Why does 4DOF want altitude?

[Antares]

Forgive me if this is a dumb question. I'm pretty new to ballistic solvers. In the past, all I ever did was take data of the box of ammo, dump it into JBM, and make trajectory cards to tape to the stocks of my hunting rifles. Now I'm fooling around with 4DOF and truing it to my actual DOPE.

Anyways, why does 4DOF want station pressure and altitude? I thought solvers just used altitude to translate baro pressure into station pressure. If you're already feeding 4DOF station pressure, what's it doing with the altitude data?

 

[CRT2]

Does this help? From Hornady: NOTE: It is necessary with the Hornady 4DOF® Ballistic Calculator to specify the altitude and atmospheric conditions (either known from a pressure at altitude table or attained from a Kestrel™ or similar device) at the firing location in order for the calculator to accurately predict trajectories. Rather than assuming a fixed atmospheric condition, the calculator calculates an altitude-based atmosphere dependent on the input conditions and allows for the atmosphere to change in air density as the projectile climbs or descends along its line of flight.

 

[Antares]

Yeah, that does help (I think).

I read that as, the software is correcting for changes in atmospheric pressure along the flight path, which becomes more influential the longer and higher angle the shot. Since the relationship between atmospheric pressure and altitude in nonlinear, the software wants to know the altitude of your muzzle so it can accurately place you on the pressure/altitude curve.

Something like that?

 

[CRT2]

Recommend you contact Hornady and ask them since it is their application and code.
Phone: 1-800-338-3220 or
Phone: 1-308-382-1390

 

[Pbgt]

I'm gonna call an Bytch, hornady asked for My calibur and even bullet weight...it's none of their business, just give me the friggin drop already..

 

[Antares]

Right!? They even wanted my MV! Mind your damn beeswax!

 

[WilburW]

Interestingly, I believe Applied Ballistics recommends the opposite. In their instructions, you either use barometric pressure, compensated for altitude; or (preferably) you only enter station pressure at your location.
I‘m going from memory on this, so I may not be exactly accurate.

I‘m confused as to why the Hornady software would need both altitude and station pressure? With actual, current station pressure at your location (from a Kestrel, for example), I thought you don’t need to compensate for altitude?

 

[Antares]

Yeah, that was my initial question. Below is my guess based on the info @CRT2 pulled of the Hornady website.

Yeah, that does help (I think).

I read that as, the software is correcting for changes in atmospheric pressure along the flight path, which becomes more influential the longer and higher angle the shot. Since the relationship between atmospheric pressure and altitude in nonlinear, the software wants to know the altitude of your muzzle so it can accurately place you on the pressure/altitude curve.

Something like that?

 

[LastShot300]

Doesn't make any sense to ask for Stat and Alt, unless what they call Stat is actually Baro

 

[Antares]

I think it's using altitude to calculate the change in atmospheric pressure along the flight path.

Even though the app knows your station pressure at the gun, it can't calculate station pressure at the target without know the altitude because the relationship between altitude and pressure isn't constant. Think of a long, high-angle shot where the target it 2,000 vertical feet below you. The change in pressure between the gun and the target will be different if your shooting from 3,000ft to 1,000ft in elevation vs. 18,000ft to 16,000ft. It's still 2,000 vertical feet in both cases, but the altitude matters.

 

[LastShot300]

I think it's using altitude to calculate the change in atmospheric pressure along the flight path.

Even though the app knows your station pressure at the gun, it can't calculate station pressure at the target without know the altitude because the relationship between altitude and pressure isn't constant. Think of a long, high-angle shot where the target it 2,000 vertical feet below you. The change in pressure between the gun and the target will be different if your shooting from 3,000ft to 1,000ft in elevation vs. 18,000ft to 16,000ft. It's still 2,000 vertical feet in both cases, but the altitude matters.

To do that, you only need the incline, not the Stat at your target location, since the Pressure gradient is easily calculated from the incline.

 

[Antares]

That would work if the pressure gradient was linear but it's not.

 

[TripleBull]

Density altitude - Wikipedia

en.wikipedia.org

.

You'd think Hornady would just take the info from my phone and extrapolate my sphincter tension and flatulence coefficient.

 

[Antares]

What version are you running? The latest update has a place to manually input your sphincter tension (ST) and flatulence coefficient (FC) parameters...

What are you wanting me to get from the wiki DA article?

 

[LastShot300]

That would work if the pressure gradient was linear but it's not.

It was never said it is linear, a gradient is what it is