Silva 70P compass question

[SZihn]

Well I got a Silva 70P compass. It came new in the box, but going on line for instructions it seems there is info on setting it for declination, which they call compensation, but when I look a the compass itself there are NO screws to adjust for east/west and north/south despite what the sheet says. Here is a link>>>

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Anyone else have any experience with this issue?

It seems there are no adjustment built into the compass at all. If not it's no big deal. I can navigate as easily from magnetic north as I can from grid north. But I have to be sure all my paddling partners are on the same "sheet of music" and all communications are designated Grid or magnetic. I have always used grid for my purposes for the last 50+ years, but if I can't make an adjustment to the compass itself I'll just use magnetic north and all headings stated from that system. So if I could dial it to grid north (16 degrees east from my location) I would not have to change anything from my land navigation to my water navigation.

So.............am I missing something here? Do any of you know what Silva is talking about in their instruction from the link above?

 

[JohnAbercrombie]

I just had a very quick look at the Silva instructions.

They don't make very clear the difference between deviation (compass error produced by magnetic effects from the vessel) and variation (16E at your location). In non-nautical navigation, variation is often called declination.

I know that orienteering compasses allow adjustment for variation/declination. Marine compasses don't provide for that, in my experience.
Consequently, the paddlers I know use magnetic headings and bearings, and adjust their GPS devices to display directions in magnetic, not true.
When doing chart work, it's common to label course lines with the magnetic heading (ex. 260M) and reciprocal (80M).

Also, I think 'Grid' is a military navigation term; most sailors will talk about 'True' vs 'Magnetic' directions.

 

[JKA]

Also, I think 'Grid' is a military navigation term; most sailors will talk about 'True' vs 'Magnetic' directions.

Can the navigation pedant in me stir the pot about True, Magnetic and Grid North?

Once when instructing some police, my army background was offended by their dismissal of true v grid so I chucked celestial north in just to wind them up. Which led to an argument about degrees and mils!

Alpha personalities eh.

 

[alexsidles]

I, too, delight in stirring the navigational pot.

“Grid north” is a feature of map projections that use a system of regularly-sized grids. Universal Transverse Mercator (UTM) and Military Grid Reference System (MGRS) are two common examples. These projections have the advantage of making it extremely easy to calculate distances between points. In the case of MGRS, you can do it in your head. These projections have the disadvantage that “up” on the grid map will not be true north but rather a distortion: grid north.

Nautical charts don’t try to shoehorn the world into evenly-sized grids, so they don’t have grid north; they just have true north. Look at the compass rose of a nautical chart - it won’t even mention grid north, because grid north is always true north. The downside to this approach is that calculating distances requires a lot more math, particulary east-west distances involving changes in longitude.

Most kayakers use nautical charts most of the time, so most of us rarely have to deal with grid north.

Funny story about setting the variation on a 70P compass earlier this year. Variation in my area is about 15 or 16 degrees east, and I thought I had it all figured out in my head. I even made up a little ditty to sing: “Whatever the compass says, subtract fifteen.”

Of course, I had it exactly backwards. You have to add easterly variation, not subtract. I was doubling the amount of “error” in the compass!

Over the course of a few days, I was getting more and more frustrated, because my compass was consistently 30 degrees off what I expected the bearing to be. I even tried reinstalling it, in case I had gotten the lubber line crooked.

It was only after long and careful thought that I figured out the problem. Now I sing a new dittty: “Whatever the compass says, you add fifteen!”

Alex

 

[CPS]

There's all sorts of little rhymes or mnemonics to remember whether to add or subtract deviation. I usually forget most of them.

Ground Onto Map, Add Deviation - GO MAD, seems to stick for whatever reason. Pretty sure it's a bastardized form of GUMA - Ground Unto Map, Add variation.

Or another thing that helps me remember is that is I take a bearing and ADD it to the map, I have to ADD the variation. Similarly I remove or SUBTRACT the compass from the map when taking a bearing from a map and using it in the field.

Given that we're on the West Coast and Variation is East, I use the rhyme East is Least, which doesn't really make sense, but reminds me to take off variation.
Or course this wouldn't work well on the other side of the agonic line.

 

[CPS]

Also, in the instructions linked the compensator is mentioned as being an optional extra. That's likely why the screws are absent.

 

[SZihn]

Military lensatic compasses never had any adjustment for declination in them. Some of us in Recon bought 'civilian" compasses for our use, because navigation was the very foundation for reconnaissance and we wanted to have redundancy, and also have the ability to use the map and compass together and not risk any mistakes on the move. Also the oscillation of the Silva, Brunton and Suunto compasses were far reduced from the military compass, so the use of them in the field "on the fly" was faster and easier.

The magnetic N. Pole is actually near the north end and a bit west of Hudson Bay. Looking at a globe, if the top of Hudson bay is to the left of the N.P. from where you are standing, you'll have declination to the left. If it's to your right of the N.P from where you are standing, your declination is to the right.
Here is a link to show what I am talking about. Once you see it illustrated the idea becomes clear. https://maps.ngdc.noaa.gov/viewers/historical_declination/

 

[JohnAbercrombie]

There's all sorts of little rhymes or mnemonics to remember whether to add or subtract deviation. I usually forget most of them.

Ground Onto Map, Add Deviation - GO MAD, seems to stick for whatever reason. Pretty sure it's a bastardized form of GUMA - Ground Unto Map, Add variation.

Or another thing that helps me remember is that is I take a bearing and ADD it to the map, I have to ADD the variation. Similarly I remove or SUBTRACT the compass from the map when taking a bearing from a map and using it in the field.

Given that we're on the West Coast and Variation is East, I use the rhyme East is Least, which doesn't really make sense, but reminds me to take off variation.
Or course this wouldn't work well on the other side of the agonic line.

Deviation is not the same as variation! I don't think any kayakers swing the compass to make a deviation table to put beside the navigation station in the boat.
See:
https://westcoastpaddler.com/community/threads/deviation-vs-variation.7459/

The aide-memoire I was taught (and teach) is: VARIATION EAST, COMPASS LEAST. VARIATION WEST COMPASS BEST.

And, don't be lubberly: Use Variation, not Declination in nautical navigation discussions!

 

[JohnAbercrombie]

The magnetic N. Pole is actually near the north end and a bit west of Hudson Bay.

Looking at a globe, if the top of Hudson bay is to the left of the N.P. from where you are standing, you'll have declination to the left. If it's to your right of the N.P from where you are standing, your declination is to the right.

Here is a link to show what I am talking about. Once you see it illustrated the idea becomes clear. https://maps.ngdc.noaa.gov/viewers/historical_declination/

That NOAA link shows that our ideas about Magnetic north and variation - which work well for navigation - are simplificatons of the actual situation. If we could 'track along' a line of variaton - say follow the 20E variation line, we wouldn't follow a straight line to the magnetic north pole.
That 'bar magnet inside the earth' isn't straight!
And the south magnetic pole is even farther from the earth's axis than the N magnetic pole- so we should consider ourselves lucky 'up here'.
Here's another view of the N magnetic pole, which is galloping across the Arctic at a good clip these days.

On a practical note, it's a good idea to check the current value for variation in the area of your nautical chart if you are using an older chart, as the modelling and projections for annual change (marked in the compass rose) may not be accurate. In practice, since we can't steer our kayaks on a heading very consistently, it doesn't matter much.

Aren't you glad we have GPS?

 

[Tongo-Rad]

As CPS mentioned the Silva 70P instructions (albeit from a 2017 model) manual say that you can "Adjust by using a compensator (optional extra) and eliminate or at least reduce the errors that iron or other sources of magnetic disturbance may have on the compass." Hopefully you don't have anything on or in your kayak contributing to a deviation error, but if so, it sounds like this optional extra could be useful if it's not too late to adjust it.

On that note, the Ritchie V-527 marine compass comes with compensation adjustment in the form of two built-in sets of magnets fixed to two adjusting rods with slotted ends, perpendicular to each other. A nice feature of this compass is that even after mounting it, you can unscrew the compass separately from the housing to service or adjust it (compensation, seating, replace o-ring, etc.) and then reinstall with the mount still in place.

 

[Tongo-Rad]

This seems like a good time and place to put out some related definitions, more for my own reference and others who are wondering rather than for the more experienced salty dogs here:

Variation - This term is preferred by mariners and pilots because the word "declination" also has an astronomical usage - the angle of a star or planet above the celestial equator. However, the word "variation" is used by geomagneticians to refer to time changes in the magnetic field.

Declination - This is the term preferred by those who study the magnetic field; it is also the term most commonly used by land navigators. Sometimes the term "magnetic declination" is used.

Example of magnetic declination showing a compass needle
with a "positive" (or "easterly") variation from geographic north.
Ng is geographic or true north, Nm is magnetic north, and δ is magnetic declination


Deviation - In a vehicle such as a ship or aircraft, a compass is influenced by the magnetism of the iron used in the construction of the vehicle as well as the Earth's magnetic field. This causes the compass needle to point in the wrong direction. This directional error is called "deviation". Many people incorrectly use deviation when they mean declination. (This is a situation when you might want to adjust the compensation feature on a compass)

Further references on compass basics: https://paddling.com/learn/compass-basics-the-marine-compass

For anyone looking to take some courses, I can recommend Skils.ca's Navigation for Paddlers and Boaters and the more in-depth Online Trip Planning Series of courses that @kayakwriter organizes with Jericho Beach Kayak.

 

[JohnAbercrombie]

In a recent issue of Paddling magazine there's a compass article by Virginia Marshall.
https://dashboard.mazsystems.com/reader/75333?page=70

She explains how to use an orienteering compass for kayaking.
The first step she recommends is using the adjustments for declination/variation in the compass so that no calculations need to be made to use the compass to follow a 'True' heading. This woujld be well and good if:
The kayaker is using that orienteering compass as the deck compass, and
The kayaker isn't paddling with other people who have (marine) deck compasses. or orienteering compasses with the declination adjustment set at zero and
That orienteering compass isn't used in areas with different variation - no paddling (or driving) to Alaska from Victoria BC.

The last point is one reason that marine (and military) compasses don't have that adjustment to 'zero out' declination/variation - the compass could be used in different parts of the world. In marine compasses, it would be almost impossible to do that adjustment anyway, since the compass would have to be disassembled completely (draining the fluid) to change the position of the magnet in the card.

My verdict on that Paddling article: it's a recipe for confusion.
YMMV, IMO, etc etc!

 

[alexsidles]

On that note, the Ritchie V-527 marine compass comes with compensation adjustment in the form of two built-in sets of magnets fixed to two adjusting rods with slotted ends, perpendicular to each other. A nice feature of this compass is that even after mounting it, you can unscrew the compass separately from the housing to service or adjust it (compensation, seating, replace o-ring, etc.) and then reinstall with the mount still in place.

That's cool the Ritchie compasses can be removed from their mount. Very convenient for cleaning and drying beneath the compass!

The Ritchie compasses also have nicer compass cards. The Ritchie cards display cardinal and intercardinal directions in large font, whereas the Brunton/Silva cards display only cardinal directions in small font. Also, the Ritchie cards are available in black or white or blue, whereas the Brunton/Silva cards are only available in black. I think a white compass card would probably be easier to read at night, a problem I encountered using my Brunton on a recent overnight trip.

One caution with the Ritchie V-527 is that it is not designed to be read from a low angle. The Ritchie V-527 is meant to be read from an angle of 45 to 90 degrees, whereas the compass recesses of some models of sea kayak create a reading angle of around 25 or 30 degrees. Apparently, you can adjust the Ritchie's compass angle so it works even from a low angle, but it sounds like a bit of a pain.

Ritchie does make other compass models that are designed for low-angle viewing.

Alex

 

[Tongo-Rad]

...using my Brunton on a recent overnight trip.

Not to deviate from the compass topic, but whoa @alexsidles, what a jarring trip report of your Pacific night paddle ! I'm glad to hear you had a VHF on your person, and that you and your kayak were ultimately saved. Thanks for sharing.

 

[kayakwriter]

In a recent issue of Paddling magazine there's a compass article by Virginia Marshall.
https://dashboard.mazsystems.com/reader/75333?page=70
She explains how to use an orienteering compass for kayaking.
The first step she recommends is using the adjustments for declination/variation in the compass so that no calculations need to be made to use the compass to follow a 'True' heading. This woujld be well and good if:
The kayaker is using that orienteering compass as the deck compass, and
The kayaker isn't paddling with other people who have (marine) deck compasses. or orienteering compasses with the declination adjustment set at zero and
That orienteering compass isn't used in areas with different variation - no paddling (or driving) to Alaska from Victoria BC.
The last point is one reason that marine (and military) compasses don't have that adjustment to 'zero out' declination/variation - the compass could be used in different parts of the world. In marine compasses, it would be almost impossible to do that adjustment anyway, since the compass would have to be disassembled completely (draining the fluid) to change the position of the magnet in the card.
My verdict on that Paddling article: it's a recipe for confusion.
YMMV, IMO, etc etc!

Being a bear of little brain, I strive to make things if not idiot-proof, at least idiot-resistant. So rather than trying to remember whether I'm adding or subtracting when going from chart to compass or compass to chart (and whether I'm in western Canada or eastern Canada, etc. etc.), what I do is use parallel rules to "walk" the magnetic North arrow around the chart and draw in additional arrows such that no matter how the chart is folded, at least one magnetic North arrow is visible. This means I can quickly use my hiker's compass to orient the chart in its case. It also means that if I use the hiker's compass as a protractor to determine the bearing from one location to another, it's already in magnetic and ready to be used as is on my deck compass. (And vise-versa - if I've used the deck compass to take a bearing from a landmark, I can transfer it to the hiker's compass for triangulating my position on the chart without adding or subtracting.)

Here's a section of the chart from my recent Hotham Sound trip showing the idea. Astute observers (that is, the entire WCP commentariat of course!) will notice that my blue drawn-in arrows are actually a few degrees different than the magnetic North arrow shown in the inner compass rose. That's because the declination has changed since that compass rose was drawn in 1996. I could have just worked it out according to the rate of change shown on the arrow (7'W per year) but I know that recently the rates of change have, well, changed, compared to what they had been predicting previously. So I went to the handy-dandy online declination calculator to get the current best guestimate for this year, then used that offset (16 degrees East from true North) to draw my arrows.
Instead of using the purple inner compass rose, I use the bezel on my hiker's compass as my "rose" and line it up with my time-corrected blue magnetic North arrows when plotting bearings or sights. Voila: declination and change in declination over time all automagically corrected for without error-prone addition or subtraction!

 

[JohnAbercrombie]

Instead of using an orienteering compass as a protractor - which I find very confusing - I use a Douglas Protractor.
https://www.avworld.ca/douglas-protractor.html?
They are cheap and stow easily in the chart case. I have used the protractor on deck in a class. It's a useful skill if one needs to make a 'change to Plan X' in the middle of a paddle, but I try to do chartwork before it's needed.

Drill a small hole in the middle of the protractor and add a string with a knot, and it's even easier to use.

 

[kayakwriter]

Instead of using an orienteering compass as a protractor - which I find very confusing - I use a Douglas Protractor.https://www.avworld.ca/douglas-protractor.html?They are cheap and stow easily in the chart case. I have used the protractor on deck in a class. It's a useful skill if one needs to make a 'change to Plan X' in the middle of a paddle, but I try to do chartwork before it's needed.Drill a small hole in the middle of the protractor and add a string with a knot, and it's even easier to use.

Yup, Mike Gilbert, who's an instructor instructor, uses the stringed protractor trick too. I've been meaning to copy him, but hadn't been able to source a Douglas Protractor (like parallel rules, they're getting harder and harder to find.) But the aviation supply store is a great idea - I'd been limiting my search to marine supply stores!

Edited to add: bingo!

 

[eriktheviking]

I am a convert to the system that "Blue Dog Dave" showed me- drawn pencil lines on the charts along the magnetic N-S lines (updated from the compass rose direction), and use the protractor above with a string to read off magnetic headings directly. This made much more sense to me than what we did in my L2 course. No math in the head needed or remembering to add or subtract the declination/variation.

I even got to use it this year- it fogged right in after we left Guise Bay so we crossed Hansen Bay etc on compass bearings. Simple to do in the field and worked out fine- though probably felt safer doing it in a group of 8 rather than solo...

Sure- my charts will not have correct lines as the declination shifts over the next decades, but I don't expect to be using that chart in 20 years.

 

[JohnAbercrombie]

But the aviation supply store is a great idea - I'd been limiting my search to marine supply stores!

I've ordered protractors (for a class I taught) from AVworld (link above) and got good service from them.
Before that I had bought them from the UVic bookstore - they used to stock them for students doing field work.
Marine stores...been there, done that, but $40 for the Weems and Plath logo is a bit steep, as you say!

 

[JohnAbercrombie]

drawn pencil lines on the charts along the magnetic N-S lines

Even though I date from 'the time of the ancients' I no longer own a 'proper' drafting table equipped with a drafting machine and protractor head, so looking at my chart pile and thinking of that task usually inspires me to find more interesting projects!
But it is a good idea IMO mainly because it requires less thought when one may be cold/tired/wet .....and the GPS has failed!