 2014 by JohnHuth

[Joyce Carpenter]

I think something's wrong. When I click your link, I see two images. I thought those were the files to open/download, but apparently it's really just two images. Double-clicking gets an error and right-clicking gives the "view / save / image" menu.

Update: I use Google Drive, so I noticed an option in the top right to "add to my Drive." From there, I seem to be able to download each file. I'm not sure what happens if one doesn't use Drive.

Edited January 29, 2014 by carpy

[JohnHuth]

Try going to the lower right hand corner and there's an arrow that will allow you to download.

[Joyce Carpenter]

Sorry, but I don't see anything in the lower right hand corner when looking at the page in Firefox, Safari or Chrome on Mac.

On a different topic: Are you interested in typos in your book. I just started reading it and noticed that Figure 9a should say "Possible locations after 1 hour" rather than "after 1 day. " (My day job: editor)

[cfolster]

To help a bit here . . .

1. Click the link John provided.

2. Click on one of the icons and it may open the document, or you may get an error. Either way, clicking the icon opens an overlay window and in that, you should see a little down arrow in the lower right hand corner.

3. Click that down arrow to download the document.

You may get a message that it has not been scanned for viruses - download if you choose. . .

Repeat for the second document.

Even though I got an error trying to open/view the document online, I was still able to download it.

[JohnHuth]

Add/subtract mnemonics I've collected:

East is least, west is best (Subtract easterly, add westerly variation)

Maps Tell Almost Everything (Mag To True Add East)

True Virgins Make Dull Company (True heading minus Variation plus or minus Magnetic variation plus or minus Deviation equals Course)

CADET - Compass Add East for True

Can Dead Men Vote Twice (Same as CADET)

Of these, I like CADET the best, as it's all purpose If you go left to right - you have "Compass Add East for True" - then if you substitute "West" - you know it's Compass Subtract West for True" - and then if you go the reverse direction it's True subtract East for compass, and True add west for compass. You have to keep the arrows in mind - left to right and right to left, but it's an easy way to remember addition and subtraction. The virgin one is just plain confusing to me.

[JohnHuth]

If you click on the icon, you'll see it trying to download. Then, you'll see an arrow in the lower right hand corner - click on that. You'll then get a message saying that the file is too large for google to process, do you want to download anyway? Click to download, and you should get it. It's a large file.

Yup, I noticed that typo already, but thanks. There are some others in there.

[djlewis]

Add/subtract mnemonics I've collected

My "mnemonic" is non-verbal -- visualize the north magnetic pole somewhere north of Chicago or Minneapolis on a map or globe of the northern hemisphere.

That tells me that magnetic north is to the left of (numerically lower then) true north, which is at the North Pole. Of course, on the west coast you get the opposite result, but that's what you want.

Actually, the visualization tells me exactly how to adjust course more or less directly, without much verbal processing, which I find distracting and prone to error, especially in bilaterally symmetric situations (yes, I am bilaterally challenged). So, for example, if I want to head true (chart) north, I know I need to point the boat to the right of what the deck compass says is north (that is make the deck compass read the actual declination) just as the north pole is to the right of Chicago.

Now that I think about (and visualize) things, it might be simpler just to imagine magnetic north at the top of the chart but slightly to the left of center.

This all probably sounds tricky, but remember that I am trying to verbalize a fundamentally non-verbal process, which always makes it sound harder than it is.

Anyway, chacun à son goût.

[gyork]

Add/subtract mnemonics I've collected:

Of these, I like CADET the best, as it's all purpose If you go left to right - you have "Compass Add East for True" - then if you substitute "West" - you know it's Compass Subtract West for True" - and then if you go the reverse direction it's True subtract East for compass, and True add west for compass.

The acronym in my head seems simpler still.

MTEL (say "metal"): Map to Terrain, East is Least (please forgive the non-nautical flavor)

Every combination spins off of this, in an opposite way: Map to Terrain, West is Best; TMEB; TMWL.

Example: Reviewing my chart at the Island Beach where I have spent the previous night camping, red nun #4 is at a northerly direction, one half mile from me. In fact, the line of direction from the edge of my compass base plate exactly parallels the right-hand edge of my chart (true North; 0°). Remember that in our neck of the woods/ocean, declination/variation is between 16 and 17° West. Now when I scout the horizon for the nun, I will use the MTWB rule. 16° added to 0° equals 16°. I now turn my compass bezel (or when asea, my kayak with mounted compass) to read 16°, hold the compass flat in front of me, standing, then rotate until the North needle positions itself squarely in the doghouse (red in the shed). Scanning the general direction with binoculars, the nun comes into view.

I am a steadfast practitioner of drawing magnetic North lines on my charts, equally spaced at one or two nautical miles, depending on the scale.

I, like David, constantly visualize the compass Rose as an imaginary "overlay" when staring at charts.

[BillyD]

If I had Nanook's kayak (from Doug's post), I would take two cadets and a virgin with me.to help me get my bearings back.

Thanks again John for sharing with us all

BD

[JohnHuth]

I pretty much do like David Lewis, and picture things in my head, but I realize that some people do better with mnemonics. So, I figured it was worth doing a survey and trying to pick out one that struck me as being the easiest. CADET seemed to be the best of the lot for me, but I can easily imagine others would work better for some.

I suppose the one thing that I would reiterate from the workshop is this: try to always keep both a magnetic and true bearing/heading going in your mind when you're on the water, and in the trip planning stages.

[JohnHuth]

Link to Boston Harbor Chart JPEG

It's 10.6 MB, and has decent resolution - the whole chart.

Link to Boston Harbor Chart with variation lines JPEG

Variation lines are spaced 1 nm apart.

Edited February 1, 2014 by JohnHuth