Long Range Hunting Online Magazine


Go Back   Long Range Hunting Online Magazine > Rifles, Reloading, Optics, Equipment > Rifles, Bullets, Barrels and Ballistics


Reply

Altitude vs. Barometric pressure

 
LinkBack Thread Tools Display Modes
  #8  
Old 06-20-2005, 05:01 AM
Platinum Member
 
Join Date: May 2001
Posts: 2,369
Re: Altitude vs. Barometric pressure

Lots of good info in this thread. I'll make it sticky for future shooters.
Reply With Quote

  #9  
Old 06-20-2005, 07:15 AM
Platinum Member
 
Join Date: Aug 2003
Location: NC, oceanfront
Posts: 3,301
Re: Altitude vs. Barometric pressure

"When altitude is selected, the pressure field dissapears and is defaulted to 29.92"

This wouldn't quite work. I really like your direction on this this though. But if the altitude block is selected in your scenario, and the pressure field disappears, the program should default to STD PRESSURE AT ALTITUDE.
Else you lock in the apparent/potential problem as started for this thread. 29.92 would only be the std pressure at sea level, ICAO conditions. If you need a very small program or specific math for standard pressure at altitude, let me know. I know I had a difficult time finding it, back when I was looking.
Reply With Quote
  #10  
Old 06-20-2005, 08:17 AM
Silver Member
 
Join Date: Aug 2003
Location: McKinney TX
Posts: 403
Re: Altitude vs. Barometric pressure

Meichele,

Your topic is very good and one that got me a couple of years back when I started using my Krestel 4000 handheld weather meter with my RSI Ballistic Lab program.

I was plugging in absolute BP numbers from the krestel and still keeping the altitude field populated. My results were wacky. A quick call to Jim Ristow at RSI and he set me straight. I think this is something a lot of shooters do who use b-programs to help with long distance shooting.

It's everyone's understanding that they need to populate all fields in their b-programs with most accurate field conditions they can provide. Sometimes, that's not always the case as in your example. If your using a weather station or hand held weather station and you know your exact BP, then you need to leave the Altitude at zero.

Great topic and thanks for sharing this one.
__________________
Jeff

Mathew 5:16

Distance is not an issue, but the wind will make it interesting!
Reply With Quote
  #11  
Old 06-20-2005, 08:35 AM
Platinum Member
 
Join Date: Jan 2003
Location: The rifle range, or archery range or behind the computer in Alaska
Posts: 3,524
Re: Altitude vs. Barometric pressure

Micke CR, I know where you are going with the standard pressure at altitude. It does calculate "standard pressure at altitude" but it NEEDS a starting point. That starting point is 29.92. From that point it calculates the differance from the altitude input. Some programs use 29.53. 29.92 works best in mine with the formulas used here. It is DEADLY accurate.

In other words, once the altitude box is selected and pressure dissapears, it has to default to a specified standard so as to cancel out any other input a user may have entered. Otherwise it calculates them together (thats bad!)

I hope that helps.

Jeff in TX thanks for the input also.
__________________
Long range shooting is a process that ends with a result. Once you start to focus on the result (where the shot goes, how big the group is, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.
Reply With Quote
  #12  
Old 06-20-2005, 12:40 PM
Platinum Member
 
Join Date: Jun 2001
Location: Palmer, Alaska
Posts: 2,539
Re: Altitude vs. Barometric pressure

Michael,

I run some numbers for a guy there in Florida a couple years ago, as he was heading to Colorado hunting and I stumbled upon the same head scratching issue you did. I've Never called Jim to ask him about it simply because It was a 10,000 foot change and I didn't anticipate having to deal with that just yet, still it did not seem correct, still doesn't, but I'm sure Jim understands why there is a difference in output on the RSI program, or could find out.

I set the Kestrel 4000 to 0 feet referance to get station pressure and use it with a 0 altitude in the ballistic program, if anything other than zero is entered for altitude when you are already using station presure (pressure at altitude) the ballistic program will reduce the pressure once again, or two fold. If all one has access to is weather report data for daily pressure it must be reduced for your altitude by the program, thus an altitude needs to be entered along with this pressure at sea level in order to be accurate.

29.92 or 29.53.... to be quite honest, I'm still a little confused over this one here and have been for a while. Anyone care to shed a little light on the origin of the two?
__________________
Brent Moffitt
Reply With Quote
  #13  
Old 06-20-2005, 01:05 PM
Platinum Member
 
Join Date: Aug 2003
Location: NC, oceanfront
Posts: 3,301
Re: Altitude vs. Barometric pressure

Well let's look at it carefully.
Say altitude is checked. Pressure disappears. You enter actual altitude, and the program uses a std pressure for that altitude in it's calcs.
What if actual pressure is higher or lower than std at that altitude? How you gonna enter that deviation?
In order to calc air density correctly you'd have to determine and enter "pressure altitude" from a CALIBRATED Krestel. I guess GPS(for actual altitude) would be best for calibrating a Krestel while observing absolute pressure from a source known to be correct.
It just seems weird and confusing.

Anyone using actual altitude from GPS or maps would likely observe corrected pressure as reported locally that day. That pressure would often be near 29.92"hg regardless of altitude, and if different(say 28.7"), there needs to be a way for them to enter this. Maybe in the form of a correction like -1.22"hg. This, to be applied to Std pressure at altitude, for the day.
It's not something I would use, but that or pressure altitude are the only ways I can picture using altitude instead of absolute pressure.
Just thought I'd throw that out there.
Reply With Quote
  #14  
Old 06-20-2005, 02:26 PM
Platinum Member
 
Join Date: Jan 2003
Location: The rifle range, or archery range or behind the computer in Alaska
Posts: 3,524
Re: Altitude vs. Barometric pressure

Mike CR,

You are correct once again in that "if the pressure is differant on a given day."

Think of it this way. If a user is entering altitude it is because he has no way to measure pressure. If he has a way to figure pressure accuratly, he would want to use the pressure option instead of the altitude. Like was stated in earlier posts if the pressure is known then you would enter 0' in the altitude field on those programs that allow you to use both.

This program enters 0' in the altitude field for you if you opt to use pressure. That way you cant accidentaly enter both.

I know several RSI users that dont measure pressure. When they use the program they enter altitude and RSI defaults pressure to the standard at that altitude. Thats all mine does. If in the RSI program, and altitude and pressure are both entered, and the pressure at your location is 24.95 at an altitude of 5000' and 59 deg. you get slightly inaccurate but close numbers. If 0' is entered and the correct pressure is entered, then it will give you correct ballistics. My program automaticly issues 0' when pressure is used.

It is of course MOST accurate to use the pressure, but if the pressure is not known, altitude can be used and will be VERY close most of the time. The BARNES X program doesnt even give you the option to use pressure. Only altitude.

In short, if you know the pressure is 27.54 at you location it does not matter what the altitude is. Its raw pressure. You simply enter enter into the pressure field 27.54 at that point altitude is defaulted to 0' so as to not create inaccuracies in the ballistics and it will generate to you ballistics the same way as RSI, JBM, ect...

Please keep them coming!! I am getting alot out of this.
__________________
Long range shooting is a process that ends with a result. Once you start to focus on the result (where the shot goes, how big the group is, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.
Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes


Similar Threads for: Altitude vs. Barometric pressure
Thread Thread Starter Forum Replies Last Post
Barometric Pressure - my attempt to learn tlk The Basics, Starting Out 3 07-10-2010 09:11 AM
Altitude or pressure on Eskimo ballistics program?? Tikka Lover Rifles, Bullets, Barrels and Ballistics 7 10-05-2008 10:32 AM
How Much Does Barometric Pressure Change? Len Backus Long Range Hunting & Shooting 13 08-04-2008 08:05 PM
altitude/pressure Dave Squires Rifles, Bullets, Barrels and Ballistics 1 07-06-2005 09:16 AM
Altitude,Pressure, Temp... Matt27 Rifles, Bullets, Barrels and Ballistics 2 12-02-2002 11:50 PM


All times are GMT -5. The time now is 05:22 PM.


Powered by vBulletin ©2000 - 2014, Jelsoft Enterprises Ltd.
Content Management Powered by vBadvanced CMPS
All content ©2010-2014 Long Range Hunting, LLC