Thursday, November 24, 2022

Shooting Azimuths

Not actually Erin.
& is used with permission.
Two years ago, David Blackard wrote a post about how to determine the distance to an object in the field without using a rangefinder. This involved knowing the distance and angle to another object and then using a calculator to do some complicated math. 
  1. In the drawing, aligning the start point with the target and then laying out a 90 degree angle for the base is important. 
  2. Measure as accurately as possible along that 90° angle to establish the base, shown as X.
  3. Here is where being able to get good angle measurements is important, since this will give you the vital number that plugs into your distance equation.

This is all very well and good, but in the un-edited version of his post he talked about using a protractor to find that angle. My immediate thought was "Why not just shoot an azimuth using a lensatic compass?" Well, it turns out that David had never heard of azimuths or lensatic compasses at that point, so I sent him some links and considered the matter solved. 

As it turns out the matter is not solved, because today David posted this:

In the comments, a friend said "I have no idea what you're talking about either, but I'd love to find out." I had a bit of time, so I started briefly explaining, and by the time I was done I had a good start on a blog post, which I figured I might as well finish here. 

What is an Azimuth?
Azimuths are used in astronomy and celestial navigation (which is just astronomy with more math) in addition to land navigation, and most explanations will give a complicated answer about three dimensions. Land navigation only uses two dimensions so a simpler application gets a simpler definition. 

Put simply, an azimuth is a compass bearing. Due north is 0°, so east is 90°,  south is 180°, and west is an azimuth of  270°. You get the idea: an azimuth is the angle between north and wherever you're looking.

What is a Lensatic Compass?
You've likely seen a lensatic compass before even if you don't know the name, as it has a very distinctive shape. Rather than describe it, here is a picture with the parts labeled. 

Land Navigation Module 2 Using a Lensatic Compass.

A lensatic compass has a useful feature where you can look through it to precisely align it with a terrain feature, then glance down to see what your azimuth to that feature is. 

Why a terrain feature? Because if you navigate towards a terrain feature instead of a compass bearing, you walk faster and safer, because your eyes are looking at where you're going rather than buried in the compass. Just be sure to check your compass every 100 yards or so to make sure you're still headed in the right direction. 

How to Shoot an Azimuth
Using a compass to find an azimuth is called "shooting", because if you do it properly you hold the compass like a firearm and aim at your target terrain feature. 

There are other ways to do it, but this method, known as the "cheek hold", is the most precise.
  1. Put your thumb through the loop with your forefinger pointing alongside the compass, as shown in the picture.  
  2. Bring your hand up to your cheek with the base of your thumb in the crease between your cheekbone and upper lip. 
  3. Look through the window in the compass cover and find your target terrain feature
  4. Line up the notch (rear sight) with the wire (front sight) and center the feature in the sights the same way you'd aim a gun.
  5. Glance down through the magnifying lens to find your azimuth.

In this example the compass reads 320°, so that is your azimuth. If you want to make your life easier, rotate the bezel ring so that the luminous line is aligned with your compass arrow. With that done, you can quickly check your bearing without having to shoot a new azimuth by aligning the arrow with the line and seeing if the fixed black line is still pointing at your target. If it is, you keep walking. If it's not, something has gone wrong and you need to shoot a new azimuth. 

Back Azimuths
If you need to go back the way you came, you don't need to shoot a new azimuth. Instead, you can calculate a back azimuth:
  • If the original azimuth is less than 180°, you add 180° to get a back azimuth. 
  • If the original azimuth is greater than 180°, you subtract 180° from the original azimuth. 
In the example above, the original azimuth is 320°, which is greater than 180°. Its back azimuth is therefore 320 - 180 = 140°. 

With a quality compass and the knowledge of azimuths, you can make very precise measurements in the field. Combined with the formula above and a calculator with the TAN function, you can quickly determine the distance to an unknown point. 

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