Question about barometric pressure vs altitude

[Alan Griffith]

When I use Shooter and enter actual barometric pressure or enter the altitude I get two different sets of dope. At 1000 yards I am just under 4, MOA off. When I use the engineeringtoolbox.com and the altitude / barometric pressure chart I use 10000 feet or 20.6 barometric pressure. I can give exact ballistic specifications but at this time I do not feel it is relevant. It is more a difference of getting one read out using barometric pressure and the other read out using exact altitude.

Alan

 

[Browninglover1]

I was having the same trouble with mine, but I needed to set the program to "absolute" pressure and then my dial ups were spot on. Absolute pressure from what I've read is more accurate then entering elevation because the elevation feature just uses an average pressure reading for that height.

See if this fixes your problem.

 

[Alan Griffith]

I know what you mean. When I use absolute pressure my come ups are dead on. I was just wondering what was up with the altitude come ups being so far off.

 

[Browninglover1]

When you tried to use the elevation feature did you also have the pressure section populated? If you want to use the elevation try and leave the pressure input blank and see what it does.

 

[Alan Griffith]

Shooter will not allow both fields to be populated. Its one or the other but not both.

 

[Browninglover1]

Shooter will not allow both fields to be populated. Its one or the other but not both.

If you turn off absolute pressure it will allow both fields to be populated (if I remember correctly). I just know that I was sick of trying to figure out the elevation feature when the absolute station pressure gave me perfect results every time.

 

[Mikecr]

I guess you missed the sticky here:
http://www.longrangehunting.com/forums/f19/altitude-vs-barometric-pressure-11950/

Altitude, including pressure altitude, density altitude, etc, are not external ballistic parameters.
For local air density you need:
-Absolute pressure
-Temperature
-Relative humidity

For your bullet BC input you need to select the atmosphere standard that qualifies your rated BC(like StdMetro, ICAO) as a baseline. These terms are again:
-Absolute pressure
-Temperature
-Relative humidity


Much of the other stuff people come up with amounts to misunderstanding of these things.

 

[Greyfox]

If you turn off absolute pressure it will allow both fields to be populated (if I remember correctly). I just know that I was sick of trying to figure out the elevation feature when the absolute station pressure gave me perfect results every time.

I turn the absolute pressure button off and enter actual attitude and temperature. My drops are dead on using doing this. Altitude can not be populated in Shooter if the absolute pressure is turned on.

 

[RTK]

Alan, if you are using those pressure charts they are "standard" pressure charts and do not always reflex non standard pressures/temps that you might be experiencing. The most important factors as I "see" it are current barometric pressure (non corrected), and temp. Humidity plays a insignificant role but you can throw that in too, at least that is what we were taught back when I was flying when it came to density altitude and ballistics are all about air density.
edit:
Playing here with my shooter program, when absolute pressure is selected, as you mentioned, it does block out altitude. You still have temp and humidity that can be entered, when changing humidity there is no change in drops but when changing temp the drops will change. So if you use "pressure is absolute" don't forget to enter temp and current baro.