Manually Calculate Distance To Target

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Manually Calculate Distance To Target

Postby MarcSpaz » Thu, 29 Nov 2018 01:59:22

I have recently started preparation for doing mid to long range shooting, 1,200+ yards. The facility I started going to has a range that is maxed out at 1,250 yards, but has a huge field of known and unknown size and distance targets.

Well, a big part of the sport for me is to be able to hit an unknown sized target at an unknown distance, the very first shot. So, how the heck do you do that??? It's easy... figure out how far away the target is! LOL

I came across some well known formulas to do this. The reality is, unless you are shooting at a random rock out the the middle of the woods or flat lands... there is a really good possibility that there are going to be some things around that are "known size". We can use that to our advantage.

The first step is, know your target and what will be around your target. If you are going hunting for deer, coyote, boar, bear, etc... adult average sizes are all available.

Here are some examples:
Buck = 42" (106cm) shoulder height
Boar = 32" (81cm) shoulder height
Coyote = 18" (46cm) shoulder height

Most common rifle scopes measure angles via Milliradian (MRADs or Mils) or Minute of Angle (MOA). Before we get too far into this, There are a few misconceptions that need to be clarified. 1.) Milliradians is neither metric nor standard. It is its own unit of measure. However, Milliradians does work well with the metric system in the math world. 2.) Minute of Angle is neither metric nor standard. It is its own unit of measure. However, Minute of Angle does work well with the standard system in the math world.

Now, the fun part. Lets calculate some distances. As mentioned above, for people who think in metric, Mils is a great scope reticle to use for ranging targets. The formula is:

Target Height (cm) / Target Height (in mils) x 10 = Distance to Target in meters.

So, lets say you have an adult deer in your sights. We know the deer is about 106cm high. So, you look at the deer through your scope, placing the reticle over the deer in such a way that you can see how many mils the deer is, from his shoulder to the ground. In this example, we'll say the buck is 11 mils tall.

So, based on our new info, we can calculate the range to the target.
Target Height (106cm) / Target Height (11 mils) x 10 = Distance to Target in meters.
106 / 11 x 10 = 96.4 meters

Now, lets assume the buck measured 5 mils, then the math would be...
106 / 5 x 10 = 212 meters

If you have a 100 meter zero, you know what the range is and now you can either adjust your dial or holdover based on your ballistic performance.



Now that we have covered Mils, lets look at the ranging math for MOA. The formula and method are very similar to what we discussed about mils. The formula is:
Target Height (inches) / Target Height (MOA) x 100 = Distance to Target in Yards.

So, making the same assumptions, but using the standard system, we would see something like this.
Target Height (42 inches) / Target Height (11 MOA) x 100 = Distance to Target in Yards.
42 / 11 x 100 = 382 yards.

If you have a 100 yard zero, you know what the range is and now you can either adjust your dial or holdover based on your ballistic performance.



How does this help me at the range with a bunch of targets at unknown distance? Good question.
I bought a scope with an MRAD / Mils reticle, but I think in standard. So I need to do a bunch of extra math.

There are "waist up" human silhouettes for targets that are "to scale". That means that the targets are about 34 inches tall, a known size. When I look at the target in my reticle, it is 2 mils tall. So here is my math to adjust my point of aim or holdover.

34 in = 86 cm / 2 x 10 = 430 meters = 470 yards.

I have a 100 yard zero and we computed the distance in yards. Now I have a very important piece of knowledge to use my ballistic data to calculate my dial or holdover.

How sensitive the math is, all revolves around the size of the target, distance and your ballistic performance.

Here is an example. I am using a 6.5mm round that has decent ballistic characteristics. Generally speaking, my drop from a 100 yard zero would be 39.2" at 470 yards. Lets assume I guessed wrong and the target is really 30 inches. Well, that puts the target at 416 yards, not 470. That gives me a drop of 27.8", not 39.2". That is a difference of 11.4 inches over my point of aim.

On a 30" steel target, that is still a hit. However, the body of an adult buck is about 21". So, I missed the deer. :-(

I hope this info is useful to you and you can have some fun at the range or improve your odds of landing those steaks you've been craving.


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Re: Manually Calculate Distance To Target

Postby jdonovan » Thu, 29 Nov 2018 08:24:04

Most common rifle scopes measure angles via

Watch out for that... many scopes that claim to be MOA, are really IPHY Inch per hundred yards. Take a look at the turrets if they read 1/4 or 1/8 per click, they are most likely IPHY. Fairly common in hunting scopes, and at the sub 300 yds they are generally used the difference between MOA and IPHY is less than shooter error.

I bought a scope with an MRAD / Mils reticle, but I think in standard. So I need to do a bunch of extra math.


You just have to do the math in one set of units vs two. If you have a mil scope work in mils exclusively. Mil @ distance is a linear unit measurement, but because its on both sides of the known/unknown formula it will zero itself out, and we can basically work exclusively in angular measurements or mil.

Take the height of known objects, and convert it to mils @ a known range.
now measure the object in mil at the unknown range.
divide, and get the ratio, multiply that by the size at the known range = unknown range.

my 36" steel plate is 10 mil @ 100 yds.
my unknown distance 36" steel plate measures 2.5 mil
10/2.5 = 4
4* 100 = 400 yds. Send it!

Because mil is a base 10 system, vs English disances being base 12, you can have even more simplicity if you choose to work exclusively in the metric system.

ie something that is 3 feet tall at 230% the distance is hard in your head math, but something that is 1 meter (100 cm) @ 230% is very easily, 2.3m or 230 cm.
so did you figure out what 3' @ 230% was?

So now that you've seen that, if you have a miss, and need to send another round, do you want your spotter to call corrections in feet/inch, or would you prefer mil? I know I prefer a straight mil call, because I can then use the information directly, and either dial, or hold the adjustment. Also if we work exclusively in mil, then there is no conversion, and that eliminates a source of error between the team. This is why all my spotting scopes all have mil reticles.

side bar... surveyors realized this a long time ago, and they generally work in feet and tenths. Took me a while to embrace that when I learned how to do basic surveying, but once you do, you realize that adding and subtracting inches over a perimeter with 100's of points is not the easy way to do it.

Once you learn to embrace metric (or any base 10 system) and start doing math in it, you'll never be happy to go back to English, non base 10 units.

Another source of error to consider is how accurate of an measurement can you make with your reticle? Can you really measure to .1 mil reliably, or is .25, or .5 or even 1.0 a more likely value. Once you know what your measurement error is, then you can figure out: average group size + measurement error, + environmental factors. That gives you your CEP (Circular error probability). Compare that to target size and distance, and you have an idea if you should even take the shot.


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Re: Manually Calculate Distance To Target

Postby MarcSpaz » Thu, 29 Nov 2018 21:36:05

jdonovan wrote:Watch out for that... many scopes that claim to be MOA, are really IPHY Inch per hundred yards. Take a look at the turrets if they read 1/4 or 1/8 per click, they are most likely IPHY. Fairly common in hunting scopes, and at the sub 300 yds they are generally used the difference between MOA and IPHY is less than shooter error.


I always heard IPHY was a rough rule of thumb. I didn't realize they were making scopes to that scale.

jdonovan wrote:You just have to do the math in one set of units vs two. If you have a mil scope work in mils exclusively. Mil @ distance is a linear unit measurement, but because its on both sides of the known/unknown formula it will zero itself out, and we can basically work exclusively in angular measurements or mil.

Take the height of known objects, and convert it to mils @ a known range.
now measure the object in mil at the unknown range.
divide, and get the ratio, multiply that by the size at the known range = unknown range.

my 36" steel plate is 10 mil @ 100 yds.
my unknown distance 36" steel plate measures 2.5 mil
10/2.5 = 4
4* 100 = 400 yds. Send it!


That is an awesome concept. I like it! I'm going to see about adopting that into my toolbox. Way easier math.

jdonovan wrote:Because mil is a base 10 system, vs English disances being base 12, you can have even more simplicity if you choose to work exclusively in the metric system.

ie something that is 3 feet tall at 230% the distance is hard in your head math, but something that is 1 meter (100 cm) @ 230% is very easily, 2.3m or 230 cm.
so did you figure out what 3' @ 230% was?


I have been struggling with the idea of going over to metric 100% for target shooting. I can "see" a mile because its all I have operated in. I can't visualize a kilometer because I just don't use it. The math is very easy, but adapting that to what I see is a challenge.

I really need to give it a go. I guess its like anything else... practice.

jdonovan wrote:So now that you've seen that, if you have a miss, and need to send another round, do you want your spotter to call corrections in feet/inch, or would you prefer mil? I know I prefer a straight mil call, because I can then use the information directly, and either dial, or hold the adjustment. Also if we work exclusively in mil, then there is no conversion, and that eliminates a source of error between the team. This is why all my spotting scopes all have mil reticles.


I think those are valid points. My current spotting scope doesn't have a reticle. I need to get a new one. I like the idea of my spotter calling mils. With the reticle right there, there is no effort to corrections.

jdonovan wrote:side bar... surveyors realized this a long time ago, and they generally work in feet and tenths. Took me a while to embrace that when I learned how to do basic surveying, but once you do, you realize that adding and subtracting inches over a perimeter with 100's of points is not the easy way to do it.

Once you learn to embrace metric (or any base 10 system) and start doing math in it, you'll never be happy to go back to English, non base 10 units.

Another source of error to consider is how accurate of an measurement can you make with your reticle? Can you really measure to .1 mil reliably, or is .25, or .5 or even 1.0 a more likely value. Once you know what your measurement error is, then you can figure out: average group size + measurement error, + environmental factors. That gives you your CEP (Circular error probability). Compare that to target size and distance, and you have an idea if you should even take the shot.


All great feedback. I appreciate it. My scope is mil dot, with half-mil ticks. Its not an exact science, for sure... while I know there is a lot of science involved, I am realizing that long range shooting is indeed an art.


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Re: Manually Calculate Distance To Target

Postby kelu » Sat, 01 Dec 2018 20:53:18

Hi Marc,
What range are you going?
All political power comes from the barrel of a gun. The communist party must command all the guns, that way, no guns can ever be used to command the party - Mao Tse Tung


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Re: Manually Calculate Distance To Target

Postby MarcSpaz » Sat, 01 Dec 2018 21:38:46

kelu wrote:Hi Marc,
What range are you going?


Peacemaker National Training Center.

https://peacemakernational.com/


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