Lensatic Compass: A Better Protractor

The normal method to transfer angles from your lensatic compass to your map and back is with a protractor and a pencil. From your known location on the map, you take your protractor, line it up to true north, find the angle to your next point, adjust for declination, and then shoot that angle with your compass. At that point you know what direction you need to head to get to your next stop, and if you pair that with an accurate distance you'll be on your way in no time.

The only problem is that this process takes time and tools. You need to write things down, draw lines, perform math, and make sure you do so with a reasonable degree of accuracy. There's a lot of room for error and mistakes, and it takes time, which is sometimes a very precious commodity.

There's a better way, I believe, from both a time and an accuracy standpoint.

Since the ultimate goal is to transfer the angle between the map and compass, why not do it directly? That way you skip the math, the protractor, the lines, and get it all done quicker.

So, take your map and lay it down on a flat surface (i.e. the ground). Take your lensatic compass and open it up all the way so it is one flat line. Lay the compass down on the map so its long side is parallel with the north/south axis of the map. I generally line it up with a north/south gridline.

Next, rotate the entire thing (map and compass) until your compass is telling you that it's pointing north. Now your map's true north is lined up to your local magnetic north, and you've just completely circumvented the whole problem of declination (also removing a potential source of error since declination changes with time).

From here it's simple. All you do is take you compass and line it up from your current point to your desired destination and look at the compass reading. That's the heading that you need to move along to reach your desired end (or way) point. Pick up your map and head out!

3 comments:

gaudio said...

I've read elsewhere that to orient your map and account for declination, you want to line your compass up with the magnetic north lines on the map legend, not the grid lines (which are likely grid north, not true north, right?).

Ranger Rick said...

Your compass is always "accounting" for your declination angle by pointing to magnetic north.

So, if you line your compass up on the N/S grid lines and rotate the map till the compass is pointing north, you are actually building the declination into the orientation.

Make sense?

bloggermouse said...

RR, the practical meaning of this (if I understand what you are saying) is if you are measuring on the map yourself or taking sightings yourself the whole process is self-contained and declination is moot. i.e. The angle between True North and the "True Endpoint" is the same as the angle between Magnetic North and the "Magnetic Endpoint" (so to speak).

The only time it would matter would be if you were using or recording "outside" information: bearings based on true north would need to be declination-adjusted.