Altitude for ballistic input

[ICLIMB]

Will altitude obtained from a GPS differ when inputting into your ballistic program the altitude obtained from a pressure reading i.e. a kestrel or some type of device. Density altitude vs gps altitude.
Will it output a different reading when entered in say, shooter.
Which is best to use (if different)

Does that qualify as a stupid question?
Thanks

 

[_Windrider_]

Will altitude obtained from a GPS differ when inputting into your ballistic program the altitude obtained from a pressure reading i.e. a kestrel or some type of device. Density altitude vs gps altitude.
Will it output a different reading when entered in say, shooter.
Which is best to use (if different)

Does that qualify as a stupid question?
Thanks

GPS altitude is only correct when receiving a WAAS signal which is a corrected signal from the satellite.

I use absolute pressure as measured from my kestrel. Your altitude isn't as important so much as knowing what pressure you are shooting in.

For example, standard atmosphere is sea level, 29.92 inHg and 15 C. There are standard lapse rates that can be used to ballpark pressure based on your altitude. But a more accurate way is to use the absolute pressure you get from your kestrel or similar device and then correct it for the actual temperature. That way I don't care if I am at 10,000 ft or 1,000 ft. If the pressure is 29.65 and 10 degrees, at both altitudes, the dope should be the same. That is obviously just an illustration because if the pressure is that high at 10k, we have problems.


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[WaltHer]

To elaborate on what td said above: a waas corrected gps signal will give you the most accurate reading for true elevation (ASL=Above Sea Level). Your standard, non corrected, kestrel will give you an elevation reading that is close enough for government work. That being said, most Kestrel users will recalibrate their devices barometric pressure setting to"0" feet ASL in the settings menu so that the device gives them Station Pressure (true pressure at the device) instead of Barometric Pressure (which is corrected for elevation).
My best recommendation though is to upgrade to the Kestrel that gives you a Density Altitude (synonymous with Pressure Altitude) reading (which is not your physical altitude, but a mathmatical combination of altitude, air pressure, temperature, and humidity.) Then simply enter your DA into your ballistic solver if it accepts DA. Dont enter the DA into the ASL Elevation box by mistake. This procedure is faster than entering all the variables independently, and is a more meaningful way to track your meteorological/environmental data. You may consider setting your humidity setting to 50% to just split the difference, since humidity will impact your firing solution the least. Hope this helps. Walt.
All that being said, unless you are shooting elr distances in dramatically different conditions, you probably won't experience much difference from shot to shot.

 

[_Windrider_]

Just one point to clarify, Humidity has nothing to do with Density Altitude.

Density Altitude is pressure altitude corrected for non standard temperature.

In my day job, we use some back of the napkin math to get these numbers:

pressure altitude = (standard pressure - your current pressure setting) x 1,000 + field elevation (or in our case, the firing position altitude)
density altitude = pressure altitude + [120 x (OAT - ISA Temp)].......ISA temp is 15C with a 2 degree lapse rate for every 1,000ft in elevation increase. So if you are at 6000 ft pressure altitude the ISA temp is 3C.

The bottomline, you cant get an accurate density altitude unless you know the pressure altitude you are firing from. But with the absolute pressure (aka Station pressure, that is the uncorrected pressure where you are at) and the current temperature, you are getting the exact same result. For you and your GPS, you can use the GPS altitude as the field elevation and get pretty damn close using the two formulas above if you choose to do it the way WaltHer does it above.

In the end, it matters not. as long as you understand the output and you are consistent in your methodology, the results are the same.

as far as humidity, the 50% rule is what I use as well.

 

[KeeblerArmy]

Slightly off topic question, how close should the altitude reading be from a Kestrel compared to GPS? I know station pressure is the way to go for ballistics, but my kestrel is giving me an altitude of 1170 where I sit right now and every other source I can find on the internet and a hand held GPS all have me at approximately 1350. Such a large discrepancy makes me have trust issues with my kestrel.

 

[_Windrider_]

Slightly off topic question, how close should the altitude reading be from a Kestrel compared to GPS? I know station pressure is the way to go for ballistics, but my kestrel is giving me an altitude of 1170 where I sit right now and every other source I can find on the internet and a hand held GPS all have me at approximately 1350. Such a large discrepancy makes me have trust issues with my kestrel.

This is why I use absolute pressure. It doesnt matter what your altitude is, only what the density of the air is at your location.

But to answer the question about why the difference between the Kestrel and GPS, I would need to know what info is being put into the kestrel. Are you sure you arent looking at density alitude? Is it colder than normal there right now?

 

[KeeblerArmy]

Altitude for ballistic input

Th model I have is a 2500 so it doesn't compute DA as far as I know.

 

[_Windrider_]

Th model I have is a 2500 so it doesn't compute DA as far as I know.

The only way a kestrel can calculate a correct altitude is for you the user to input a correct reference altitude. If you put that gps altitude in and followed theses instructions right out of the manual, I bet it will be correct. It's because we don't always know our exact altitude that using station pressure or absolute pressure is better in the field.

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[_Windrider_]

Th model I have is a 2500 so it doesn't compute DA as far as I know.

You could also set your baro pressure to 29.92 and then see what your altitude reads. It should be pretty close to the GPS.


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[KeeblerArmy]

The only way a kestrel can calculate a correct altitude is for you the user to input a correct reference altitude. If you put that gps altitude in and followed theses instructions right out of the manual, I bet it will be correct. It's because we don't always know our exact altitude that using station pressure or absolute pressure is better in the field.

Reference barometric pressure you mean? I put in the baro pressure from noaa.gov as my reference pressure on the altitude screen and got a much more accurate altitude reading. I'll double check it with my GPS when I get off of work.

Thanks for the help

 

[KeeblerArmy]

You could also set your baro pressure to 29.92 and then see what your altitude reads. It should be pretty close to the GPS.


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That is what I was doing and was getting the funky altitude readings. Using the corrected baro pressure from NOAA as my reference pressure gave me a better altitude reading, I think

 

[_Windrider_]

That is what I was doing and was getting the funky altitude readings. Using the corrected baro pressure from NOAA as my reference pressure gave me a better altitude reading, I think

Things to consider when comparing altitudes/elevation between devices is what kind of altitude am i actually getting

Thin about the various altitudes/elevations this way:

1. True altitude - this is your height above MSL (Mean Sea Level). On the ground, we call this elevation. This is what your GPS is giving you as it has no way to compemsate for pressure, unless you have one with that capability.
2. Pressure altitude - this is what you get when 29.92 inHg is set as your reference baro.
3.Density Altitude - pressure altitude corrected for non-standard temperature.

My guess as to why the dfference between the Kestrel and the GPS, one has a pressure correction and the other does not.

 

[KeeblerArmy]

Things to consider when comparing altitudes/elevation between devices is what kind of altitude am i actually getting

Thin about the various altitudes/elevations this way:

1. True altitude - this is your height above MSL (Mean Sea Level). On the ground, we call this elevation. This is what your GPS is giving you as it has no way to compemsate for pressure, unless you have one with that capability.
2. Pressure altitude - this is what you get when 29.92 inHg is set as your reference baro.
3.Density Altitude - pressure altitude corrected for non-standard temperature.

My guess as to why the dfference between the Kestrel and the GPS, one has a pressure correction and the other does not.

Thanks for that. I didn't know there was a difference between true altitude and pressure altitude.

So is pressure altitude used for anything?

 

[_Windrider_]

We use it in aviation and you need it if you are trying to calculate DA by hand.

In the shooting world, it's not as relevant.


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