Hornady 4DOF App vs. Kestrel

[derek1ee]

It appears that the 4DOF app and the Kestrel unit want slightly different input:

On the iOS app, it wants:

• Temperature
• Humidity
• Pressure
• Altitude
• Wind Speed / Direction
• Target Range / Direction

But on the 4DOF Kestrel unit:

• Temperature
• Humidity
• Density Altitude
• Latitude
• Wind Speed / Direction
• Target Range / Direction

My understanding is that density altitude is calculated using station pressure, temperature, and dew point (https://www.weather.gov/epz/wxcalc_densityaltitude), of of which is available on the unit.

On the iOS app though, all measurements except altitude is available when connected to a Kestrel meter. However, the altitude is not:

• The device can estimate the altitude if a reference baro value is entered, which one will need to find out via internet search / calling local airport.
• Or one can just look up the current altitude via iOS compass app / Google Earth
• The app will try to use the altitude reported from Kestrel, and will not try to get it from iPhone

The app also doesn't seem to need latitude info.

Question:

• Why does the 4DOF Kestrel meter not care about altitude, while the app does?
• Similarly, why does the 4DOF Kestrel meter want to know latitude, while the app doesn't?
• Why does the app not try to get altitude from the mobile device?

 

[6.5SH]

Latitude is under the "Earth Based Effects" menu button in the 4DOF phone app.

As to the other two questions perhaps @Kestrel Ballistics or @Ledzep might have answers.

 

[_Windrider_]

It appears that the 4DOF app and the Kestrel unit want slightly different input:

On the iOS app, it wants:

• Temperature
• Humidity
• Pressure
• Altitude
• Wind Speed / Direction
• Target Range / Direction

But on the 4DOF Kestrel unit:

• Temperature
• Humidity
• Density Altitude
• Latitude
• Wind Speed / Direction
• Target Range / Direction

My understanding is that density altitude is calculated using station pressure, temperature, and dew point (https://www.weather.gov/epz/wxcalc_densityaltitude), of of which is available on the unit.

On the iOS app though, all measurements except altitude is available when connected to a Kestrel meter. However, the altitude is not:

• The device can estimate the altitude if a reference baro value is entered, which one will need to find out via internet search / calling local airport.
• Or one can just look up the current altitude via iOS compass app / Google Earth
• The app will try to use the altitude reported from Kestrel, and will not try to get it from iPhone

The app also doesn't seem to need latitude info.

Question:

• Why does the 4DOF Kestrel meter not care about altitude, while the app does?
• Similarly, why does the 4DOF Kestrel meter want to know latitude, while the app doesn't?
• Why does the app not try to get altitude from the mobile device?

Because the kestrel is not capable of getting altitude

There are earth based effects tab in app

Because the mobile device may not be connected to the internet to get altitude. You should know the general altitude of where you are

 

[st1650]

Well it’s because the phone apps and the meter are using different methods to achieve the same thing. To get the most accurate solution we need air pressure, temperature and humidity. Your phone sensors most likely won’t have these but the kestrel does. What your phone has and the kestrel meter doesn’t is a 4D GPS fix (lat long plus altitude). From that information then the app can derive a density altitude/station pressure and humidity) from looking up the closest airport/weather station or use your phones baro sensor (if there’s one) and a manual temperature input to determine the solution - or if you enter the altitude manually it could derive a pressure from the difference between real altitude and GPS MSL altitude)
The kestrel meter doesn’t need to do any of that because it has it own pressure, temperature and humidity sensor.

The actual altitude doesn’t matter, what matters is the actual baro pressure which is what the kestrel measures as well as the temperature and humidity.
Density altitude is simply station pressure altitude corrected for temperature. When objects fly in the air, the two main variables are “how heavy the air column is pushing down (pressure) and how slippery the air is (humid vs dry, hot vs cold). For example, a jet taking off will perform better in cold dry air because the air is more dense. Bullets works the opposite way, they have less drag in low pressure/high altitude and hot and humid air.

So density altitude is used as a single variable instead of 2, for example maybe you’re using a basic non ballistic kestrel and you want to enter the environmentals in Strelok or another ballistic app you’d only have to enter density altitude instead of pressure and temperature to obtain an accurate solution.
Or say you use printed ballistic charts, it’s way easier to have 1 chart per 1000feet of DA vs having chart for different temps and pressure altitudes. Same idea for like ballistic turrets, you could have a turret set up for 0 DA for hunting in Florida and another for 8000’ DA for the Rockies or Alaska.

Latitude doesn’t really matter either, it’s most likely there as an input to calculate so-called earth rotational effects such as coriolis. The solution change amounts to 0.1 MPH wind input so unless you’re shooting in a wind tunnel you shouldn’t loose much sleeps about latitude inputs in your calculator.

Latitude can be grabbed directly from the phone geolocation so that’s why the app doesn’t ask you for the input but the meter doesn’t have its own GPS and the Bluetooth protocol is not passing that information to the meter when it’s calculating a solution.

 

[_Windrider_]

Well it’s because the phone apps and the meter are using different methods to achieve the same thing. To get the most accurate solution we need air pressure, temperature and humidity. Your phone sensors most likely won’t have these but the kestrel does. What your phone has and the kestrel meter doesn’t is a 4D GPS fix (lat long plus altitude). From that information then the app can derive a density altitude/station pressure and humidity) from looking up the closest airport/weather station or use your phones baro sensor (if there’s one) and a manual temperature input to determine the solution - or if you enter the altitude manually it could derive a pressure from the difference between real altitude and GPS MSL altitude)
The kestrel meter doesn’t need to do any of that because it has it own pressure, temperature and humidity sensor.

The actual altitude doesn’t matter, what matters is the actual baro pressure which is what the kestrel measures as well as the temperature and humidity.
Density altitude is simply station pressure altitude corrected for temperature. When objects fly in the air, the two main variables are “how heavy the air column is pushing down (pressure) and how slippery the air is (humid vs dry, hot vs cold). For example, a jet taking off will perform better in cold dry air because the air is more dense. Bullets works the opposite way, they have less drag in low pressure/high altitude and hot and humid air.

So density altitude is used as a single variable instead of 2, for example maybe you’re using a basic non ballistic kestrel and you want to enter the environmentals in Strelok or another ballistic app you’d only have to enter density altitude instead of pressure and temperature to obtain an accurate solution.
Or say you use printed ballistic charts, it’s way easier to have 1 chart per 1000feet of DA vs having chart for different temps and pressure altitudes. Same idea for like ballistic turrets, you could have a turret set up for 0 DA for hunting in Florida and another for 8000’ DA for the Rockies or Alaska.

Latitude doesn’t really matter either, it’s most likely there as an input to calculate so-called earth rotational effects such as coriolis. The solution change amounts to 0.1 MPH wind input so unless you’re shooting in a wind tunnel you shouldn’t loose much sleeps about latitude inputs in your calculator.

Latitude can be grabbed directly from the phone geolocation so that’s why the app doesn’t ask you for the input but the meter doesn’t have its own GPS and the Bluetooth protocol is not passing that information to the meter when it’s calculating a solution.

If the actual altitude doesn't matter then explain to me why in every DA chart ive ever used requires me to know my pressure altitude?

The reason is because you don't know what standard is where you are unless you know your ALTITUDE/ELEVATION. You don't have to be exact, +- 500 ft is more than adequate, even within 1k is fine. But elevation absolutely matters

 

[st1650]

If the actual altitude doesn't matter then explain to me why in every DA chart ive ever used requires me to know my pressure altitude?

View attachment 8408172

The reason is because you don't know what standard is where you are unless you know your ALTITUDE/ELEVATION. You don't have to be exact, +- 500 ft is more than adequate, even within 1k is fine. But elevation absolutely matters

I meant altitude doesn’t matter to compute a solution through the kestrel because of the onboard baro pressure / temperature and humidity sensor will give you an accurate DA.

But say you are using your phone as a ballistic calculator then yes you’ll need a pressure altitude which you can either derive from a GPS fix which gives you a MSL altitude or paper topo map and then use the temperature (and altimeter setting if you have it) to find your DA.

 

[derek1ee]

Well it’s because the phone apps and the meter are using different methods to achieve the same thing. To get the most accurate solution we need air pressure, temperature and humidity. Your phone sensors most likely won’t have these but the kestrel does. What your phone has and the kestrel meter doesn’t is a 4D GPS fix (lat long plus altitude). From that information then the app can derive a density altitude/station pressure and humidity) from looking up the closest airport/weather station or use your phones baro sensor (if there’s one) and a manual temperature input to determine the solution - or if you enter the altitude manually it could derive a pressure from the difference between real altitude and GPS MSL altitude)

I should clarify that I'm comparing between using iOS App paired with Kestrel vs. Kestrel itself with the 4DOF solver built-in.

While you're correct that the phone has GPS, my point is that the app doesn't use it (it doesn't even request permission to use it).

The kestrel meter doesn’t need to do any of that because it has it own pressure, temperature and humidity sensor.

The actual altitude doesn’t matter, what matters is the actual baro pressure which is what the kestrel measures as well as the temperature and humidity.

But according to Hornady, it does though. And it's a required input on both the web app as well as their iOS app, but not required in the Kestrel w/ 4DOF solver.

"Height above sea level. Unlike other ballistics programs, altitude and pressure must both be input into the 4 DOF. Based on altitude and pressure value, a custom table is built within the program to properly model atmospherics."

 

[Baron23]

Ah, it does? My Kestrel with 4DOF want SP…station pressure which along with temperature and humidity from which it can calculate density altitude.

My 4DOF app on my phone (to me, much better interface) does want altitude (MSL) but if you go on Hornady’s 4DOF site, the technical document, it says they do so in order to modify the shag curve due to pressure drop as the projectile climbs and descends along its flight path.

To me, this is a waste of code unless you are shooting very ELR and launching a bullet upward like a mortar round.

 

[robb_va]

Ah, it does? My Kestrel with 4DOF want SP…station pressure which along with temperature and humidity from which it can calculate density altitude.

My 4DOF app on my phone (to me, much better interface) does want altitude (MSL) but if you go on Hornady’s 4DOF site, the technical document, it says they do so in order to modify the shag curve due to pressure drop as the projectile climbs and descends along its flight path.

To me, this is a waste of code unless you are shooting very ELR and launching a bullet upward like a mortar round.

I think the the 4DOF app calculates the station pressure from what you enter for altitude unless you bypass it and manually and enter the station pressure. Go into the app and enter 0 for the altitude (sea level) and note the pressure. It will be 29.92. Sea level corrected pressure. Change the altitude to 3,000 feet and the app will change the pressure to 26.81. That is pretty close to what you will get if you calculate it - [760-(altitude X .026)] divided by 760 and multiplied by sea level pressure of 29.92.

the technical document, it says they do so in order to modify the shag curve due to pressure drop as the projectile climbs and descends along its flight path.

If you bypass the altitude input, and enter the station pressure you can input 0 or 10,000 for altitude and it does not change the come up projection.

 

[_Windrider_]

I think the the 4DOF app calculates the station pressure from what you enter for altitude unless you bypass it and manually and enter the station pressure. Go into the app and enter 0 for the altitude (sea level) and note the pressure. It will be 29.92. Sea level corrected pressure. Change the altitude to 3,000 feet and the app will change the pressure to 26.81. That is pretty close to what you will get if you calculate it - [760-(altitude X .026)] divided by 760 and multiplied by sea level pressure of 29.92.



If you bypass the altitude input, and enter the station pressure you can input 0 or 10,000 for altitude and it does not change the come up projection.

2 things. First 29.92 is standard atmosphere at sea level, not corrected. If it’s corrected it would be barometric and not station. Second, that formula is correct but way harder than it needs to be. How about 1” per 1000 feet.

 

[robb_va]

2 things. First 29.92 is standard atmosphere at sea level, not corrected. If it’s corrected it would be barometric and not station. Second, that formula is correct but way harder than it needs to be. How about 1” per 1000 feet.

Points taken. I was only trying to point out that you either have to use a GPS device to determine elevation or rely on a pressure sensor from a device to get station pressure. Either will work. I have used both the 4DOF and AB/Kestrel solver solutions out to fair distances with pretty good results.

 

[robb_va]

It appears that the 4DOF app and the Kestrel unit want slightly different input:

On the iOS app, it wants:

• Temperature
• Humidity
• Pressure
• Altitude
• Wind Speed / Direction
• Target Range / Direction

But on the 4DOF Kestrel unit:

• Temperature
• Humidity
• Density Altitude
• Latitude
• Wind Speed / Direction
• Target Range / Direction

My understanding is that density altitude is calculated using station pressure, temperature, and dew point (https://www.weather.gov/epz/wxcalc_densityaltitude), of of which is available on the unit.

On the iOS app though, all measurements except altitude is available when connected to a Kestrel meter. However, the altitude is not:

• The device can estimate the altitude if a reference baro value is entered, which one will need to find out via internet search / calling local airport.
• Or one can just look up the current altitude via iOS compass app / Google Earth
• The app will try to use the altitude reported from Kestrel, and will not try to get it from iPhone

The app also doesn't seem to need latitude info.

Question:

• Why does the 4DOF Kestrel meter not care about altitude, while the app does?
• Similarly, why does the 4DOF Kestrel meter want to know latitude, while the app doesn't?
• Why does the app not try to get altitude from the mobile device?

The Kestrel is calculating the solver projection based on density altitude it calculates that from the Kestrel meter reading (temp, humidity and station pressure). It doesn't need the altitude input to back into the station pressure because the meter has captured that (the station pressure). Look at your altitude input in iOS 4DOF, as you change the altitude input, the station pressure changes. That program is calculating your station pressure based on the altitude you input. The app has no way to read or capture station pressure so it is relying on you to input either it or the altitude.

The 4DOF app has a tab "Earth Based Effects" for you to input the latitude.

The 4DOF app doesn't reach out to your phone to obtain the altitude. Your iOS compass app will provide you an approximate altitude that you can input into the solver.

 

[derek1ee]

It doesn't need the altitude input to back into the station pressure because the meter has captured that (the station pressure).

That's what I thought but the tooltip shown, "Unlike other ballistics programs, altitude and pressure must both be input into the 4 DOF." Based on your explanation it should need either/or not both?

Look at your altitude input in iOS 4DOF, as you change the altitude input, the station pressure changes. That program is calculating your station pressure based on the altitude you input. The app has no way to read or capture station pressure so it is relying on you to input either it or the altitude.

If you bypass the altitude input, and enter the station pressure you can input 0 or 10,000 for altitude and it does not change the come up projection.

This is interesting and I didn't notice that changing altitude affects the pressure value before. Looks like the inverse isn't true (changing pressure doesn't affect altitude value). But the most important thing is, like you said, for a given pressure value, changing altitude doesn't actually affect the result. This makes me think that the tooltip about needing both altitude and pressure isn't really accurate.

Thinking about this more and this make sense - the bullet "feels" pressure (which is related to altitude and other things), but it can't "feel" altitude.

The reason I started the thread is... When I pair the Kestrel meter with Hornady iOS app, the app will retrieve both station pressure and altitude reading from the meter. The station pressure will always reflect the actual, uncorrected reading where the meter is at, but the altitude reading won't be accurate without either manually setting it, or by setting a reference baro value.

Because of this, I was wondering if I need to either make sure to manually set it correctly in the meter, or overwrite the altitude reading in the app. But based on the finding above, it appears that I can just ignore the altitude reading since it will not actually affect the result.

 

[robb_va]

That's what I thought but the tooltip shown, "Unlike other ballistics programs, altitude and pressure must both be input into the 4 DOF." Based on your explanation it should need either/or not both?





This is interesting and I didn't notice that changing altitude affects the pressure value before. Looks like the inverse isn't true (changing pressure doesn't affect altitude value). But the most important thing is, like you said, for a given pressure value, changing altitude doesn't actually affect the result. This makes me think that the tooltip about needing both altitude and pressure isn't really accurate.

The reason I started the thread is... When I pair the Kestrel meter with Hornady iOS app, the app will retrieve both station pressure and altitude reading from the meter. The station pressure will always reflect the actual, uncorrected reading where the meter is at, but the altitude reading won't be accurate without either manually setting it, or by setting a reference baro value.

Because of this, I was wondering if I need to either make sure to manually set it correctly in the meter, or overwrite the altitude reading in the app. But based on the finding above, it appears that I can just ignore the altitude reading since it will not actually affect the result.

When I was using the app, I didn't link the meter. I just input my environmental data from the meter manually and left the altitude at zero.

 

[robb_va]

That's what I thought but the tooltip shown, "Unlike other ballistics programs, altitude and pressure must both be input into the 4 DOF." Based on your explanation it should need either/or not both?





This is interesting and I didn't notice that changing altitude affects the pressure value before. Looks like the inverse isn't true (changing pressure doesn't affect altitude value). But the most important thing is, like you said, for a given pressure value, changing altitude doesn't actually affect the result. This makes me think that the tooltip about needing both altitude and pressure isn't really accurate.

Thinking about this more and this make sense - the bullet "feels" pressure (which is related to altitude and other things), but it can't "feel" altitude.

The reason I started the thread is... When I pair the Kestrel meter with Hornady iOS app, the app will retrieve both station pressure and altitude reading from the meter. The station pressure will always reflect the actual, uncorrected reading where the meter is at, but the altitude reading won't be accurate without either manually setting it, or by setting a reference baro value.

Because of this, I was wondering if I need to either make sure to manually set it correctly in the meter, or overwrite the altitude reading in the app. But based on the finding above, it appears that I can just ignore the altitude reading since it will not actually affect the result.

I reached out to Hornday in 2021-

I have another question for you. I am starting to use the 4DOF program. The program wants both the uncorrected pressure and altitude. Why does it need the altitude if the Baro input is the uncorrected pressure and not corrected pressure?

I ran two calculations tonight for a 250 ATIP at a MV of 3009 and calculated dial up at 1,987 yards-

The first one I ran, I used an uncorrected pressure of 28.43 and an altitude of 0. Dial up was 54.76
The second one I ran, I used an uncorrected pressure of 28.43 and an altitude of 15,000. Dial up was 54.82.

It doesn't look like it matters as long as you are using the uncorrected pressure. Is this just a glitch?
Response -

I recommend you read through this document for the information. However if you do not see what the tolerance variations you are seeing, then we may need you to send a email into our contact us in the tech side so it can be forwarded to the lead on the 4 DOF system for a better answer.

 

[GAl08]

The nice thing about a Kestrel is its a stand alone unit and is bullet proof compared to a cell phone and it will always work. A cell phone is hit and miss, they lock up, doesn't load, battery dies quick. As far as which ballistic software I would go AB.

 

[Ledzep]

Question:

• Why does the 4DOF Kestrel meter not care about altitude, while the app does?
• Similarly, why does the 4DOF Kestrel meter want to know latitude, while the app doesn't?
• Why does the app not try to get altitude from the mobile device?

1. They're both looking for air density and temperature in similar but different ways. I'm pretty sure that the "Altitude" input in the mobile device is a handy dandy tool to get you 'altitude-corrected STP' pressure. When you input your altitude it gets you close to the station pressure, then if you have actual station pressure you enter that. I haven't found a case where changing the altitude while maintaining the station pressure (and all other parameters) causes a change in the solution. The solver doesn't need you to input the altitude to adjust for gradient air density for high-trajectory or high-angle shots.

2. The App will ask for latitude if you turn on earth-based effects. Generally, I suggest keeping earth-based effects turned off because most people aren't shooting far enough for coriolis to make significant changes on the solution, and it serves to cause inaccuracies if you neglect to update latitude and azimuth of fire for each target/location. It is more accurate with it turned on, but requires more work that a lot of people forget to do which ends up causing problems. Definitely use it if you're primarily shooting 1000yd+. YMMV.

3. Do you mean pull info from the GPS location of the phone? That might be a feature that could be added that would require location permissions from the user. I can run that up the chain.


The altitude itself isn't so much important as the barometric pressure and I think in general it's purpose in the app is mostly for folks that might not have access to a barometer but know generally what altitude they're at. I could be wrong here but that is my understanding. I'll ask Jayden and verify.

 

[6.5SH]

3. Do you mean pull info from the GPS location of the phone? That might be a feature that could be added that would require location permissions from the user. I can run that up the chain.


The altitude itself isn't so much important as the barometric pressure and I think in general it's purpose in the app is mostly for folks that might not have access to a barometer but know generally what altitude they're at. I could be wrong here but that is my understanding. I'll ask Jayden and verify.

FWIW the Android version of the app does ask for Location permissions already. It just doesn't seem able to populate that info.

 

[Baron23]

I think the the 4DOF app calculates the station pressure from what you enter for altitude unless you bypass it and manually and enter the station pressure. Go into the app and enter 0 for the altitude (sea level) and note the pressure. It will be 29.92. Sea level corrected pressure. Change the altitude to 3,000 feet and the app will change the pressure to 26.81. That is pretty close to what you will get if you calculate it - [760-(altitude X .026)] divided by 760 and multiplied by sea level pressure of 29.92.



If you bypass the altitude input, and enter the station pressure you can input 0 or 10,000 for altitude and it does not change the come up projection.

Hi Robb - well, I think we agree on some things but I think we may not agree on all.

First, we both seem to agree that the Kestrel 5700 w-4DOF does not use altitude in any way for ballistic solutions. It does use temp, station pressure, and humidity to calculate density altitude which is pretty much all that's relevant to a ballistic solution.

Kestrel does have in its weather portion altitude and baro pressure. This seems to be there to allow one to calculate changes by making one or the other as primary and the other calculated thereof (see below).

From Kestrel docs:

​

"For Kestrel 4500 Ballistic and 5700 Ballistic meters, the ballistics calculator will ALWAYS use station pressure despite the altitude number that is entered.​

• If you want station pressure, set your reference altitude to ZERO on the Baro screen (4500 Series) or just view the Pressure screen (5000 Series).
• If you want your altitude to display correctly and have station pressure, you will need to enter your current barometric pressure as the reference barometric pressure. The reference altitude should remain ZERO, and Sync Alt should be OFF."

So, for our ballistic solution purposes, altitude in the Kestrel is not relevant. My 5700 measures temp, humidity, and station pressure and so I'm good to go. Right?

Now to the 4DOF app.

Yes, the app has a field for altitude (MSL) but as far as I can ferret out, its for a very specific purpose and not applicable unless you have a very high max ordinance in...for example...ELR...very ELR or shooting at very high angles (mountain goats maybe??).

This is what their tech doc says:

Most ballistic programs available today compute an air density value based on the input​

atmospheric conditions (note added by me...this means temp, humidity, and SP, right?). This air density is then assumed for all predictions of the projectile’s flight. The Hornady 4 DOF creates an internal altitude based atmospheric table of conditions based on the provided inputs and accounts for the air density changes at the projectile’s altitude during flight (underlined added by me). Changes in air density during the bullet’s flight can become important if long range high angle shooting is conducted. A 1000 yard shot at a 30 degree angle results in a 1,500 foot change in altitude from launch to target. It is very important to know or have a very good estimate of the firing altitude in combination with atmospheric condition​

And yes, in the apps environmental section, if you vary the altitude input (which is the first input) then it changes the pressure reading shown in the next field down. HOWEVER, set your altitude and THEN modify the pressure and the altitude does NOT change. That is, they are not tied together in both directions. Change altitude, pressure changes. Change pressure, altitude does not change.

I have zero idea wtf they did this and I am tending to view this as a coding mistake. OR, it could be like the Kestrel weather function and provide a way to see SP value with a change in altitude....but not like the Kestrel which will show you a change in altitude with changing pressure. But, I still just don't see the utility of this.

Also, varying station pressure with altitude implies an standard atmosphere pressure lapse rate and I see absolutely zero reason to assume this is true. In order to calculate a change in pressure with a change in altitude, there has to be an assumed lapse rate.

This is the way I see it. If I'm wrong somewhere in this, please do point it out.

Cheers and this has been an interesting discussion.

 

[Baron23]

Applied Ballistics without a doubt. My Kestrel 5700E has always been spot on with AB. 4DOF is fine if you only shoot Hornady factory ammo, for everyone else, you need AB.

I have not found that to be true. Example, my Berger 6.5 140 HT factory ammo is in their 4DOF listing of ammo.

 

[Ledzep]

....

I have zero idea wtf they did this and I am tending to view this as a coding mistake. OR, it could be like the Kestrel weather function and provide a way to see SP value with a change in altitude....but not like the Kestrel which will show you a change in altitude with changing pressure. But, I still just don't see the utility of this.
...

Again, I'm pretty sure it's a stop-gap tool for the user that doesn't have a barometer but knows their general altitude. Plug in altitude and it auto-populates barometric Station pressure for a "standard" day at that altitude. When you manually input baro, altitude doesn't play a part in the solver that I can see, even at high altitude and high angle trajectories. The solver, as near as I can tell, uses the barometric Station pressure to get air density and uses that as the reference point, not the manual altitude input, for the density gradient for high angle or high max ordinate.

 

[Baron23]

Uh, you mean when you ,manually enter station pressure? Below is from the 4DOF web site and the ? in the circle on the app says similar...SP.

I'm pretty sure it's a stop-gap tool for the user that doesn't have a barometer but knows their general altitude. Plug in altitude and it auto-populates barometric pressure for a "standard" day at that altitude.

I guess that's a reasonable work around for not having SP available. Sort of.

The solver, as near as I can tell, uses the barometric pressure to get air density and uses that as the reference point, not the manual altitude input, for the density gradient for high angle or high max ordinate.

Again, I'm guessing that you really meant station pressure. And yes, it seems reasonable that if you have SP, temp, and humidity (everything needed for DA), and you use the shooting angle function in the app, that it can then calculate a solution even accounting for a high angle/high increase in altitude shot by using standard atmosphere lapse rates.

This is all pretty much down in the noise for me but its fun to try to understand the 4DOF software better. Mostly I take the environmentals from my Kestrel and enter them into the app on my phone as I find the interface to be easier/more clear.

But I do also have the guns built in the Kestrel and can use that also.

I appreciate the information and expertise you bring to the forum. Please continue to do so.

 

[Ledzep]

Yeah yeah my bad. Station pressure.

 

[bulletbluesky]

Station pressure, temperature and humidity inputs are required for calculating air density, which is the most important air parameter for drag force calculation. However, point mass apps I know of assume the drag coefficient is independent of humidity alone. This is quite correct in supersonic flow regime where pressure drag is dominant. However, what about subsonic region where sheer stress (frictional drag) might be important component of overall drag: does Hornady 4DOF account viscosity effects changes?