Advanced Marksmanship Determining Altimeter Setting? Density Altitude Calculation

[demolitionman]

I'm trying to find some ways to figure my Density Altitude using simple paper charts rather than a Kestrel, one because I don't have a Kestrel, and two I often find guys using them are concerned their DA reading on their Kestrel is way off; which is usually because they don't realize DA isn't the true altitude but more of a "felt altitude" that affects the bullet.

Here's my main question; Is BAROMETRIC PRESSURE of the area that I am shooting at also the same exact pressure that I need to input into my "Altimeter Setting" in a Density Altitude Calculator? So check this chart out:
http://www.asiresource.com/DensityAltitudeChart.PDF

I'm talking about the "Altimeter setting in Hg" <- is that the same as my locations Barometric Pressure? If so I'd just find the corresponding number on that setting list, do the alitmeter conversion factor by subtracting my shooting areas actual altitude from that pressure altitude conversion factor then follow the chart to my estimated DA.

I use this website to determine my Barometric Pressure. I use a simple topgraphy map on my phone to determing actual altitude.
http://www.usairnet.com/weather/maps/current/ohio/barometric-pressure/

 

[alpine44]

What influences the bullet path via drag is the air density. Density is influenced by three parameters: barometric pressure, temperature, and -to a lesser degree- humidity.

Density altitude is a way to express the combination of these three parameters as the density that would exist at that altitude if the other two parameters were "standard" for this altitude. Density altitude is a good way for pilots to get a "feel" how an airplane will perform since they experience vastly different altitudes as part of their business/hobby and can relate this to performance via experience and by consulting operating manuals. Example: Las Vegas (LAS) is at 2181 feet which most hobby pilots would not worry much about in terms of performance decrease. However, when the temperature is around 110 degrees there, which is not unusual in the summer, the density altitude is now over 6000 feet which would be higher than taking off in Denver on a "normal" day. These altitudes are something any prudent pilot will factor into his/her takeoff planning. (Airline pilots will perform detailed calculation for every flight.)

For ballistic calculations it is IMO simpler to input barometric pressure, temperature and humidity directly. If you care about a ballpark density altitude number you could use this chart

 

[_Windrider_]

What influences the bullet path via drag is the air density. Density is influenced by three parameters: barometric pressure, temperature, and -to a lesser degree- humidity.

Density altitude is a way to express the combination of these three parameters as the density that would exist at that altitude if the other two parameters were "standard" for this altitude. Density altitude is a good way for pilots to get a "feel" how an airplane will perform since they experience vastly different altitudes as part of their business/hobby and can relate this to performance via experience and by consulting operating manuals. Example: Las Vegas (LAS) is at 2181 feet which most hobby pilots would not worry much about in terms of performance decrease. However, when the temperature is around 110 degrees there, which is not unusual in the summer, the density altitude is now over 6000 feet which would be higher than taking off in Denver on a "normal" day. These altitudes are something any prudent pilot will factor into his/her takeoff planning. (Airline pilots will perform detailed calculation for every flight.)

For ballistic calculations it is IMO simpler to input barometric pressure, temperature and humidity directly. If you care about a ballpark density altitude number you could use this chart

Nothing ball park about that chart. Its in every operating handbook of every aircraft in the US...and probably the world. The thing about DA is we dont have to be accurate to the nth degree. a 500 foot change will hardly be noticed, heck even 1000 foot change. so this chart is more than accurate.

There are some very simple rules of thumb to help you.

1. DA is simply station pressure corrected for non standard temperature.
2. Standard atmosphere is 29.92 inHg at 59F (15C).
3. Standard Lapse Rate (Dry Adiabatic) is 2C decrease (3.5F...Its actually 3.6 but 3.5 is much easier to work with) per 1000 ft gain in elevation/altitude
4. Standard pressure change (decrease) is 1.00 inHg per 1000 ft increase in elevation/altitude
5. DA increases 1000ft per 15F (approximate) above standard temp

if you keep all those in mind, you can figure you DA within 500 feet in your head.

Examples:

Denver, 70F and station pressure is 26.42

Standard temp for 26.42 (3500 ft) is 47F (59-(3.5*(29.92-26.42))

Standard atmosphere in Denver (using the elevation at KDEN airport) is 5434' (round it to nearest 500') we will call it 5500. This makes the standard atmosphere 24.42 inHg (29.92 - 5.5) and standard temp 40F (59-(3.5*5)). at those exact numbers, DA equals 5500 . So lets say it was 60 degrees outside in Denver. Whats the DA? 60-40=20......20/15 = 1.33333 (call it 1.3). so DA is 5500*1.3 = 7150 (call it 7000). That coincides with the DA chart alpine 44 linked:

DA = 5500((70-47)/15))=8433 feet

That’s the shortcut if you have station pressure

If you have an altimeter setting from an airport or other weather source you need to calculate pressure altitude and then you can find DA. Alpine was 100% correct meanwhile I was tripping over my dick because I kept saying the wrong thing and doing that wrong thing.

 

[alpine44]

The "ballpark chart" comment was meant with respect to ballistic calculations not aviation. The DA chart is great for pilots but a shooter would be off by about 5 to 10 inches (depending on altitude) @1000 yards with a .308Win 175 FGMM if the DA is rounded to the next 1000 feet.

I'll keep the DA chart in my airplane and use barometric pressure, temperature, and humidity for the ballistic calculator.

 

[_Windrider_]

The "ballpark chart" comment was meant with respect to ballistic calculations not aviation. The DA chart is great for pilots but a shooter would be off by about 5 to 10 inches (depending on altitude) @1000 yards with a .308Win 175 FGMM if the DA is rounded to the next 1000 feet.

I'll keep the DA chart in my airplane and use barometric pressure, temperature, and humidity for the ballistic calculator.

you mean station pressure right?

i was merely illustrating how to do it without any kestrel or phone.
I do disagree though that if you rounded 7150 to 7000 that even witha 308 you would miss an MOA target (provided your wind call is accurate). if you rounded up to 8000, now we may have an issue. again, this is for shooting steel targets or movers that simulate human sized. there is a wide margin of error on a 36" tall steel plate. if you are trying to hit a coke can at 1400 yds, yeah a bit more precise calculations are required.

 

[alpine44]

you mean station pressure right?

i was merely illustrating how to do it without any kestrel or phone.
I do disagree though that if you rounded 7150 to 7000 that even witha 308 you would miss an MOA target (provided your wind call is accurate). if you rounded up to 8000, now we may have an issue. again, this is for shooting steel targets or movers that simulate human sized. there is a wide margin of error on a 36" tall steel plate. if you are trying to hit a coke can at 1400 yds, yeah a bit more precise calculations are required.

Yes, station pressure.

In practice, I use a best case (lowest density) and worst case (highest density) drop chart for any given region and season of the year as a paper backup.

What I wanted to discourage is to SWAG or generously round input values, use a sophisticated program, and then take the full accuracy of the output as the gospel. GIGO - garbage in, garbage out.

To summarize the answers to the OP's question:

Known:
Barometric pressure or altimeter setting (NOT actual pressure at shooting station)
Topographic elevation
Temperature

-> Find the barometric pressure in this this chart and add the corresponding conversion factor to the topographic elevation. Then use the graph to determine the density altitude.

Known:
Station pressure
Temperature

-> Input directly

 

[demolitionman]

Thanks for the replies I'll look it over tonight.

 

[demolitionman]

This is what I used before my Kestrel and still use often. Courtesy of Zak Smith ThunderBeast arms.
http://demigodllc.com/icao.pdf

I looked high and low on Google for just such a chart. Thank you Max.

 

[_Windrider_]

Yes, station pressure.

In practice, I use a best case (lowest density) and worst case (highest density) drop chart for any given region and season of the year as a paper backup.

What I wanted to discourage is to SWAG or generously round input values, use a sophisticated program, and then take the full accuracy of the output as the gospel. GIGO - garbage in, garbage out.

To summarize the answers to the OP's question:

Known:
Barometric pressure or altimeter setting (NOT actual pressure at shooting station)
Topographic elevation
Temperature

-> Find the barometric pressure in this this chart and add the corresponding conversion factor to the topographic elevation. Then use the graph to determine the density altitude.

View attachment 6870728

Known:
Station pressure
Temperature

-> Input directly

i was typing and got distracted by a shiny penny. The chart Max just posted is correct. compare it to my methodology. 1k increase in DA for every 15 degrees above standard. who cares what the pressure actually is at 2000 feet standard temp is 52F, in your example of 70F thats 1.2 times standard temp.....or 1200ft increase over elevation for your DA. that gets about 3200 feet. The chart says 3171. thats pretty damn close if you ask me. you dont need anything but your topo map and some kind of thermometer to get close enough to make a hit.

demoman.....dont get lost in the weeds here. its easily possible to get DA without a kestrel.

 

[_Windrider_]

This is what I used before my Kestrel and still use often. Courtesy of Zak Smith ThunderBeast arms.
http://demigodllc.com/icao.pdf

I put this in the resources section. This is a good tool to keep handy.

 

[Basher]

Warms my heart to see DA charts being used outside the world of aviation.

 

[_Windrider_]

Hopefully a link to it with clear credit to Zak Smith.....

His name is on the chart.

 

[demolitionman]

Basher, how do you determine the pressure input for correctly setting your altimeter before takeoff?

 

[Basher]

Basher, how do you determine the pressure input for correctly setting your altimeter before takeoff?

By dialing up the AWOS or ASOS frequency at your departure airport (or the closest one) and listening to the info it provides. An altimeter setting is provided for you, and you simply adjust the kollsman window on your altimeter for the stated pressure.

 

[alpine44]

i was typing and got distracted by a shiny penny. The chart Max just posted is correct. compare it to my methodology. 1k increase in DA for every 15 degrees above standard. who cares what the pressure actually is at 2000 feet standard temp is 52F, in your example of 70F thats 1.2 times standard temp.....or 1200ft increase over elevation for your DA. that gets about 3200 feet. The chart says 3171. thats pretty damn close if you ask me. you dont need anything but your topo map and some kind of thermometer to get close enough to make a hit.

demoman.....dont get lost in the weeds here. its easily possible to get DA without a kestrel.

Except, we forgot to account for barometric pressure changes due to weather systems. This week the barometric pressure at my location is forecasted to range from 29.87"Hg to 30.5"Hg. That pressure change is like changing your elevation by about 600 feet. In aviation that difference is significant and therefore altimeters have the Kollsman window where you input the barometric pressure ( via the knurled knob on the bottom left in the pictures below).

You decide whether that influence is significant for your shooting. Elevation, temperature, barometric pressure and even humidity will influence air density. The "Altitude" in Zak's chart is pressure altitude NOT topografic elevation.

For those who do not like graphs (like posted above) I will try to expand Zak's chart to include barometric pressure input.

 

[demolitionman]

Alpine thanks. Can you elaborate on The pressure altitude conversion factor? This is directly related to Barometric Pressure and your ACTUAL altitude at takeoff right?

 

[alpine44]

Here is another chart showing the correction that you have to add to your physical/topographic elevation to get Pressure Altitude. This accounts for high or low pressure systems moving over your location. The physical/topographic elevation obviously stays the same but the weather changes the pressure and with it the air density. When a low pressure system moves in, the air becomes less dense the same way as if you had increased your elevation before the Low moved in.

Barometric Pressure ["Hg]    Altitude Correction [ft]
30.94    -1000
30.83    -900
30.71    -800
30.63    -700
30.51    -600
30.39    -500
30.31    -400
30.20    -300
30.12    -200
30.00    -100
29.96    -50
29.92    0
29.84    50
29.80    100
29.69    200
29.61    300
29.49    400
29.41    500
29.29    600
29.21    700
29.09    800
29.02    900
28.90    1000
28.82    1100
28.70    1200
28.62    1300
28.50    1400
28.43    1500
28.31    1600
28.23    1700
28.11    1800
28.03    1900
27.91    2000

Elevation + Altitude Correction = Pressure Altitude

Then use this Pressure Altitude in Zak's chart to get Density Altitude.

(The values are slightly different than the previous correction factor list. Both are approximations but are precise enough for what we try to accomplish)

 

[_Windrider_]

See my corrections above. My method was right, my terminology was wrong. We are coinciding a little closer now alpine. I kept saying standard pressure and I meant station pressure.

I ignore humidity it’s effects are negligible unless we are talking ELR distances.

 

[_Windrider_]

Here is another chart showing the correction that you have to add to your physical/topographic elevation to get Pressure Altitude.

Barometric Pressure ["Hg]    Altitude Correction [ft]
30.94    -1000
30.83    -900
30.71    -800
30.63    -700
30.51    -600
30.39    -500
30.31    -400
30.20    -300
30.12    -200
30.00    -100
29.96    -50
29.92    0
29.84    50
29.80    100
29.69    200
29.61    300
29.49    400
29.41    500
29.29    600
29.21    700
29.09    800
29.02    900
28.90    1000
28.82    1100
28.70    1200
28.62    1300
28.50    1400
28.43    1500
28.31    1600
28.23    1700
28.11    1800
28.03    1900
27.91    2000

Elevation + Altitude Correction = Pressure Altitude

Then use this Pressure Altitude in Zak's chart to get Density Altitude.

(The values are slightly different than the previous correction factor list. Both are approximations but are precise enough for what we try to accomplish)

Here are the real formulas as well for your further knowledge.

pressure altitude = (standard pressure - your current pressure setting) x 1,000 + elevation

Standard pressure =29.92

density altitude = pressure altitude + [120 x (OAT - ISA Temp)]

OAT (C)
ISA Temp (C) is at your pressure altitude

Alpine, sorry for the back and forth. I was tripping over myself. Sometimes the simplest thing becomes the most difficult.

 

[alpine44]

Here are the real formulas as well for your further knowledge.

pressure altitude = (standard pressure - your current pressure setting) x 1,000 + elevation

Standard pressure =29.92

density altitude = pressure altitude + [120 x (OAT - ISA Temp)]

OAT (C)
ISA Temp (C) is at your pressure altitude

Alpine, sorry for the back and forth. I was tripping over myself. Sometimes the simplest thing becomes the most difficult.

No problem. I know what you mean, I also wrote one thing and meant another.

To everybody who got utterly confused by the discussion, here is the low-down.

As precision shooters we need to account for the density of the air we are shooting through. However, no weather station gives us that value directly and even if one did we would not be happy with a number that needs about 5 decimals to be of any value.

So we use our Elevation and adjust for barometric pressure, which gives us Pressure Altitude. We now have compensated for high and low pressure systems in our area.

We can take this one step further and compensate for how much our local temperature differs from what would be standard. (Remember it is supposed to get colder the higher we go in the Atmosphere).

Compensating Pressure Altitude for non-standard temperature gives us Density Altitude.

Density Altitude is another way to express the local air density by saying: If we had standard barometric pressure (29.92"Hg) and standard temperature (depends on altitude) at that Density Altitude then we would find the same air density that we are actually dealing with.

PS: Make sure you do not confuse the terms in bold when inputting values in your ballistic calculator.

If you print out your backup tables for various Density Altitudes you do not need different tables for different temperatures (or different barometric pressures). Both influences are already factored in. And as TacticalDillhole pointed out, Density Altitude is easily calculated on the back of an envelope if you know Elevation, Temperature and Barometric Pressure.

Also, remember that we neglected the influence of humidity so far and ask yourself whether that is acceptable for your firing solution.

 

[SubOptimal]

Another source for density/altitude charts might be found in drag racing calculators. They use it for jetting changes to optimize HP output. EFI has taken most of that away but not all drag cars/bikes use EFI.

 

[308pirate]

Density-Altitude charts are all over the internet. They are nothing new, us aviators were using them for takeoff performance calculation long before shooters ever knew such a concept existed.

 

[alpine44]

Density-Altitude charts are all over the internet. They are nothing new, us aviators were using them for takeoff performance calculation long before shooters ever knew such a concept existed.

As a commercial pilot I am not going to argue with that. However, that does not mean that any non-aviator understands the concept and the benefit of accounting for air density changes via Density Altitude.

There are a lot of things that aviation and precision shooting have in common. Maybe we should suggest a pilot's license as a good investment for becoming a better shooter. It is not just the physics but also the visceral understanding of how important the fundamentals are for a repeatable, pleasant outcome. Any time Frank preaches the fundamentals, I hear a distance voice talking about stick and rudder.

 

[308pirate]

Not saying there's no need for education of what D-A is and means. Just pointing out that there's no mystery in figuring out what it should be on any given day. Print a chart and run your inputs.

 

[_Windrider_]

Not saying there's no need for education of what D-A is and means. Just pointing out that there's no mystery in figuring out what it should be on any given day. Print a chart and run your inputs.

For some it doesn’t matter for others it does. That’s the difference between being a master at your craft or just a guy who likes to shoot and knows how to get it done.

 

[Basher]

Any time Frank preaches the fundamentals, I hear a distance voice talking about stick and rudder.

Hahaha. Any time I was out of trim, my instructor woukd start to tap the appropriate anti-torque pedal. If I didn't notice or pay attention, the tapping got progressively harder until I noticed. He also used it as a good lesson for paying attention to unusual vibrations in the aircraft. Funny how important those fundamentals are though, no?

 

[308pirate]

For some it doesn’t matter for others it does. That’s the difference between being a master at your craft or just a guy who likes to shoot and knows how to get it done.

Do you think anyone sits around wondering how to derive DA charts? No, you do a lookup, get the answer, and get to work.

Could I do it? Yes, I know the formula. Am I going to waste my time if I have a pre-printed chart? Nope. I don't need to know what DA is within 10 feet to get the bullet where it needs to go.

 

[_Windrider_]

Do you think anyone sits around wondering how to derive DA charts? No, you do a lookup, get the answer, and get to work.

Could I do it? Yes, I know the formula. Am I going to waste my time if I have a pre-printed chart? Nope. I don't need to know what DA is within 10 feet to get the bullet where it needs to go.

Oh I totally agree. Even within 500-1k and you will be fine. I’m an engineer at heart. I like to know the minutia. Some of us are weird like that. But I do exactly what you say, I use a chart. But if I lost the chart I also know how to get what I need if all else fails. That’s all I’m getting at.

 

[alpine44]

Oh I totally agree. Even within 500-1k and you will be fine. I’m an engineer at heart. I like to know the minutia. Some of us are weird like that. But I do exactly what you say, I use a chart. But if I lost the chart I also know how to get what I need if all else fails. That’s all I’m getting at.

Absolutely!
Aviators are expected to understand the concept AND crank the equations AND work the charts. That creates redundancy and avoids finding yourself in a pickle if/when a gadget fails or was left behind. I have not found one facet of life yet where this approach does not create more confidence and better results. (Yes, I am lazy and use the simplest tool available but I know the day of punishment will come if I stop there.)

 

[Zak Smith]

First of all, a copyright refresher. A copyright means you cannot reproduce it (that is copy or re-host it) without the author's permission, regardless of if "it has his name of it". Post a link, that's how this works.

The version I "shipped" with datacards when I was actively selling those had the corresponding station pressure along the bottom matched with the altitudes. Otherwise the altitude uses the base pressure for that altitude in the ICAO model.

http://demigodllc.com/icao_column.pdf

This was also incorporated into the Magpul data book when Cody was getting that done.

Note the date on the PDF-- this was "new" 13 years ago, LOL.

ETA-- the chart is split in the middle so you can fold it in half and then laminate it, FYI.

 

[demolitionman]

Guys, I'm reallying enjoying getting my learn on here. I'm not a pilot, I don't know jack shit about air density and I have no formal schooling outside of highschool so I apologize if I'm being dense.

Please Alpine, Basher, TacticalDillhole, confirm that I've got this figured out:
Looking at a topo map of an area I shoot at alot, the peaks are about 1000ft above sea level.
If I'm shooting on a day that the barometric pressure is 30.4 and the temperature is 40F degrees; I would subtract 440 feet of elevation from 1000ft to get my corrected altitude from the pressure conversion factor which in this case is 560feet.

Now I simply follow the Density altitude chart from my temp of 40Fdegrees up to 560feet and over to the corresponding DA which in my area is so low it appears I barely show up on the chart at all.

This https://wahiduddin.net/calc/calc_da_rh.htm calculator says my DA to be about -576ft

 

[_Windrider_]

First of all, a copyright refresher. A copyright means you cannot reproduce it (that is copy or re-host it) without the author's permission, regardless of if "it has his name of it". Post a link, that's how this works.

The version I "shipped" with datacards when I was actively selling those had the corresponding station pressure along the bottom matched with the altitudes. Otherwise the altitude uses the base pressure for that altitude in the ICAO model.

http://demigodllc.com/icao_column.pdf

This was also incorporated into the Magpul data book when Cody was getting that done.

Note the date on the PDF-- this was "new" 13 years ago, LOL.

ETA-- the chart is split in the middle so you can fold it in half and then laminate it, FYI.

Chill man. I deleted my link mecause max did exactly that. I wasn’t trying to steal your work just putting something out there that pops up in any google search under images on the subject. It’s not secret squirrel info either. Apologies if you felt otherwise. Not my intention. Only putting common knowledge info out there for free. Next time I will be sure to use the MLA handbook and cite the source good and proper.

Understanding basic copyright law, I would say this falls under the fair use doctrine. No one here is profiting of your excel spreadshet. Sharing it isn’t a detriment to the market value of your product. Again I apologize if I ruffled your feathers.

 

[_Windrider_]

Guys, I'm reallying enjoying getting my learn on here. I'm not a pilot, I don't know jack shit about air density and I have no formal schooling outside of highschool so I apologize if I'm being dense.

Please Alpine, Basher, TacticalDillhole, confirm that I've got this figured out:
Looking at a topo map of an area I shoot at alot, the peaks are about 1000ft above sea level.
If I'm shooting on a day that the barometric pressure is 30.4 and the temperature is 40F degrees; I would subtract 440 feet of elevation from 1000ft to get my corrected altitude from the pressure conversion factor which in this case is 560feet.

Now I simply follow the Density altitude chart from my temp of 40Fdegrees up to 560feet and over to the corresponding DA which in my area is so low it appears I barely show up on the chart at all.

This https://wahiduddin.net/calc/calc_da_rh.htm calculator says my DA to be about -576ft

Yes

 

[damoncali]

Poor man's kestrel:
https://www.amazon.com/Sun-Company-...rts-and-fitness&ie=UTF8&qid=1517888508&sr=1-2

Just put that number in for station pressure and you're good to go (I'm assuming you've got a grip on temperature).

 

[308pirate]

I’m an engineer at heart. I like to know the minutia. Some of us are weird like that. But I do exactly what you say, I use a chart. But if I lost the chart I also know how to get what I need if all else fails. That’s all I’m getting at.

+1 Same here