Advanced Marksmanship Density Altitude calculation......

S

Skytteren

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Minuteman
Jan 12, 2012
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#1
If this is posted in the wrong forum, please move it to where it should be....

My question is kind of short and simple...
What is the DA comprised of(temp? humidity? altitude? pressure?) and is there a formula for calculating it yourself based on those data, or do I have to buy a gizmo (I.e Kestrel 4000 or similar) Or can I get by with a calculator and a Kestrel 2000 for instance...?
smile.gif
 
#2
Re: Density Altitude calculation......

I have found this, but will this be sufficient to calculate a firing solution?


Density altitude in feet = pressure altitude in feet + 118.8 x (OAT - ISA_temperature)

Where:

OAT = Outside air temperature in °C
ISA_temperature = 15 °C - 1.98ºC / 1000ft x PA

considering that temperature drops at the rate of 1.98 °C each 1000 ft of altitude until the Tropopause (36000ft), usually rounded to 2ºC

Or simply:

DA=PA+118.8([PA/500]+OAT-15)

(This was from Wikipedia......)
smile.gif
 
#6
Re: Density Altitude calculation......

Honestly, the formulas are more precise, but for shooting you can print out a chart similar to the FDAC. You can find a few online, or just buy the FDAC for your caliber and then you will have elevation and wind adjustments.



Edit: Looks like LL beat me to it

FDAC.jpg
 
#8
#14
Re: Density Altitude calculation......

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lowlight</div><div class="ubbcode-body">Rather than use Math, most use something like this

[url="http://www.americanflyers.net/aviationlibrary/pilots_handbook/images/chapter_9_img_12.jpg"]http://www.americanflyers.net/aviationlibrary/pilots_handbook/images/chapter_9_img_12.jpg[/url][img]

If you come across something that requires long hand math in the field, chances are there is an easier way to do it, if not I would suspect you don't need it.

</div></div>

That's okay to use, but you have to account for pressure altitude, which is known elevation +/- air pressure factor. Depending on how accurate you're trying to be, neglecting air pressure could put you off by a few thousand Use this instead, I use this when I'm out flying and it's more exact. Pretty simple too. Even with dirty math, you'll be far more accurate and it only take ~15 seconds to do.

1 determine known elevation
2 read barometer and find pressure altitude conversion factor
3 if barometer is BELOW 29.92, add to known elevation, if ABOVE, subtract from known elevation. This is your pressure altitude
4 determine air temp
5 draw a line up from air temp to the pressure altitude contour you calculated
6 draw a horizontal line, this is your density altitude

Enjoy

[img]http://www.cfinotebook.net/graphics/performance/Density-Altitude.jpg
 
#16
Re: Density Altitude calculation......

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Killer Spade 13</div><div class="ubbcode-body">7. Get a Kestrel . . .
</div></div>

There's that...
 
#19
If you have a barometer, you don't need to convert to standard pressure and pressure altitude.

Humidity is NOT a factor in density altitude.

Most ballistics programs will take actual pressure as an input.

And at the risk of starting a storm again, if you have altitude or actual pressure, you can just use them. The difference between makes all the corrections and using just pressure or altitude (assuming 29.92 for standard pressure).
 
#20
What ballistic programs take DA directly and use G7 ballistic profiles?

And let me get this straight for myself. DA is just a composite value of barometric pressure and temperature, right? So essentially I can grab the current DA off of a kestrel and that takes the vast majority of the trajectory-altering weather variables into account. Thus I can print out every 500m DA charts specific to my rifle/load, or get a whiz wheel or slider like above and be pretty much set, except for very small factors?
 
#21
Another option is 'DenAlt'. It is a free iPhone application that I sometimes use for work to calculate aircraft performance.
It is free and easy to use like the generic charts others have suggested, if you have known weather parameters to enter.

To 'roughly' answer your original question, pressure altitude is elevation corrected for station pressure and density altitude is pressure altitude corrected for temperature.
 
Last edited:
#25
Ledzep said:
What ballistic programs take DA directly and use G7 ballistic profiles?

And let me get this straight for myself. DA is just a composite value of barometric pressure and temperature, right? So essentially I can grab the current DA off of a kestrel and that takes the vast majority of the trajectory-altering weather variables into account. Thus I can print out every 500m DA charts specific to my rifle/load, or get a whiz wheel or slider like above and be pretty much set, except for very small factors?
Click to expand...

The short and sweet, Density Altitude is pressure altitude corrected for non-stnadard temperature.
 
#26
To put a differnt spin on it, you can radically change to two primary factors (pressure and temperature) and wind up with the same result.

I shoot in Sacramento and our DA in February is around 200. We went to Oklahoma that same month, and we were at a higher altitude and the termperature was about 30 degrees cooler and our DA was around...200.

Point being, our dope was essentially the same in two dramatically different environments. The DA aggregated to the same.

It's good to understand how you get there, but get a Kestrell. I learned long divisionion in grade school, but I use a calculator as an adult.
 
#27
So once you find density altitude, how do you adapt that to your bullet and velocity combination? Is this where trial and error comes into play? Or could I use a program like Shooter to do the work for me?
 
#28
The actual answer to your question is that it accounts for temperature, pressure and water vapor pressure in the air (aka humidity, aka dew point). It's a pain - details here: http://www.srh.noaa.gov/images/epz/wxcalc/densityAltitude.pdf

As others said, you can also look it up on a chart and skip the math.

A better answer is to just measure absolute pressure, temperature, and humidity (optional) and not bother worrying about DA at all. It's an unnecessary complication. You can go cheap and use a mechanical altimeter to measure pressure and thermometer to get temperature or you can get spendy and spring for a kestrel. Either one will get you what you need. You can just guess on humidity since it doesn't play a big role.
 
#30
I have a Kestrel, and can easily get DA readings. But I want to take it the next step. With a given DA, is there a chart to show how many [minutes/mils/clicks] to add/subtract from my range card (which is based on zeroing weather DA)?

For example, let's say my range card shows that at my zeroing weather a 600 yard shot needs 11.0 minutes of elevation. My zeroing weather was at 150 DA. Now I am shooting at 3500 DA -- what additional increment of elevation do I need to add or subtract?

I would like to build a reference card showing just the +/- values for different DA's, keeping all other ballistic solution info the same. Is there an easy way to build this?
 
#32
A couple problems with the FDAC:

1. It costs money
2. I see this in their description: "The FDAC comes with 5 cards. The ballistic data for these cards were designed around the 175 grain Sierra MK. Card velocities are in 50 FPS increments at the following velocities 2500, 2550, 2600, 2650, 2700."

I am using a .260 Rem AI with a SMK 142gr at 2945 fps muzzle velocity. Doi they have a card for that?

3. Even if they do, if I change loads/rifles and have a different set of external ballistics then I need another oddball card. Then see #1.
 
#33
They do have other calibers for the FDAC or Impact Data sells the inserts for them...

Pretty much everything with a precision rifle costs money.

if you want a dedicated DA Card for your caliber / load, you can make it with different software. For example, ColdBore gives you DA Cards that you can print based on your specific load. You can build multiple tracks and have a nice big database of bullets you use, and at any time print the cards.

The old Rule of Thumbs don't really work, at the same time we have learned to measure the changes to each specific load better. Remember we have better Bullets, Barrels and Powders nowadays so you can't say, 20 degrees = X amount of change, unless you test you individual load to get the solution to that question. Which is where many turn to software.
 
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