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Now, to solve for true altitude, we need to know a few additional items. We need to know that: ISA at the surface is 15°C; Standard lapse rate is -2°C per thousand feet; To find true altitude, the difference from indicated altitude is 4 ft per 1°C deviation from ISA for every 1,000 ft; Knowing all this, we can calculate the following:
You need to correct your pressure altitude (FL85) for the barometer setting QNH first before you apply the temperature correction. I assume a 30ft / hPa offset here: $\textrm{barometric altitude} = \textrm{pressure altitude} + (\textrm{QNH} - 1013.25)\cdot 30 = 8500 -907.5 = 7592.5 \textrm{ft}$
Learn how to use lines and shapes to check distances and estimate sizes of different features on Earth. Find out how to save, open, and edit measurements, and how to find a path’s elevation profile in Google Earth Pro.
There are two separate answers to this question, depending on what altitude the flight is conducted. At lower altitudes, below the transition altitude (18,000' in the US, but it varies from country to country), it is the responsibility of the pilot to receive the local "altimeter setting" -- which is the local barometric pressure, corrected to sea level.
How to calculate density altitude using this chart? Are there more different ways Motorola 68000 A-line and F-line traps were used than the Classic Mac way and the Lisa way? My client keeps changing password and denying me access before the project is fully complete.
You will notice that all aircraft performance calculations require pressure altitude and temperature and in effect is calculating density altitude. To calculate pressure altitude you will calculate $$ \left( 29.92 - \frac{\mathrm{current~altimeter~setting}}{\mathrm{inHg}} \right) \cdot 1000 \, \mathrm{ft} + \mathrm{current~field~elevation}$$
According to that formula, I calculated pressure altitude difference for 28.92 and it's 938 feet, as opposed to 1000 feet. Additionally, as you can tell from the formula, the relationship is not a constant linear value. So difference of 1 inch of HG is not always 938 feet, it's changing. So, the FAA guide is accurate.
The $300ft/NM$ used to calculate your height above the runway at a particular distance is just an approximation that is easy to remember, and is close enough for our purposes. If you want a more exact number, you can use some trigonometry:
If your barometric altitude were 2000 ft at an altimeter setting of 29.70", then you would first need to calculate your pressure altitude. This would be done using the table on the right. The correction value of 205 needs to be added to the barometric altitude to convert to a standard setting of 29.92". This would yield a pressure altitude of ...
If you want to know what your altitude is at a certain distance then you can reverse the calculation. Divide the distance by 3. So, 10 nm is (10 ÷ 3) = 3.3. Multiply this by 1000 which will give you an altitude of 3300 ft. You can also use trigonometry. That is a triangle with a 3 degree angle with 10 nm as adjacent, we can find the opposite.