View Single Post
Old 10-25-2015, 07:39 AM   #1
TwoBlocked
Member
 
Join Date: Oct 2015
Posts: 3
New Member - Baseplate Compass

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?
TwoBlocked is offline   Reply With Quote