Range Report Which Altitude for Ballistic Program?

[RobertHode]

New guy question.

When a ballistic program requests altitude data, should I enter the firing point elev., the target elev., an elev. halfway between the two, or does it matter?

 

[Lindy]

Re: Which Altitude for Ballistic Program?

It depends on your source of barometric pressure data. You only use the altitude if you have no source of local pressure. See:

Barometric Pressure and Ballistic Software

 

[Mag 300]

Re: Which Altitude for Ballistic Program?

Best to use local pressure, anyt flight service can get it to you from local airport or use a Kestral
Bill

 

[azimutha]

Re: Which Altitude for Ballistic Program?

If you only have pressure from the airport (which, when I get it from the weather.gov site is corrected) and you are shooting at a different altitude would it be helpful to enter the difference in altitude into JBM's altitude field? In other words, if my corrected pressure is 29.95 @ 3935' I typically just enter that pressure, put zero (0) in the altitude field and check the "Pressure is Corrected" checkbox. But I shoot 1600 feet higher. At 300 yards this is just .2" difference in drop, but at 600 it's nearly 3".

I understand that:

1) At 600 yards I have worse problems to worry about than a 3" drop

2) Pressure can vary widely/wildly in a small region, particularly one that is mountainous.

So I'm just wondering if this is even a valid way to "iron out" some of the variation in altitude.

 

[Lindy]

Re: Which Altitude for Ballistic Program?

You might wish to read the link I provided above. It answers your question.

As the article notes,

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">If "Pressure is corrected" is checked, the density is calculated using the pressure corrected for whatever altitude is entered in the altitude box. It means that the pressure you entered is from a source which corrected the station pressure to the sea-level-referenced barometric pressure, and the program corrects that pressure for altitude.</div></div>

So, you will need to enter the elevation at which you are shooting.

 

[azimutha]

Re: Which Altitude for Ballistic Program?

Oops. Hold the phone. I really had some kind of moment there. I got that all backwards (even from what I previously posted here: http://www.snipershide.com/forum/ubbthreads.php?ubb=showflat&Number=1531806). Anyway, if I get a corrected pressure of 29.95 @ 3935 feet should I go ahead and use that pressure (if it's the best I have) and enter the shooting altitude (5500)?

 

[Greg Langelius *]

Re: Which Altitude for Ballistic Program?

Firing point.

Don't put all your faith in a ballistic program. Some things elude precise computation.

Greg

 

[Jaeger308]

Re: Which Altitude for Ballistic Program?

Well as Lindy point’s out station pressure is what most people use, myself included, and that is measured at the FFP. Your question is if you input altitude for correction in the program which one do you use, in a high altitude and high angle shot of 1000 yards at 45 degrees the change in altitude from the FFP to target is 2100+/- ft, does that come into play for the solution? Theoretically sure. But in that shot I would be much more concerned in getting the slope and wind calls correct for a solution, and how much time will you have to compute a solution until the target may move off?

 

[Lindy]

Re: Which Altitude for Ballistic Program?

Put in the corrected pressure, put in the altitude at which you are shooting, and check the box which says, "Pressure is Corrected."

 

[Graham]

Re: Which Altitude for Ballistic Program?

Use one variable instead of two: get a Kestrel, set it to measure the actual pressure where you are, and enter your altitude on the ballistic program as zero.