Calculating altitude MSL

[Salty]

I've got pressure altitude and baro, is this correct?

Amsl = Ap + (baro - 29.92) x 1000

Example:
Pressure Altitude: 1141
Baro: 30.03

Amsl = 1141 + (30.03 - 29.92) * 1000 = 1251

 

[Half Fast]

I've got pressure altitude and baro, is this correct?

Amsl = Ap + (baro - 29.92) x 1000

Example:
Pressure Altitude: 1141
Baro: 30.03

Amsl = 1141 + (30.03 - 29.92) * 1000 = 1251

Looks correct, with the caveat that the formula uses an approximation of 1 inch Hg = 1000 ft altitude, which is pretty close.

 

[Larry in TN]

I've got pressure altitude and baro, is this correct?

Amsl = Ap + (baro - 29.92) x 1000

Example:
Pressure Altitude: 1141
Baro: 30.03

Amsl = 1141 + (30.03 - 29.92) * 1000 = 1251

A quicker way, for me at least, to do this in my head, or with very minimal writing down, is converting the altimeter settings into "feet" by removing the decimal and adding a zero. i.e 29.92 becomes 29920. So 30.03 is 30030 - 29920 to get change in feet.

I would just write down the 30030 above the 29920 and subtract.

I remember the +/- by remembering that when I increase the altimeter setting, by turning the knob clockwise, the altitude increases.

We all think about these things differently, though, so that might work at all for you.

 

[midlifeflyer]

A quicker way, for me at least, to do this in my head, or with very minimal writing down, is converting the altimeter settings into "feet" by removing the decimal and adding a zero. i.e 29.92 becomes 29920. So 30.03 is 30030 - 29920 to get change in feet.

I would just write down the 30030 above the 29920 and subtract.

I remember the +/- by remembering that when I increase the altimeter setting, by turning the knob clockwise, the altitude increases.

We all think about these things differently, though, so that might work at all for you.

I like that.

I’m horrible at mental math so all those extra zeroes will be guaranteed to mess me up. A similar mental or pen/paper method is to drop the decimals and add a zero to the result.
3003-2992=11
Use 110.

OTOH, if you’re using a calculator, no reason not to use the decimals

 

[Salty]

A quicker way, for me at least, to do this in my head, or with very minimal writing down, is converting the altimeter settings into "feet" by removing the decimal and adding a zero. i.e 29.92 becomes 29920. So 30.03 is 30030 - 29920 to get change in feet.

I would just write down the 30030 above the 29920 and subtract.

I remember the +/- by remembering that when I increase the altimeter setting, by turning the knob clockwise, the altitude increases.

We all think about these things differently, though, so that might work at all for you.

I don’t think my computer cares. And on that note, what would be the more accurate formula rather than the approximation?

 

[luvflyin]

Why are we doing this calculating? I do it by setting baro in the box and reading the Altimeter. What am I missing here?

 

[Salty]

Why are we doing this calculating? I do it by setting baro in the box and reading the Altimeter. What am I missing here?

I have some telemetry that does not provide indicated altitude, but does provide pressure altitude and baro setting. This is not for in flight purposes.

 

[midlifeflyer]

I don’t think my computer cares. And on that note, what would be the more accurate formula rather than the approximation?

Pressure Altitude Correction = 145442.2*(1- (Baro/29.92126)^0.190261)

You'll still have to account for whether you add or subtract. This comes out with a negative number. If you use a sperad sheed, just wrap it in the abs function to geta a positive number if that's what you prefer.

 

[asicer]

I have some telemetry that does not provide indicated altitude, but does provide pressure altitude and baro setting. This is not for in flight purposes.

And it's not in contact with the ground in any way? Slant range and declination are not available?

 

[Salty]

Pressure Altitude Correction = 145442.2*(1- (Baro/29.92126)^0.190261)

You'll still have to account for whether you add or subtract. This comes out with a negative number. If you use a sperad sheed, just wrap it in the abs function to geta a positive number if that's what you prefer.

Where is pressure altitude in that formula? I’m guessing 145442.2

 

[Salty]

And it's not in contact with the ground in any way? Slant range and declination are not available?

The readings are taken in flight. I’m doing the calculations after the flight not during.

 

[Larry in TN]

I don’t think my computer cares.

I didn't see a reference to a computer (or calculator). I described my method for making such calculations in the cockpit.

 

[Salty]

I didn't see a reference to a computer (or calculator). I described my method for making such calculations in the cockpit.

No worries, just explaining my use case.

 

[midlifeflyer]

Where is pressure altitude in that formula? I’m guessing 145442.2

I'm not sure what you mean. All this is calculating is the correction based on non-standard baro, the full formula that the (baro - 29.92) x 1000 rule of thumb is based on. In your example, the rule of thumb gives you the 110' approximation; the full calculation gives you 100.418' In both cases, you need to add or subtract the result from the pressure altitude you say you already have.

The formula is from Ed Williams "Aviation Formulary." I don't pretend to understand it any more than I understand the "real" density altitude calculation using Kelvin (or 90% of the calculations Ed shows)

 

[Salty]

I'm not sure what you mean. All this is calculating is the correction based on non-standard baro, the full formula that the (baro - 29.92) x 1000 rule of thumb is based on. In your example, the rule of thumb gives you the 110' approximation; the full calculation gives you 100.418' In both cases, you need to add or subtract the result from the pressure altitude you say you already have.

The formula is from Ed Williams "Aviation Formulary." I don't pretend to understand it any more than I understand the "real" density altitude calculation using Kelvin (or 90% of the calculations Ed shows)

I needed this to understand what you posted:
P_alt= Ind_Alt + P_alt_corr

 

[midlifeflyer]

I needed this to understand what you posted:
P_alt= Ind_Alt + P_alt_corr

OK. Sorry. I just understood that if I am given P_Alt, the correction gets applied to it and if I have Ind_Alt, the correction gets applied to that, just like in the rule of thumb version, so I missed that you needed that.

Since you did, I guess I'll mention that in your case, whether the correction is positive or negative depends on whether Baro is higher or lower than standard and that the algebraic equation needs to be turned around to:

Ind_Alt = P_Alt ± P_alt_corr

since you already know P_Alt.

 

[Salty]

OK. Sorry. I just understood that if I am given P_Alt, the correction gets applied to it and if I have Ind_Alt, the correction gets applied to that, just like in the rule of thumb version, so I missed that you needed that.

Since you did, I guess I'll mention that in your case, whether the correction is positive or negative depends on whether Baro is higher or lower than standard and that the algebraic equation needs to be turned around to:

Ind_Alt = P_Alt ± P_alt_corr

since you already know P_Alt.

yeah, the link you sent was perfect. Thanks.