Range Report altitude adjustment ????

[ker2222]

I need some help with altitude adjustment

when adjusting for a change in altitude, can you just change the zero or is the entire trajectory effected. I have a bout a 2moa difference in my zero (200yrds) when moving between 800' and 3200'.

However, most of my shooting at the higher elevation is on steel so its hard for me to get a real solid feel for what is happening down range in terms of precision elevation corrections. So, if i add 2MOA to my zero, am I good to go throughout the trajectory, or has the actual trajectory changes?

 

[Lindy]

Re: altitude adjustment ????

Your zero does not change in that altitude range.

That's because the muzzle velocity is a function of temperature, not of altitude, and because if you have a zero at 100 yards or meters, that's too short a distance for it to be affected by the atmospheric drag.

At distances beyond 400 or 500 yards, your elevation will need correction, but any ballistic program will handle that.

That's why a short-range zero is simpler to deal with than an extended one.

 

[Outlaw45]

Re: altitude adjustment ????

So when he zeros at 800', then goes to 3200' his zero is still on?

But his range card out to longer ranges (400, 500, 600, etc) will be different from 800' and 3200', right? That's where the ballistics program helps out.

 

[Rhys]

Re: altitude adjustment ????

That would pretty well cover it. I wouldn't really expect that much of a change even at that. I normaly shoot at 4700 feet, and when going to sea level my change out to the 200 we were shooting to was still less than .1 mil.

 

[azimutha]

Re: altitude adjustment ????

If you moved out to a 400-600 yard zero, you would see a difference. I checked this out after consulting with the author of Ballistic and it didn't amount to enough with my loads (.2 mils @ 600) to give up all the advantages of a 100 yard zero. I was, incidentally, comparing a zero obtained at 34F, 30.39 hg, 5350' with shooting at 85F, 29.70 hg, 6000'). A more radical difference in altitude, e.g. 1350' vs. 6000' would make it more like .5 mils - enough to consider a change.

But it still didn't make sense to me because no matter what, the drop between 100 and 600 yards was always 3.9 mils. Stands to reason that if the 100 yard zero is indistinguishable from one atmosphere to another, then why worry? I'll be testing that out more, but right now still can't get out to a range of more than 300 yards because of the snow. (Not complaining, we need it more than I need to go shooting).

 

[landcbeitner]

Re: altitude adjustment ????

I live in an area where one day you may shoot at sealevel and the next you may be at 6000ft. If I know I may change elevation frequently I'll bring several drop charts for different elevations. To keep an altimiter accurate (not including GPS) you have to re-set it for weather changes... I don't like to do this. I pay more attention to barometric pressure (station pressure) than elevation per se. I have a chart attached to my portable weather station, so I know which chart to use or how to make approximate adjustment...

Elev/ft.....Standard Pressure
Sea Level...29.92
1000ft......28.86
2000ft......27.82
3000ft......26.82
4000ft......25.84
5000ft......24.90
6000ft......23.99
7000ft......23.10
8000ft......22.23
9000ft......21.39

It's worked well so far.

For my 260 I keep a 2000ft drop chart and on the back of my MOA ranging card I keep this...

2000ft chart..500yds.......1000yds
4000ft.......-1/4 MOA.....-1 MOA
6000ft.......-1/2 MOA.....-2 MOA

 

[Lindy]

Re: altitude adjustment ????

Station pressure may be substantially different from the standard pressure at a given altitude.

You might find this useful reading:

Barometric Pressure and Ballistic Software

 

[landcbeitner]

Re: altitude adjustment ????

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lindy</div><div class="ubbcode-body">Station pressure may be substantially different from the standard pressure at a given altitude.

You might find this useful reading:

Barometric Pressure and Ballistic Software
</div></div>

Exactly... my drop charts will say 2000ft... 3000ft.... 4000ft etc. I use the actual station pressure as mentioned to determine which drop chart is most appropriate... just because you're at 2000ft doesn't necessarily mean that's the best one to use (although it often is). When bad weather moves in (usually with a low pressure system) it's like being at higher elevation.

Or am I still missing something? It's been working well for me, I re-read my origional post again and I can't find anything wrong... open to suggestions though.

 

[Lindy]

Re: altitude adjustment ????

Air density is a function of three things: atmospheric pressure, temperature, and humidity.

At practical distances, humidity may be ignored. But both temperature and presssure must both be taken into consideration.

If you're not familiar with the concept of density altitude, you might find this useful: Making Ballistic Cards Using Density Altitude

 

[oneswithfunes]

Re: altitude adjustment ????

Great page Lindy.

 

[landcbeitner]

Re: altitude adjustment ????

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lindy</div><div class="ubbcode-body">
If you're not familiar with the concept of density altitude, you might find this useful: Making Ballistic Cards Using Density Altitude
</div></div>

Now <span style="font-weight: bold">that's</span> what I'm after right there.... thanks. I'm tring to avoid having to use a PDA in the field (or avoid having to rely on a PDA). It seems the guys who spend the most time geeking on PDA's and ballistic calculators at the matches are often the least likely to make hits at the longer ranges (just an observation).

 

[steve223]

Re: altitude adjustment ????

Heres a site that will convert sea level pressure to station pressure. but the problem is it gives the ststion pressure in millibars.
http://www.pages.drexel.edu/~brooksdr/DRB_web_page/Aerosols/pressure.htm
Steve

 

[gafferq2xl]

Re: altitude adjustment ????

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: steve223</div><div class="ubbcode-body">Heres a site that will convert sea level pressure to station pressure. but the problem is it gives the ststion pressure in millibars.
http://www.pages.drexel.edu/~brooksdr/DRB_web_page/Aerosols/pressure.htm
Steve </div></div>

Problem?? What Problem?? I don't see no stinking problem!!- http://www.onlineconversion.com/pressure.htm