Help me make sure I'm reading this right, RE: JBM ballistics

[TheGerman]

For the first time since I've been using it, I noticed something weird in the line item descriptions for something on JBM and wanted to make sure I was actually looking at it the right way. For reference, its the descriptions you can see when clicking the ' ? ' behind any of the individual line items.

I'm specifically looking at the Range Card - Trajectory card and am a bit confused on how they are getting density altitude from this screen.

Everything is pretty straightforward until you get about 3/4 of the way down to these line items:

Zero Range (1 to 4000 yd) [100] [?]
Zero Height (-100.00 to 100.00 in) [0.0] [?]

Zero Offset (-100.00 to 100.00 in) [0.0] [?]

Zero Altitude (-4000.0 to 15000.0 ft) [0.0] [?]

Zero Temperature (-40.0 to 140.0 °F) [59.0] [?]

Zero Wind Speed (0.0 to 100.0 mph) [0.0] [?]

Zero Wind Angle (0 to 360 deg) [90.0] [?]


Zero range, offset, temp, wind make sense. Zero altitude is the beginning of the confusion. When clicking on the ' ? ' JBM lists Zero Altitude as "The altitude at which the firearm was zeroed. This value is used to find the atmospheric density."

Then, at the bottom, there is a line item to figure out which altitudes you want the range card for. JBM describes this as "A comma separated list of altitudes. These altitudes are used to calculate the standard density at the entered altitude. You should enter altitude densities (in feet or meters) here. It is important to understand that this is the only value used to find the density of the atmosphere. The program assumes that whatever instrument provides the density altitude measurement takes into account all necessary variables. Temperature is not used to calculate the density since it is already taken into account in the density altitude.

So 2 questions:

- Is JBM using some sort of 'generic' numbers for Temp, Pressure, Humidity and just putting this against my actual sea level altitude? If so, does Zero Altitude actually just mean my above sea level altitude?

- On the bottom where I can put what altitude outputs I want, 2000,4000,6000, etc, is this output just meant as sea level altitude (which would be kind of useless) or as actual density altitude?

 

[_Windrider_]

It seems to me it's just reverse engineering the formula to get the station pressure based on the temp altitude and humidity you input. Maybe it's assuming you don't have a device that's capable of measuring that (also since In That section it doesn't ask for pressure it seems that's what it's doing).

As as for the altitudes on the range card, it Looks like you find your MSL and current temp (which would roughly be DA) and go down.

Seems counterintuitive. I could be way off though.

 

[Deleted member 10043]

- Is JBM using some sort of 'generic' numbers for Temp, Pressure, Humidity and just putting this against my actual sea level altitude? If so, does Zero Altitude actually just mean my above sea level altitude?

Zero altitude is the altitude you zeroed the rifle at. the calculator uses this as a base for subsequent altitudes yet to be determined.

- On the bottom where I can put what altitude outputs I want, 2000,4000,6000, etc, is this output just meant as sea level altitude (which would be kind of useless) or as actual density altitude?

Determined altitudes you anticipate shooting.

Put a link to the calculator in your original post.

 

[TheGerman]

- Is JBM using some sort of 'generic' numbers for Temp, Pressure, Humidity and just putting this against my actual sea level altitude? If so, does Zero Altitude actually just mean my above sea level altitude?

Zero altitude is the altitude you zeroed the rifle at. the calculator uses this as a base for subsequent altitudes yet to be determined.

- On the bottom where I can put what altitude outputs I want, 2000,4000,6000, etc, is this output just meant as sea level altitude (which would be kind of useless) or as actual density altitude?

Determined altitudes you anticipate shooting.

Put a link to the calculator in your original post.

For the zero altitude, I left it at zero for right now. The question is, by JBM's description, this would be your actual above/below sea level altitude and NOT the density altitude. Is that correct? It just confuses me because normally there are other input lines for humidity, pressure, etc plus the sea level altitude in order to figure density altitude.

The second part about determining the altitudes to shoot at, that's obvious, but is the output on the range card you generate from this in density altitude? It would make sense that it would be because having it simply give it to you for sea level altitude that never changes would be useless, but JBM's descriptions are confusing with these 2 things.

Go to http://www.jbmballistics.com/ballistics/calculators/calculators.shtml , range cards, trajectory card

 

[_Windrider_]

For the zero altitude, I left it at zero for right now. The question is, by JBM's description, this would be your actual above/below sea level altitude and NOT the density altitude. Is that correct? It just confuses me because normally there are other input lines for humidity, pressure, etc plus the sea level altitude in order to figure density altitude.

The second part about determining the altitudes to shoot at, that's obvious, but is the output on the range card you generate from this in density altitude? It would make sense that it would be because having it simply give it to you for sea level altitude that never changes would be useless, but JBM's descriptions are confusing with these 2 things.

Go to http://www.jbmballistics.com/ballistics/calculators/calculators.shtml , range cards, trajectory card

If it was DA it wouldn't have temperature columns. DA is already corrected for temperature.

 

[TheGerman]

Here is what the JBM description for the altitude says on that specific calculator (I think its different per each of their calculators):

Altitudes A comma separated list of altitudes. These altitudes are used to calculate the standard density at the entered altitude. You should enter altitude densities (in feet or meters) here. It is important to understand that this is the only value used to find the density of the atmosphere. The program assumes that whatever instrument provides the density altitude measurement takes into account all necessary variables. Temperature is not used to calculate the density since it is already taken into account in the density altitude.

The way I am reading this, says that the outputs/columns on the range card are density altitude, altitudes, not sea level altitudes. The temperate sub columns per density altitude are there because JBM doesn't use them for DA, but instead (I'm assuming) uses the change in temperature to adjust your muzzle velocity, hence why you have to reference your current density altitude plus your current temperature range.

Did I just answer my own question?

From what I am gathering, your zero altitude is your sea level altitude for this specific calculator (on their regular trajectory calculator for a single DA its all different) and the altitude on the actual range card it generates are density altitudes based on whatever formula JBM uses via your input for sea level altitude for your zero and your high/low muzzle velocities at two temperatures. Assuming I have a Kestrel that lists density altitude (I do), I should then use whatever temp and DA columns my Kestrel tells me my current conditions are.

 

[_Windrider_]

Here is what the JBM description for the altitude says on that specific calculator (I think its different per each of their calculators):

Altitudes A comma separated list of altitudes. These altitudes are used to calculate the standard density at the entered altitude. You should enter altitude densities (in feet or meters) here. It is important to understand that this is the only value used to find the density of the atmosphere. The program assumes that whatever instrument provides the density altitude measurement takes into account all necessary variables. Temperature is not used to calculate the density since it is already taken into account in the density altitude.

The way I am reading this, says that the outputs/columns on the range card are density altitude, altitudes, not sea level altitudes. The temperate sub columns per density altitude are there because JBM doesn't use them for DA, but instead (I'm assuming) uses the change in temperature to adjust your muzzle velocity, hence why you have to reference your current density altitude plus your current temperature range.

Did I just answer my own question?

From what I am gathering, your zero altitude is your sea level altitude for this specific calculator (on their regular trajectory calculator for a single DA its all different) and the altitude on the actual range card it generates are density altitudes based on whatever formula JBM uses via your input for sea level altitude for your zero and your high/low muzzle velocities at two temperatures. Assuming I have a Kestrel that lists density altitude (I do), I should then use whatever temp and DA columns my Kestrel tells me my current conditions are.

I don't know man but what I do know is if I'm at 1000' MSL at say 67 degrees my DA is X, but if I'm at 1000' MSL at 100 degrees, my DA is Y. That's why I don't think those altitudes are DA. You can't be at 1000 DA at 67 and 100 at the same time at the same MSL altitude. It's impossible.

 

[Deleted member 10043]

Ok, yes actual altitude above sea level without regard to atmospheric variables. Also, in the past I've always just sent the author an email with questions he replies to his email.

 

[TheGerman]

Ok, yes actual altitude above sea level without regard to atmospheric variables. Also, in the past I've always just sent the author an email with questions he replies to his email.

I emailed him yesterday, hope I hear from him.

So the real confusion now is the output columns. When you run the calculator, input the info and then you get the trajectory data that looks something like:

3000 altitude, 60 degrees, 80 degrees

6000 altitude, 60 degrees, 80 degrees,

And so on.....are the altitude numbers sea level numbers (which to me would be totally useless) or DA altitudes?

My goal here was to have one rifle/bullet combo entered and have a separate sheet for whatever DA altitude/temperature readings I got on my Kestrel that day, and always have that for reference.

 

[_Windrider_]

I emailed him yesterday, hope I hear from him.

So the real confusion now is the output columns. When you run the calculator, input the info and then you get the trajectory data that looks something like:

3000 altitude, 60 degrees, 80 degrees

6000 altitude, 60 degrees, 80 degrees,

And so on.....are the altitude numbers sea level numbers (which to me would be totally useless) or DA altitudes?

My goal here was to have one rifle/bullet combo entered and have a separate sheet for whatever DA altitude/temperature readings I got on my Kestrel that day, and always have that for reference.

Mean Sea level (MSL). Then you find the temp (which is actually DA, go down the column to the range.

 

[Deleted member 10043]

I agree.

 

[TheGerman]

So all of the atmospheric variables that dictate density altitude are 'hidden' in the temperature column?

For example, I am right now at 2860 above sea level where I live. if its 80 degrees outside when I'm shooting, I'd use the 3000 (rounding up, unless I make an altitude of 2850, etc) column with the 80 degree sub column and that would give me the trajectory data I'd need for whatever the DA is at my location?

So as long as I know what my actual altitude is where I am shooting, as well the the current temperature, I'd have the data on the card I'd need assuming I have it in the range of data I keep with me?

If thats how it works, seems I got the data I need, just the way its worded and the columns made it confusing.

 

[_Windrider_]

So all of the atmospheric variables that dictate density altitude are 'hidden' in the temperature column?

For example, I am right now at 2860 above sea level where I live. if its 80 degrees outside when I'm shooting, I'd use the 3000 (rounding up, unless I make an altitude of 2850, etc) column with the 80 degree sub column and that would give me the trajectory data I'd need for whatever the DA is at my location?

So as long as I know what my actual altitude is where I am shooting, as well the the current temperature, I'd have the data on the card I'd need assuming I have it in the range of data I keep with me?

If thats how it works, seems I got the data I need, just the way its worded and the columns made it confusing.

That's the way I see it, all DA is barometric pressure corrected for nonstandard temperature. I can try putting all your data in AB Analytics and see if it lines up at all.

 

[TheGerman]

That's the way I see it, all DA is barometric pressure corrected for nonstandard temperature. I can try putting all your data in AB Analytics and see if it lines up at all.

If you could please, I'd appreciate it so we can have this figured out.

I was hoping to have heard from the author, but I'm sure he has other things to do than monitor his email for random ballistics questions. Just wish the descriptions made more sense than 'the temperature isn't used in density altitude because its used in density altitude' lol

 

[Deleted member 10043]

The best way to understand it is take your output from JBM and compare it to your own input with a similar but different app. Range cards are not intended for anything other than basic output. Altitude is actually altitude. Temperature is only input for muzzle velocity.

"The altitude at which the firearm was zeroed. This value is used to find the atmospheric density."

 

[_Windrider_]

If you could please, I'd appreciate it so we can have this figured out.

I was hoping to have heard from the author, but I'm sure he has other things to do than monitor his email for random ballistics questions. Just wish the descriptions made more sense than 'the temperature isn't used in density altitude because its used in density altitude' lol

Ok, here is what I need (AB asks for a lot of extra stuff here because it takes into account coriolis, spin drift and everything else):

Bullet info, MV, zero range, sight height, barrel twist, your heading (direction of fire), latitude. Next ill need Temperature, pressure and humidity (while im here I need all the same for your zero). Now keep in mind AB doesn't use DA either, BUT temp, pressure and humidity is DA. I need the pressures in station pressure (uncorrected). from this we should be able to compare. you can PM if you need to.

 

[Deleted member 10043]

I did an example this morning using Strelok. I used DA to get the same results. If you don't have DA then just use standard altitude above sea level.

 

[_Windrider_]

ok, so I spent the morning doing some digging into this. I am not very familiar with JBM Ballsitics, however I think I have figured this out.

The Range Card - Trajectory calculator you are using is used to provide you with a quick reference on the change to your zero. so if you zeroed your rifle at home and say its elevation (MSL) is 1000ft and the temp was 20C, you could use this quick reference card and find the change to your zero if you were now shooting at say 2500' MSL and 23C degrees. IT IS MOST DEFINATELY NOT DA. however, what its doing is reverse engineering it using the standard atmosphere at those altitudes ISA (International standard atmosphere) is 29.92" Hg at 15C at sea level. the standard lapse rates are 1.0"Hg per 1000/ft and 2C/1000ft.

SO in a perfect world if the lapse rate always followed standard, DA would always = MSL. THIS RARELY happens. remember, DA is standard pressure corrected for non standard temp. so in my above example, JBM is using 27.42"Hg (standard pressure at 2500') and 23C (nonstandard temperature) to calculate your zero change. as an aside, 2500ft at 23C = ~4051 Density Altitude (assuming RH=50%)

Culpepper, does this jive with your thoughts as well?

Now the regular trajectory calculator is using DA, only it asks for the exact environmental data (pressure, temp, humidty) that give you DA instead of an altitude.

Its very confusing in my opinion. what JBM should say instead of altitude, is elevation (for this card only) since it seems one would likely be looking at a topographic map of some kind which is usually referenced as an elevation and not an altitude since you are on the ground. but that's really splitting hairs as the bullet is in the air and not on the ground. But that makes more sense in my mind for this particular function of JBM.

 

[Deleted member 10043]

Nah, it is not that intense. The inputs have variables such as high and low MV based on temperature and zero altitude to use as a base. The other altitude variables only need to be the same type of altitude used for zero altitude to be consistent. Also, the temperature variables for each altitude is only used for MV purposes. I used DA to back into the JBM output. But you may see a .1 mil difference at 100 yards at the other altitudes. That will correspond up and down the line. This is just a "dope" card. It is better than having nothing at all going from one extreme to another. Once you are at the different altitude you still need to verify it and make the necessary adjustments.

 

[_Windrider_]

Nah, it is not that intense. The inputs have variables such as high and low MV based on temperature and zero altitude to use as a base. The other altitude variables only need to be the same type of altitude used for zero altitude to be consistent. Also, the temperature variables for each altitude is only used for MV purposes. I used DA to back into the JBM output. But you may see a .1 mil difference at 100 yards at the other altitudes. That will correspond up and down the line. This is just a "dope" card. It is better than having nothing at all going from one extreme to another. Once you are at the different altitude you still need to verify it and make the necessary adjustments.

we are on the same page. All this table does is back into DA and give you a shift based on your change in MV due to alt and temp.

 

[TheGerman]

So JBM uses a generic variable for pressure and applying it at its generic variable/value for each altitude, which doesn't always happen. I figured as much when the DA was built into the temperature line and not a specific DA column; the other thing that confused me was that for this specific card, JBM didn't allow you to enter pressure, humidity, etc separately like it does for the basic trajectory card that is only built for one specific set of atmospheric conditions (whatever you enter at that time).

The wording and description on JBM is what threw me off as well.

So am I right to think this range card would be way off for me; for example, I'm at 2860 elevation where I shoot. From my Kestrel (station pressure), just on the 2 strings I recorded and used for high/low muzzle velocity my DA is 5251 @ 89 degrees and 3676 @ 60 degrees. I get a huge swing here throughout the year as it can be as cool as in the 40s and 120+ in the summer. From just the 2 muzzle velocity DA amounts, compared to the 2860 elevation I can see there being a huge issue if I'm understanding that JBM's model is DA=MSL

 

[_Windrider_]

So JBM uses a generic variable for pressure and applying it at its generic variable/value for each altitude, which doesn't always happen. I figured as much when the DA was built into the temperature line and not a specific DA column; the other thing that confused me was that for this specific card, JBM didn't allow you to enter pressure, humidity, etc separately like it does for the basic trajectory card that is only built for one specific set of atmospheric conditions (whatever you enter at that time).

The wording and description on JBM is what threw me off as well.

So am I right to think this range card would be way off for me; for example, I'm at 2860 elevation where I shoot. From my Kestrel (station pressure), just on the 2 strings I recorded and used for high/low muzzle velocity my DA is 5251 @ 89 degrees and 3676 @ 60 degrees. I get a huge swing here throughout the year as it can be as cool as in the 40s and 120+ in the summer. From just the 2 muzzle velocity DA amounts, compared to the 2860 elevation I can see there being a huge issue if I'm understanding that JBM's model is DA=MSL

No it's not saying DA is MSL. You simply enter the table at your elevation and current temperature and go down to the range to get the difference in drop.

But we scientifically know what standard pressure is at any elevation. So by using that and adjusting for temp it's using DA in the background (approx since humidity isn't considered) to determine that MV change and drop difference. Does that make sense?

 

[Deleted member 10043]

TheGerman, be sure and not miss this part at the top of the input data fields.

"Calculate a trajectory card from bullet, velocity, firearm and atmospheric parameters. Density is calculated using input density altitudes. Muzzle velocity is found by using linear interpolation between two velocities measured as a fuction of temperature."

I discovered a typo on his page...

Anyway, if you are shooting under 800 yards the data output will be close if your zero related and MV/temp input is good.

 

[TheGerman]

TheGerman, be sure and not miss this part at the top of the input data fields.

"Calculate a trajectory card from bullet, velocity, firearm and atmospheric parameters. Density is calculated using input density altitudes. Muzzle velocity is found by using linear interpolation between two velocities measured as a fuction of temperature."

I discovered a typo on his page...

Anyway, if you are shooting under 800 yards the data output will be close if your zero related and MV/temp input is good.

lol the more I read stuff the more I'm confused on JBM's wording.

Dillhole and I are going to compare his data from AB using all of my specific data and variables. I'm curious to see what happens as far as how close/far away they are.

The <800 yards isn't too big an issue, its the 1k+ stuff I don't need things adding .1MIL here, .1MIL there, etc until you end up missing by 11 feet because of it. First round is the best round.

May be time to just get AB since I literally have everything else and have been sandbagging on a ballistics app.

 

[Deleted member 10043]

Get a ballistic app would be so much better.

 

[_Windrider_]

Get a ballistic app would be so much better.

I think that's the conclusion he came too. JBM is a good resource if u have nothing else but with apps like shooter being like 9.99 or AB at 19.99(?) why wouldn't you.

 

[pell1203]

TRASOL, currently at $9.99 in the App Store is a pretty good deal. Did notice the last update was early October of of 2016 and I'm wondering if Desert Tech is going to continue updating/supporting it. User Interface is really good!

 

[JCCinOhio]

http://www.arcanamavens.com/LBSFiles/Shooting/Downloads/DA/

 

[TheGerman]

http://www.arcanamavens.com/LBSFiles/Shooting/Downloads/DA/

Actually emailed Lindy 2 days ago about this. He responded and said the columns are actually DA; also suggested a ballistics app.

But its great to see him promptly return a random email about a page that's 7+ years old, and take the time to go over it with you.

 

[TheGerman]

Yeah I know, I know, ballistics app.

I've just been using JBM for most things and logging data/updating data for a long time. Apps are now way more prevalent, and while I see that they are useful, I always wanted to make sure I had a hard copy of what I needed and worked off of, as well as did not rely solely on something electronic for the reason that if your battery went to shit you were in trouble, and that you shouldn't just use electronics because you learn very little of the how/why/what. This is why when I show people how to shoot or spot, they learn to reticle range for weeks before even touching the LRF; ironically they get really good at hashing out target sizes to that last .1MIL this way. Of coarse we're going to range with the LRF when we need to, but don't rely on it and don't have a dumb look on your face as to what you need to do if the LRF all of a sudden stops working.

The other thing is, where I shoot, I'm going to need to buy a damn Trimble with a ballistics app, so it was always in the back of my head that its going to cost me to finally make the jump lol