Range Report Standard Atmosphere vs Actual Atmosphere

[Deleted member 10043]

I'm shooting at an altitude of 3600' out to 500 meter range. I'm using the ammo's BC value which is .460 G1 or its G7 equivalent. When I set the current actual atmosphere I am not getting predicted results per JBM or an other ballistics program for PC or iPod. When I set to standard atmosphere I am getting actual results with predicted results per software. Velocity inputs are based on chronographed data. I've tinkered with numerous setting to include Litz's values. So, all things relevant what gives with the whole actual current atmosphere settings?

 

[Lowlight]

Re: Standard Atmosphere vs Actual Atmosphere

If you use the actual, the altitude needs to be set to 0

Using the standard pressure you include the altitude.

 

[Deleted member 10043]

Re: Standard Atmosphere vs Actual Atmosphere

Okay, thanks. I got it backwards in my head.

 

[azimutha]

Re: Standard Atmosphere vs Actual Atmosphere

Lowlight's logic is correct, but the terminology is not. Station pressure is the uncorrected pressure (the one you'd use 0 altitude with) while corrected pressure, which is also known as sea level pressure or altimeter setting is what you commonly see reported in weather forecasts. With that, you have to use altitude (which just means having to mess with entering more data).

For example, where I'm sitting right now the corrected pressure is 30.21 (and I'd have to enter my altitude of X feet). The station pressure is 26.23 and I can leave altitude in my calculator set to 0. Note: if using JBM make sure you uncheck the "Pressure is corrected" checkbox.

 

[Deleted member 10043]

Re: Standard Atmosphere vs Actual Atmosphere

Yeah, but all he had to say was station pressure to get me back on track. Some people call it absolute pressure as well. As for my OP my zero was off about .5" at 100 yards. It was just a coincidence or something that the standard metro settings was giving me the correct MOA adjustments at the longer range. Also wasn't accounting for chrono error as well. My OP was premature.

Thanks

 

[azimutha]

Re: Standard Atmosphere vs Actual Atmosphere

My point was not to make anybody look bad, but to correct the terminology. Inputs for ballistics calculators are confusing enough as it is. Best to make sure people understand which is which, especially if they then have to relate that to some device. For example, Garmin likes to refer to uncorrected station pressure as "ambient pressure" (see my post on this @ http://www.snipershide.com/forum/ubbthreads.php?ubb=showflat&Number=1696382).

 

[Lowlight]

Re: Standard Atmosphere vs Actual Atmosphere

Youre right, I went to use his own terminology and said station instead of standard. No excuse but was simply trying to use his words back for ease.

 

[Deleted member 10043]

Re: Standard Atmosphere vs Actual Atmosphere

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Inputs for ballistics calculators are confusing enough as it is. </div></div>

Yep it got me this time. Lowlight and I were talking in pig latin.

Appreciate the clarification for posterity. You made the thread informative and helpful. Thanks.

BTW, Example, the Hornady online ballistics calculator wants station pressure and not weather report pressure. It doesn't have an input for altitude but I wonder how many people use it with altimeter pressure?

 

[bronco9588]

Re: Standard Atmosphere vs Actual Atmosphere

I actually just got done working on a density calculator that compensates for humidity, temperature, and pressure. To me, it makes more sense to bring a barometer, thermometer, etc. to the rifles location rather than convert from the weather station, but it definitely is an option if you don't own these instruments. If you are converting from sea level, you can use the equation under Altitude Atmospheric Pressure Variation at:

http://en.wikipedia.org/wiki/Atmospheric_pressure

That equation is what your ballistic calculator is using and will allow you to switch back and fourth and check out your shooting solution.

 

[Greg Langelius *]

Re: Standard Atmosphere vs Actual Atmosphere

OK, I guess I'll bite.

I'm neither a good enough shooter, nor a dedicated enough believer in the concept that calculations will deliver a precise hit, to consider using more than to basic inputs or counting on calculational results to put me any closer than on paper at what I consider to be reasonably shootable distances (1Km or closer).

So my question is, do the differences between using map altitude vs observed station pressure result in a POI difference that actually stands out from the 'background noise' of unavoidable minimum dispersion?

I'm trying to give the process reasonable credibility, and also trying to figure out what the extra effort buys the committed shooter. More succinctly, is the result worth the effort on a practical level. I am concerend that any additional time involved might compromise a tasked sniper's window of exposure.

Greg

 

[Lowlight]

Re: Standard Atmosphere vs Actual Atmosphere

Greg,

It is the same information in a short hand form. The issue was, as people are wording it, "The weather report"

When you woke in NY they gave you the weather report, and the weatherman said it was 29.92 barometric pressure along with the temperature of the day. When I woke in Denver 2 hours later the weather man here said the same thing. Except really Denver doesn't have a Barometric pressure of 29.92, they corrected that, understand we are at 5280. So now figure that 29.92 is gonna lose an inch of pressure for every 1000 feet, so really my weather man should said 24.92

That 24.92 makes the difference up, along with the temp. Because if we are both experiencing a 65 degree day the real difference is the altitude which is big.

It's actually less effort as well you don't had to reference your altitude with a Kestrel.. doing it the traditional way requires more effort not less. It' s why people are using density altitude because it tells the bullet with one number what the air feels like when it is gonna fly. That has actually become the quicker and easier way of doing it.

Really it's all the same, just how you process the information.

 

[azimutha]

Re: Standard Atmosphere vs Actual Atmosphere

Lowlight's example would mean around a 6" difference @ 600 yards and a 32" difference @ 1000 (assuming Greg lives @ 1000 ft). So whatever calculational aid you use should take altitude into account if using corrected pressures. If you have a handheld device, then it seems unnecessary to use density altitude...unless that helps you learn if it you're going to rely on a DA chart in the field.

 

[Lowlight]

Re: Standard Atmosphere vs Actual Atmosphere

Just as an FYI for those using standard datebooks,

I put DA in the "humidity" section, because with most it is more noise than anything else and DA actually includes Humidity. So when filling out a datebook I use Temp, Baro, and DA...

My back up is the FDAC so that number is something I use, and find easy.

An aside, I was talking David Tubb 2 weeks ago, his new scope and reticle will actually be zeroed and adjusted in DA, and not have traditional MOA or Mils but DA and MPH for Wind.

 

[cali_tz]

Re: Standard Atmosphere vs Actual Atmosphere

Lowlight, what does a Kestrel unit give you... the sea level pressure, the real air pressure, or what exactly?

 

[Mike]

Re: Standard Atmosphere vs Actual Atmosphere

tag

 

[Lowlight]

Re: Standard Atmosphere vs Actual Atmosphere

The Station Pressure, or as the Ballistic programs put it Absolute Pressure...

A Kestrel gives you everything you need its a very valuable tool and I put it on par if not a bit ahead of owning a chronograph. There are ways to work MV backwards, but you absolutely need the information from the Kestrel.

I went hiking today around Mount Evans CO... we hiked up to Mt Spaulding which is just over 13,000 ft, I carry my Kestrel on these hikes so you can see the numbers provided.

This shows wind speed, uncorrected Barometric Pressure and Density Altitude, now this is just one User Screen, you get 3 User Screens you can set up any way you want.

The Barometric Pressure up around 13,000ft is 18.15 as opposed to Sea Level pressure which is 29.92. Now with the Air Temperature, Humidity and Barometric Pressure combined you can see the Density Altitude is 15,099ft. What that means is, that is where the <span style="font-style: italic">"bullet thinks" </span> it is flying at, so it will not fly like you are at 13,000ft but fly like you are at 15,000ft. See the difference. This is the number Air Crews use to determine how a plane will fly and how much weight it can carry for things like take off. For a military shooter he can ask his Air Crew what the DA is and they will have it readily available.

<span style="font-style: italic">As a quick side note, I think I took this kestrel reading showing the DA at 15k around 12.8k feet and not at 13,000, I believe at 13,000ft it was closer to 17,000ft DA. I know the wind was stronger at 12,000ft as the wind at the summit of Spaulding was only about 3MPH. </span>

This is an old Kestrel, I use the 4500NV versions now, so this is just in my pack. Specs on this model include:

The Kestrel Meter 4000 Weather Meter measures/features

Barometric pressure
Pressure trend
Altitude
Relative humidity
Heat stress index
Dewpoint
Wet bulb temperature
Density altitude
Wind chill
Air, water, and snow temperature
Current wind speed
Average wind speed
Maximum wind gust
Waterproof and floats
Easy to read, backlit display
Time and date
Exterior temperature, humidity, and pressure sensors for fast and accurate readings
Humidity sensor can be recalibrated in the field with our Relative Humidity Calibration Kit.
Minimum, maximum, and average values
Automatically store measurements, even when the unit is turned off
Manually store measurements with the press of a button
Customizable data storage - 4000 data points
Graph and recall trends
Customize screens to display user-selected measurements
Data charting
Data upload (with optional interface)
Five languages (English, French, Spanish, German, and Italian)
User-replaceable impeller
Flip-top impeller cover allows use of other functions while protecting the impeller
US Patent Nos. 5,783,753 and 5,939,645 and 6,257,074
All instruments and accessories are completely assembled in the USA

 

[THUNDERBOLT68]

Re: Standard Atmosphere vs Actual Atmosphere

tag

 

[bronco9588]

Re: Standard Atmosphere vs Actual Atmosphere

That is pretty awesome. I want...

 

[cali_tz]

Re: Standard Atmosphere vs Actual Atmosphere

a 'hike' to 13K feet??? For me that would be called an expedition, but good for you

Thanks for the detail, much appreciated. Looking into getting a Kestrel now.
I haven't tested the sensitivity of my ballistic solutions to small DA variations, but I suspect it will make a non trivial difference to get this perfectly right.

 

[ctressler]

Re: Standard Atmosphere vs Actual Atmosphere

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Greg Langelius *</div><div class="ubbcode-body">OK, I guess I'll bite.

I'm neither a good enough shooter, nor a dedicated enough believer in the concept that calculations will deliver a precise hit, to consider using more than to basic inputs or counting on calculational results to put me any closer than on paper at what I consider to be reasonably shootable distances (1Km or closer).

So my question is, do the differences between using map altitude vs observed station pressure result in a POI difference that actually stands out from the 'background noise' of unavoidable minimum dispersion?

I'm trying to give the process reasonable credibility, and also trying to figure out what the extra effort buys the committed shooter. More succinctly, is the result worth the effort on a practical level. I am concerend that any additional time involved might compromise a tasked sniper's window of exposure.

Greg </div></div>

I think you will find that if you really want to be a commited shooter you can't help but take into acount as many details as possible. Map altitude won't get you nearly as close as a Kestral.

 

[Greg Langelius *]

Re: Standard Atmosphere vs Actual Atmosphere

OK I have my answer, and all you guys have been helpful. Thanks again.

And yes, I do live rather close to 1000ft ASL.

Greg

 

[bronco9588]

Re: Standard Atmosphere vs Actual Atmosphere

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ctressler</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Greg Langelius *</div><div class="ubbcode-body">OK, I guess I'll bite.

I'm neither a good enough shooter, nor a dedicated enough believer in the concept that calculations will deliver a precise hit, to consider using more than to basic inputs or counting on calculational results to put me any closer than on paper at what I consider to be reasonably shootable distances (1Km or closer).

So my question is, do the differences between using map altitude vs observed station pressure result in a POI difference that actually stands out from the 'background noise' of unavoidable minimum dispersion?

I'm trying to give the process reasonable credibility, and also trying to figure out what the extra effort buys the committed shooter. More succinctly, is the result worth the effort on a practical level. I am concerend that any additional time involved might compromise a tasked sniper's window of exposure.

Greg </div></div>

I think you will find that if you really want to be a commited shooter you can't help but take into acount as many details as possible. Map altitude won't get you nearly as close as a Kestral. </div></div>

Again, IMO Altitude is rather pointless. Air density is a function of a local barometer and humidity sensor. The bullet doesn't care about altitude.

 

[cali_tz]

Re: Standard Atmosphere vs Actual Atmosphere

can someone advise me... which Kestrel can I get away with? Do I have to spend on the 4500, or can I save $s and go with the 3500? Or less? I don't need Bluetooth and all the bells/whistles... I'll enter data into Shooter manually...

thanks for any advice.

 

[Mutt]

Re: Standard Atmosphere vs Actual Atmosphere

You'll need the barometer. I think the 3500 is the cheapest model that measures pressure...don't quote me on that though, ask mr Google to be sure.
On a side note, I cheaped out when I upgraded to the 4500nv and passed on the bluetooth. Now I get pissed whenever I see someone making use of the bluetooth version...I don't "need" it but it sure would've been nice!

Anyone with BT want to trade straight across? No? Okay, just checking.

 

[cali_tz]

Re: Standard Atmosphere vs Actual Atmosphere

thanks Mutt... yeah it looks like 3500 has all the basics. 4000 series adds data logging and bluetooth and 4500 adds digital compass and crosswind stuff.
I was concerned that 4000 Series had something about pressure that the 3500 didn't.

Hard to tell if the 3500 provides Density Altitude...

edit: what ballistics software actually uses Density Altitude?

 

[howdydoit]

Re: Standard Atmosphere vs Actual Atmosphere

tag

 

[ctressler]

Re: Standard Atmosphere vs Actual Atmosphere

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: cali_tz</div><div class="ubbcode-body">
Hard to tell if the 3500 provides Density Altitude...</div></div>

The 3500 does not have the Density Altitude feature.

 

[azimutha]

Re: Standard Atmosphere vs Actual Atmosphere

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bronco9588</div><div class="ubbcode-body">Again, IMO Altitude is rather pointless. Air density is a function of a local barometer and humidity sensor. The bullet doesn't care about altitude. </div></div>
It depends on what data you have available to you. Since my GPS unit can report uncorrected station pressure, altitude isn't necessary to enter. If all you have is the corrected pressure reported from the nearest weather station then it's better than nothing to use that...if you know the altitude where you are and the weather is the same.

BTW, humidity barely matters in ballistics calculations. You could nearly always leave it set @ 50% and split a 1.5" difference @ 1000 yards (and that's between the extremes of 0 and 100%).

 

[bronco9588]

Re: Standard Atmosphere vs Actual Atmosphere

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: azimutha</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bronco9588</div><div class="ubbcode-body">Again, IMO Altitude is rather pointless. Air density is a function of a local barometer and humidity sensor. The bullet doesn't care about altitude. </div></div>
It depends on what data you have available to you. Since my GPS unit can report uncorrected station pressure, altitude isn't necessary to enter. If all you have is the corrected pressure reported from the nearest weather station then it's better than nothing to use that...if you know the altitude where you are and the weather is the same.

BTW, humidity barely matters in ballistics calculations. You could nearly always leave it set @ 50% and split a 1.5" difference @ 1000 yards (and that's between the extremes of 0 and 100%). </div></div>

You can judge for yourself:

P_abs is in milliBar from local barometer
T_db is the temperature of a thermometer in kelvin
phi is the percent humidity

rho_dry_air = P_abs*100/(R_dry_air*T_db);
rho = rho_dry_air* (1 + phi) / (1 + .622*phi ) ;

between dry air and saturated, there is a 2/1.622 difference with humid air being lighter.

That is not going to show up in your 100 yard shot, but definitely in your 1000.

 

[19818119]

Re: Standard Atmosphere vs Actual Atmosphere

tag

 

[azimutha]

Re: Standard Atmosphere vs Actual Atmosphere

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bronco9588</div><div class="ubbcode-body">That is not going to show up in your 100 yard shot, but definitely in your 1000. </div></div> "definitely" would seem to imply that it matters. Sorry, I cannot shoot well enough to notice a .75" discrepancy @ 1000 yards. But whatever rhos your boat tail gently down the airstream.

 

[timelinex]

Re: Standard Atmosphere vs Actual Atmosphere

So if the information I got from someones kestrel the other day is:
altitude:5526 ft
pressure:24.39
temperature:84
humidity 19.3

Can someone tell me what do I input into "ballistics FTE" on the iphone.
The inputs are
altitude:
Barometric pressure:
pressure is absolute: ON or OFF (on disables the altitude feature)
temperature:
relative humidity:

I guess my main question is whether the pressure given by the kestrel is absolute or not. It seems by what frank is saying it IS absolute pressure. I just wanted to confirm this, because the numbers I'm getting for estimated dopes is way off compared to real life.... And Im including exact yardages and slopes into account... I've been trying to make it fit but no combination fits it all.

 

[Lowlight]

Re: Standard Atmosphere vs Actual Atmosphere

Kestrels can be set up either way.

it depends what the reference altitude is set for, and what the barometric pressure reference is set to.

Let's assume they have it set correctly, then the Kestrel is giving you "absolute" in terms of Ballistics FTE. So you would use the Baro, temp and humidity, which is really the same as the DA. (Which in your settings you can tell Ballistics to use DA)

Turn on Absolute, and don't use the altitude.

 

[azimutha]

Re: Standard Atmosphere vs Actual Atmosphere

timelinex, in this case you would enter 0 for altitude. The pressure you listed is uncorrected. I don't have the FTE edition, but find that if you're using a 100-yard (meter) zero, it's better to turn off the Zero Atmosphere and just use the Current Atmosphere. I'd advise using the JBM engine and also use the JBM website to see that you're getting the same results. In other words, use and understand JBM's website and then get your ballistics calculator to agree with it.

 

[timelinex]

Re: Standard Atmosphere vs Actual Atmosphere

Ok thanks for the help guys.

My last big issue is with the mile target. With the correct atmosphere I seem to be within 1-2 moa of actual values. The big thing thats off is the mile target. Now it is a 308, which I fully understand isn't transonic past 1100 and isn't meant for those distances. But at a mile my calculator says I should need about 80 moa but my actual was about 102. Now the only reason I even give any weight to my actual moa at this distance, is even though it was getting carried by the wind heavily, the elevation was right on for 20-30 shots(within 2 moa or so that is) so there obviously wasnt alot of chaos with the bullet. Should I just scrap that data point at a mile, or is it exposing something fundamentally wrong with my inputted data.

 

[azimutha]

Re: Standard Atmosphere vs Actual Atmosphere

timelinex, going off the environmental data you listed and that you're shooting a .308, a 175 gr. SMK would (with the right inputs in JBM) and assuming 2600 fps would be 101.9 MOA. Ballistic, with the same inputs gave me 101.7. (Incidentally, my new calc for Windows Phone 7 also gives 101.7).

Sounds like you need to doublecheck the correct muzzle velocity and also the correct BC for your bullet. The best ways to do that are to get a hold of a decent chrony and get a copy of Bryan Litz' Applied Ballistics for Long Range Shooting, which gives more realistic BCs for a large variety of bullets.

 

[ArcticLight]

Re: Standard Atmosphere vs Actual Atmosphere

All that's fine and dandy but how many people are competing at say 5800 feet?

And when you do, do you get practice shots?

I see the logic behind it but I am sooo old school about that stuff...
I forget exactly but there's a rough estimate on the .308 round for every 1000 feet in altitude add x amount of inches and swag it..

On the other hand if you are behind a rock in Afghanistan this stuff is critical to a good first shot hit.

 

[timelinex]

Re: Standard Atmosphere vs Actual Atmosphere

Something is going on with my ballistics software then! I get around 80 moa even when I lower my MV to 2600. I'm using Southwest 175's, so Im using litz's G7 values of .243. The listed MV for those bullets are 2650 out of a 24" barrel(and pretty sure its a savage as well). I know it could be different for me, but its not going to make 20+MOA difference between 2600 or even 2700.

Im gonna try to use computer software as recommended earlier and see if some settings are off on my iphone app.

EDIT:I put in the wrong pressure.. and somehow kept overlooking it!! Can't believe I was overlooking such a crucial part. The numbers still don't all match as close as I would like, but atleast its closer.

 

[Lowlight]

Re: Standard Atmosphere vs Actual Atmosphere

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ArcticLight</div><div class="ubbcode-body">All that's fine and dandy but how many people are competing at say 5800 feet?

And when you do, do you get practice shots?

I see the logic behind it but I am sooo old school about that stuff...
I forget exactly but there's a rough estimate on the .308 round for every 1000 feet in altitude add x amount of inches and swag it..

On the other hand if you are behind a rock in Afghanistan this stuff is critical to a good first shot hit.</div></div>

Colorado Rifle Club is at 5000ft, DA hovers around 8000 in the summer,

Whittington Center in Raton is 6600ft,

5800 isn't really a big deal.

 

[azimutha]

Re: Standard Atmosphere vs Actual Atmosphere

timelinex, if you're using a 100-yard zero, make sure you turn off the Zero Atmosphere in Ballistic. Then just focus on getting the right inputs into Current Atmosphere. It should read:

Enable Atmosphere [ON]
Altitude 0
BP 24.39 Hg.
Pressure is Absolute [ON]
Temp 84 F
Rel Hum 19.3

 

[ctressler]

Re: Standard Atmosphere vs Actual Atmosphere

Your right the pressure is absolute should be on.

I was at Ashfork the other day when you were going for the mile target, we gave you a ride up the hill. That was a lot of wind to be shooting in and it's tough anyway at a mile with a 308, so don't lose faith in the Ballistic program just yet. Come to the next match and I'll help you figure it out.

 

[cali_tz]

Re: Standard Atmosphere vs Actual Atmosphere

just acquired Kestrel 4000... be interesting to see how much more accurate that makes Shooter than using standard atmosphere.

 

[timelinex]

Re: Standard Atmosphere vs Actual Atmosphere

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ctressler</div><div class="ubbcode-body">Your right the pressure is absolute should be on.

I was at Ashfork the other day when you were going for the mile target, we gave you a ride up the hill. That was a lot of wind to be shooting in and it's tough anyway at a mile with a 308, so don't lose faith in the Ballistic program just yet. Come to the next match and I'll help you figure it out. </div></div>
Deal. Just went over to the dark side and bought a mil/mil scope, so will have to make new drop chart anyways(yea I know I can just convert moa to mil)

 

[ctressler]

Re: Standard Atmosphere vs Actual Atmosphere

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: cali_tz</div><div class="ubbcode-body">just acquired Kestrel 4000... be interesting to see how much more accurate that makes Shooter than using standard atmosphere. </div></div>

Have you been using standard atmosphere (station pressure/uncorrected pressure) alone for your calcs?