Altitude vs. Barometric pressure
- Thread starter Michael Eichele
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Mikecr
Well-Known Member
This statement here: "Gravity is proportional to your elevation. It is decreasing as you move away from the center of the earth. The force of gravity is reduced by 1/R^2 where R represents your distance from the center of the earth" is a generalization, which does not hold true for us.
Earth is not uniform in density but changes in density with topography. With this, gravity is a local affect, with center of gravity changing from position to position. So if you reached 5000'ASL by standing on a rocky mountain, gravity would likely be a little higher there, due to higher density of the mountain, even while further from Earth's center.
A gyro stabilized platform, which normally points to near center of Earth, will tilt toward a mountain as you pass by it.
Since it's tiny and we have no way to practically measure it while hunting, we dismiss the variance.
Earth is not uniform in density but changes in density with topography. With this, gravity is a local affect, with center of gravity changing from position to position. So if you reached 5000'ASL by standing on a rocky mountain, gravity would likely be a little higher there, due to higher density of the mountain, even while further from Earth's center.
A gyro stabilized platform, which normally points to near center of Earth, will tilt toward a mountain as you pass by it.
Since it's tiny and we have no way to practically measure it while hunting, we dismiss the variance.
RTK
Well-Known Member
It affects both, Less molecules in the air to interact with in "High Density altitude".
You would think something with a High Density is more dense than something with low density, not true in this case.
Mikecr
Well-Known Member
It's not high density, but high altitude density (per standard conditions for altitude). Or another way to see it: (High) Density-Altitude.
Again, all of the confusion with it validates my assertion that DA holds little to no value in ballistics.
That's one matter
The other bigger matter is that DA is based on standard conditions per altitude, and it would be extremely rare for conditions (wherever you stand) to actually be at standard conditions. This means your conditions must be measured to know your DA (as calculated). But if you're measuring, it's logical to just enter what you measure in ballistic software (without extraneous calcs).
After all, if you don't understand this stuff, you will screw it up.
Again, all of the confusion with it validates my assertion that DA holds little to no value in ballistics.
That's one matter
The other bigger matter is that DA is based on standard conditions per altitude, and it would be extremely rare for conditions (wherever you stand) to actually be at standard conditions. This means your conditions must be measured to know your DA (as calculated). But if you're measuring, it's logical to just enter what you measure in ballistic software (without extraneous calcs).
After all, if you don't understand this stuff, you will screw it up.
RTK
Well-Known Member
DA in aviation is pressure altitude corrected for "non standard conditions".
For our needs just use pressure, temp and if wanted humidity. Use a chart or some other electronic means to figure out.
Most newer electronic range finders will figure all that out along with shooting angle.
For our needs just use pressure, temp and if wanted humidity. Use a chart or some other electronic means to figure out.
Most newer electronic range finders will figure all that out along with shooting angle.
That situation can be pretty hairy, I got the tee shirt
RTK
Well-Known Member
Us sea level dwellers don't have that problem Although I am very conscious of it when flying to Tahoe.
Although I am very conscious of it when flying to Tahoe.
Mikecr
Well-Known Member
Yes DA and PA are equal under standard conditions for altitude. And since standard conditions couldn't ever be relied on, the separate parameters must be measured -> for ballistics.
Those measured parameters can be used to calculate DA, but inside ballistic software, the individual parameters are better taken for their separate contributions to drag calcs & lookups. This, rather than forcing the software to rely solely on air density as a general term.
For instance, temperature and relative humidity affect Mach, and every lookup on a drag table is w/resp to Mach#. This individual attribute is independent of air density itself. So if you force software to set temperature based on DA, the Mach#s/Drag will be wrong all the way.
It won't affect a local G7 BC much, but it is another inaccuracy added.
Well said and explained.Thanks
This stuff is confusing and way over my pay grade...lol
VICTORNTN
Well-Known Member
Has anyone figured "air density" like pilots use?
Mikecr
Well-Known Member
You want to reference an industry that actually loses their most hi-tech commercial planes. They lose them!
Hey OnStar, where is MH370?
Hey OnStar, where is MH370?
VICTORNTN
Well-Known Member
Because they lost a few planes doesn't mean the science that keeps them up there is bad or flawed.
