New Member - Baseplate Compass

[TwoBlocked]

Hi Folks:

I am looking at compasses and am afraid I am turning into a bit of a gear head. Having a marine navigation background I am most comfortable with a compass with a compass card rather than a compass needle. I will surely pick up a Cammenga Lensatic. But a baseplate sighting compass with a compass card would be super. The only ones are the R&K Dakar which does not have a jeweled pivot, and the R&K Alpin Pro which I am returning due to +/- 4 degree needle drag and the pivot point not being concentric with the azimuth ring causing multiple problems, so no thank you R&K! Cammegna makes a baseplate compass (Destinate) with their standard compass card and index line, but no sighting device.

The few compasses with compass cards instead of needles have reviews about how they cannot be use as a protractor because there are no parallel indexing lines within the rotating azimuth ring. Well, yes and no. And this is one of the reasons for this post.

The typical way to measure a bearing on a map is to either use a drafting device, such as a protractor or a baseplate compass, or to use a compass itself. To use the compass itself, the map must be oriented. You can either orient the map to true north, and correct the bearings for magnetic declination, or orient it to magnetic north and use the magnetic bearings directly.

But what if you set the adjustable azimuth ring or index line on the compass to read either true or magnetic north when the baseplate is aligned with a meridian regardless of how the map might be orientated. You would then be able to plot and measure bearings just as if the map was oriented by using the reading of the magnetic compass as indicated by the azimuth ring or index line. Using a compass in this manner would do away with the need to use it either as a protractor or to precisely orient the map. Let me give a couple examples.

Assume you have a modern lensatic military compass, the kind with a straight edge, and you are only concerned with true bearings. Lay your map out on whatever is convenient without regard to orientation. Place the compass on the map with the straight edge on a meridian. Turn the index line to magnetic north on the compass. You can now use the compass as a poor-man's parallel motion protractor. You can measure bearings or plot bearings by simply using the reading of the index line on the compass card.

With an additional correction, this technique could also be used with a typical baseplate compass with a compass needle and azimuth ring. After aligning the baseplate with a meridian turn the azimuth ring to align 0 degrees with the compass needle. Then when measuring or plotting bearings, subtract the bearing indicated by the compass needle on the azimuth ring from 360 degrees. Obviously, this would be a pain and prone to error!

Using this technique, of orienting the index line instead of the map, can correct for magnetic or even grid north. Simply apply the declination correction when setting the index line. Rather than setting the index line according to 0 degrees on the compass card, set it to what the compass card would read when pointed true north (east is least, west is best). So if the magnetic declination is 12 degrees west, set the index line to 012 degrees magnetic.

I had not read of this technique anywhere and thought this Forum would be a good place to mention it. I look forward to comments from you fine folks. It may be a technique that would only appeal to few. As I said, I have an extensive marine navigation background.

But also, does anyone know of any base plate sighting compasses that have a compass card rather than a compass needle - other than the R&K Dakar or R&K Alpin Pro?

[Wldrns]

Originally Posted by TwoBlocked

The typical way to measure a bearing on a map is to either use a drafting device, such as a protractor or a baseplate compass, or to use a compass itself. To use the compass itself, the map must be oriented. You can either orient the map to true north, and correct the bearings for magnetic declination, or orient it to magnetic north and use the magnetic bearings directly.

It is not necessary to orient the map when using a baseplate compass to measure azimuths/bearings. You can if you wish, but don't have to. It may be helpful to take the extra time to orient the map to true earth if you have visible objects in the terrain, but that requires a flat surface and taking care that no magnetic materials are nearby (such as not on the hood of a car or a table with metal supports). That is only one of several viable methods of using a compass with a map.

A compass is basically a protractor, one that "remembers" the measurement that is set under the index line. Angles are measured from some defined reference line, usually either magnetic north or true north. Most topographic maps printed in recent years will have UTM grid squares printed on them. In most cases, depending on where located within the particular grid zone, the "grid declination", is less than 2 degrees from true north (check the declination diagram to find out how much). When measuring an azimuth on a map with a modern baseplate compass it is easy to simply lay the long edge of the compass along the desired course line, then turn the bezel so that the bezel's meridian orienting lines end up parallel with the map's grid north lines. The grid azimuth will then be under the compass index mark. If your compass is of the type that has been properly adjusted for declination (the combination of magnetic and grid declination), you are done. Pick up the compass, hold it properly with the direction of travel pointing straight out of your chest, rotate your body to "box the needle", and go. NOTE: When measuring azimuths with this protractor method the map may be in any random orientation and you must IGNORE the magnetic needle whenever the compass is on the map. You can even use this method with the map on the hood of your car. Just step away when you pick up the compass.

If your compass is not adjusted for declination, simply look at the declination diagram (or draw one if it is not printed on the map). Ask yourself: "is the desired azimuth measured from magnetic north greater or lesser than the measurement from true/grid north?" Then apply that correction to the azimuth under the index line. In more than 40 years of teaching navigation (9 years in the air, 31 years on land), even I get confused when I hear the silly "east is least west is best" mnemonic phrase. Sometimes you will need to convert from magnetic to true, other times you convert from true to magnetic. The mnemonic gets confusing unless you understand what it really means. Understanding the declination diagram with a visual graphic leaves no question in how to apply the correction.

Many of my older topographic maps do not have the grid overlay printed on them. For those I will often draw magnetic north lines on them during my map study session at home. I use a real protractor, the larger the better. Then with a long ruler or yardstick I simply draw parallel lines spaced the width of the ruler across at the map at the magnetic north angle. It only takes a few minutes. After accurately drawing the first line with a protractor you can advance the ruler from one line to the next by eye and stay very accurate. Then measure azimuths with compass on the map using these reference lines as in the method I suggest above. The declination is automatically compensated, the compass will read the correct magnetic heading. Pick up the compass and go.

Originally Posted by TwoBlocked

Assume you have a modern lensatic military compass, the kind with a straight edge, and you are only concerned with true bearings. Lay your map out on whatever is convenient without regard to orientation. Place the compass on the map with the straight edge on a meridian. Turn the index line to magnetic north on the compass. You can now use the compass as a poor-man's parallel motion protractor. You can measure bearings or plot bearings by simply using the reading of the index line on the compass card.

Sure, that works as you suggest with a magnetic carded compass. It saves the step of orienting the map true to earth, but otherwise is not much different from that method. You are still using the magnetic needle to define a magnetic reference line, which requires the map to remain stationary and stable in whatever orientation it is in throughout the measurement, (could be an issue in a drifting canoe) and to be free of other magnetic influences.

Originally Posted by TwoBlocked

With an additional correction, this technique could also be used with a typical baseplate compass with a compass needle and azimuth ring. After aligning the baseplate with a meridian turn the azimuth ring to align 0 degrees with the compass needle. Then when measuring or plotting bearings, subtract the bearing indicated by the compass needle on the azimuth ring from 360 degrees. Obviously, this would be a pain and prone to error!

Seems unnecessarily complicated and very prone to error. No reason to do this, use the direct method instead.

Originally Posted by TwoBlocked

But also, does anyone know of any base plate sighting compasses that have a compass card rather than a compass needle - other than the R&K Dakar or R&K Alpin Pro?

Even though the military seems stuck on using a card style compass (very heavy and rugged), most people have migrated to the baseplate orienteering style compass. A two-day Land Navigation course is currently being taught several times a year to law enforcement and SAR personnel at the NY State Preparedness Training Center in Oriskany NY. The number of classes has been doubled since the Dannemora prison escape incident. The course was originally developed from techniques taught in the Army Land Nav field manual, but SPTC students are taught with orienteering baseplate style compasses and encouraged to purchase their own if they don't already have one of that type.

I have more than 20 orienteering compasses, mirrored, unmirrored, baseplate, and military carded. Some are basic student compasses, some are declination adjustable. My favorite by far is the Suunto M-3D. i have 10 of those.

[TwoBlocked]

Thanks, Wldrns, I hadn't thought about the senario with a drifting canoe. I guess the same would apply to hang gliding.

Over 20 compasses if quite a few. Do you know of any baseplate sighting compasses with a compass card instead of a needle?

The full mnemonic phrase I like is:

Compass best error west,
compass least error east.
If that makes it worse,
you did the reverse.
If you are still lost,
call the boss.

[TwoBlocked]

I found what I was looking for! The Silva Expedition 54. Has to be shipped from overseas, though. Silva no longer makes them, so it must be old stock. Gonna take about a month, hope it's worth it.

[wiiawiwb]

Suunto no longer lists the M3-D model. Has it been replaced by the M-3NH (Northern hemisphere)?

Also, I've heard some people comment that the M-3G (Global) needle is a little smoother floating. Is there any truth to that?

[Wldrns]

Originally Posted by wiiawiwb

Suunto no longer lists the M3-D model. Has it been replaced by the M-3NH (Northern hemisphere)?

Also, I've heard some people comment that the M-3D (Global) needle is a little smoother floating. Is there any truth to that?

I recently did a search for the M3-D to add to my personal and training collection after I gave one away during a visit to a cousin in the Carpathians. Still listed on Amazon, and REI, but unavailable. The Compass Store lists it for $29, but at check out it is also unavailable. I did just order a M-3DL from Everestgear.com for $32.85, along with a non-dec-adjustable A-10 as another light stowable backup/loaner.

I still have more than a dozen M-3D compass, plus another dozen or more similar other baseplate style compasses. I did note the change in model number. You can still find the M-3D/L on Amazon for as much as $67. That's nearly twice what I paid for each in my collection, usually available for around $35 prior to this season.

I do have one M-3G global version compasses that I bought for travel to Australia. I do not notice that it operates any differently or any smoother than any other M-3 model. It is recognizable by a larger center pivot for global balance to counter dip, and an orange north end of the needle (instead of red). In the past it has always been more expensive than the M-3D, but now I see the price has dropped to $38 on the Suunto web site.

I do not like that in the new M-3NH version they have covered up most of the lower half of the orienteering arrow with "Suunto".

[wiiawiwb]

Thanks for your excellent thoughts.

When I went to the Suunto website, I also noticed that almost every of their baseplate compasses had the "SUUNTO" stamp which blocks out lower orienteering lines. They should have found another place on the baseplate for that.