The 5 Types of Altitude in Aviation: Indicated, True, Absolute, Pressure, and Density Explained
- Nathan Hodell
- Aug 27, 2025
- 10 min read
Updated: May 3
If a controller tells you to "maintain 7,000 feet," what altitude is that? If your altimeter shows 7,000 feet but it's an unusually cold day, are you actually at 7,000 feet? When you're flying over a 6,000-foot mountain at 7,000 feet on the altimeter, how much terrain clearance do you really have?
These are the questions the five altitude types answer. Pilots need a clear mental model for what each altitude means, when each one matters, how they relate to each other, and how to convert between them. This isn't just academic — choosing the wrong altitude reference can put you into terrain, get you in trouble with ATC, or cause performance miscalculations on takeoff.
This post covers the five altitude types as a complete system: indicated, true, absolute, pressure, and density altitude — what each means, how to calculate each, how they relate, and when each matters in real-world flying.
Study this full length lesson (video, podcast, flashcards, and quiz) here: Full Length Lesson >
Why Multiple Altitude Types Exist
The atmosphere doesn't have a single "altitude" — it has multiple ways of measuring height depending on what reference you're using:
Surface elevation as the reference: How high above the ground are you?
Sea level as the reference: How high above mean sea level are you?
Pressure as the reference: What altitude does your pressure correspond to?
Density as the reference: What altitude does your air density match?
Just what the altimeter reads: What does the instrument show?
Each reference is useful in specific situations. Mixing them up creates confusion and danger. The five altitude types are essentially a vocabulary for talking precisely about height in different contexts.
Indicated Altitude: What the Instrument Reads
Indicated altitude is exactly what the altimeter displays when set to the local altimeter setting.
How to obtain it:
Set the local altimeter setting in the Kollsman window (e.g., 30.05 inHg)
Read the altitude shown on the altimeter
That number is indicated altitude
When indicated altitude is accurate:
When the local altimeter setting is correct
When atmospheric conditions are at standard (15°C at sea level, 2°C/1,000 ft lapse rate)
For most everyday flying, indicated altitude is approximately equal to true altitude
When indicated altitude can be misleading:
In very cold weather (your actual altitude is lower than indicated)
In very hot weather (your actual altitude is higher than indicated)
When you've forgotten to update the altimeter setting
When the altimeter has mechanical errors
ATC use:
All ATC altitude clearances reference indicated altitude
When ATC says "maintain 7,000," you set the local altimeter setting and fly at 7,000 indicated
This works because everyone in the same area uses the same altimeter setting, providing consistent vertical separation
The simple definition: Indicated altitude is what you read on the altimeter. It's the practical altitude pilots talk about most of the time.
True Altitude: Actual Height Above Sea Level
True altitude is the actual physical height of the aircraft above mean sea level (MSL).
Why it matters:
Charts and maps reference altitudes to MSL
Mountain peaks, towers, and obstacles are listed in MSL altitudes
Instrument approach minimums are MSL altitudes
Sectional chart contour lines and terrain elevations are MSL
The relationship to indicated altitude:
In standard atmospheric conditions, indicated altitude approximately equals true altitude
In non-standard conditions, the two diverge:
Cold weather: True altitude is LOWER than indicated (the air is denser, so you're physically lower than the altimeter shows)
Hot weather: True altitude is HIGHER than indicated (the air is less dense, so you're physically higher)
Low pressure regions: True altitude is LOWER than indicated
High pressure regions: True altitude is HIGHER than indicated
**The "high to low, hot to cold, look out below" rule applies to both pressure and temperature differences from standard.
Calculating true altitude:
For practical purposes, the corrections are:
Temperature correction (cold weather): At extreme cold temperatures, true altitude can be hundreds of feet below indicated altitude. The FAA publishes specific cold-temperature altitude corrections in the chart supplement for IFR operations below approximately -15°C. These corrections are:
Added to minimum published altitudes (MEAs, MOCAs, MDAs, DH/DA)
Required for IFR operations in cold temperatures
Applied to ensure adequate obstacle clearance
Pressure correction:
Standard altimeter setting is 29.92 inHg
For each 0.10 inHg below 29.92, true altitude is about 100 feet below indicated
For each 0.10 inHg above 29.92, true altitude is about 100 feet above indicated
Practical rule: Update your altimeter setting frequently. Most "true altitude vs. indicated altitude" issues come from outdated altimeter settings, not unusual atmospheric conditions.
Absolute Altitude: Height Above the Ground (AGL)
Absolute altitude is the actual height above the terrain directly below the aircraft.
Why it matters:
Terrain clearance during low-altitude operations
Pattern altitude (typically 1,000 feet AGL)
Approach to landing operations
Flight near obstacles
Some FAR regulations specify altitudes in AGL terms
How to determine absolute altitude:
Without a radar altimeter:
True altitude (MSL) minus terrain elevation = AGL
Example: Flying at 7,000 feet MSL over terrain at 5,000 feet MSL gives 2,000 feet AGL
With a radar altimeter:
The radio altimeter directly measures distance to ground using radio waves
Most common in airliners and turboprops
Some advanced GA aircraft have them
Provides direct AGL reading without calculation
The challenge for GA pilots:
Most GA aircraft don't have radio altimeters
AGL must be calculated by subtracting terrain elevation from indicated altitude
This is why pilots must constantly check sectional charts for terrain elevations
Pattern altitudes are calibrated by the airport elevation in the chart supplement
Common AGL references:
Pattern altitude: typically 1,000 feet AGL
Class B/C/D ceiling: typically 4,000 feet AGL (B), 4,000 feet AGL (C), 2,500 feet AGL (D)
Mountain obstacle clearance: 1,000-2,000 feet AGL minimum (FAR 91.119)
Cruise altitude in FAR 91.159: above 3,000 feet AGL = even/odd thousands rules
The practical insight: At a high-elevation airport (say, Telluride at 9,078 feet field elevation), a pattern altitude of 10,078 feet MSL (1,000 AGL) feels much higher than at sea level even though it's the same AGL. The aircraft also performs much worse at this density altitude — illustrating how AGL doesn't tell you everything about flight conditions.
Pressure Altitude: The Performance Reference
Pressure altitude is the altitude in the standard atmosphere (ISA) that corresponds to the current atmospheric pressure.
How to obtain pressure altitude:
Method 1 (most common):
Set the altimeter to 29.92 inHg
Read the altitude shown
That's pressure altitude
Reset to local altimeter setting when done
Method 2 (calculation):
Pressure altitude = field elevation + [(29.92 - current altimeter setting) × 1,000]
Example: Field elevation 4,000 feet, altimeter setting 29.82 inHg
Pressure altitude = 4,000 + [(29.92 - 29.82) × 1,000] = 4,000 + 100 = 4,100 feet
When pressure altitude is used:
Performance calculations:
Takeoff distance from POH charts
Climb rate calculations
Cruise performance
Maximum operating altitude
All POH performance charts use pressure altitude (with temperature) as the reference
Above the transition altitude:
Above 18,000 feet MSL in the U.S., all aircraft set 29.92
All flight levels (FL180 = 18,000 feet pressure altitude, FL250 = 25,000 feet, etc.) are pressure altitudes
This standardization ensures all aircraft above transition altitude are using the same reference, providing consistent vertical separation
The relationship to indicated altitude:
When local altimeter setting equals 29.92, indicated altitude equals pressure altitude
When local altimeter setting differs from 29.92, indicated altitude is offset from pressure altitude
Density Altitude: The "Performance Altitude"
Density altitude is pressure altitude corrected for non-standard temperature.
Why it's the most important altitude for performance:
The aircraft doesn't care about indicated altitude or true altitude — it cares about air density. Air density determines:
Lift production at any given airspeed
Engine power output
Propeller efficiency
Fuel-air mixture
When the air is less dense (because it's hot, high, or both), the aircraft performs as if it's at a much higher altitude.
Calculating density altitude:
The rough formula: Density altitude = Pressure altitude + (120 × ISA temperature deviation in °C)
Where ISA temperature deviation = actual temperature - ISA temperature at that pressure altitude
Worked example:
Pressure altitude: 6,000 feet
Outside air temperature: 30°C
ISA temperature at 6,000 feet: 15°C - 12°C = 3°C
Deviation: 30 - 3 = 27°C
Density altitude: 6,000 + (120 × 27) = 6,000 + 3,240 = 9,240 feet
The aircraft performs as if at 9,240 feet in standard conditions — even though the airport elevation is 6,000 feet.
The "hot and high" scenario:
High-elevation airport (high pressure altitude)
Hot temperature (significant ISA deviation)
Result: Density altitude potentially 10,000+ feet
Severe performance degradation
Can compromise takeoff performance
When density altitude matters most:
Takeoff at high-elevation airports
Hot summer days
Heavy aircraft loads
Short runways
High terrain near departure/arrival
The Five Altitude Types Working Together
Here's how the five altitudes relate in a typical flight scenario:
Departure airport: Las Vegas (KLAS), field elevation 2,181 feet MSL
Conditions:
Altimeter setting: 29.85 inHg
Temperature: 32°C (90°F)
Calculations:
Indicated altitude on takeoff: Set 29.85, read altimeter → reads 2,181 feet (matches field elevation if altimeter is correct)
True altitude: 2,181 feet MSL (assuming altimeter set correctly and no significant deviations)
Absolute altitude: 0 feet AGL (you're on the ground)
Pressure altitude:
2,181 + [(29.92 - 29.85) × 1,000] = 2,181 + 70 = 2,251 feet
Density altitude:
ISA temperature at 2,251 feet pressure altitude: 15 - (2.25 × 2) = 10.5°C
Deviation from ISA: 32 - 10.5 = 21.5°C
Density altitude: 2,251 + (120 × 21.5) = 2,251 + 2,580 = 4,831 feet
The takeaway: Even though field elevation is only 2,181 feet, on this hot day the aircraft will perform like it's at nearly 5,000 feet pressure altitude in standard conditions. This affects takeoff distance, climb rate, and overall performance significantly.
In flight at 8,000 feet indicated:
Indicated altitude: 8,000 feet (what the altimeter shows)
True altitude: approximately 8,000 feet (assuming altimeter setting is accurate)
Absolute altitude: variable (8,000 minus terrain elevation below)
Pressure altitude: 8,000 + 70 = 8,070 feet (if altimeter setting is 29.85)
Density altitude: significantly higher than indicated due to temperature
Common Misconceptions
"Indicated altitude is always what I'm flying at." Only when the altimeter setting is correct and conditions are near standard. Real altitude can differ.
"True altitude doesn't matter as long as ATC tells me what to fly." ATC clearances use indicated altitude. But true altitude is what clears terrain. In cold weather IFR especially, the difference can be significant.
"Density altitude is just for takeoff." No — it affects every phase of flight. Climb performance, cruise efficiency, single-engine performance in twins, and even landing distance can all be affected by density altitude.
"Pressure altitude is what the altimeter shows on a non-standard day." No — pressure altitude requires you to set 29.92. The altimeter setting on a non-standard day could be anything.
"Above 18,000 feet, indicated and true altitude are the same." No — at FL180 and above, you're using pressure altitude (29.92 set) which may differ from true altitude. The standardization above transition altitude prevents conflicts between aircraft, but absolute terrain clearance still requires understanding true altitude.
ATC and the Altitude Reference Question
When ATC says "maintain 7,000 feet," they mean indicated altitude with the local altimeter setting:
Below 18,000 feet MSL: Local altimeter setting is used; indicated altitude is the reference
At and above 18,000 feet MSL (Class A airspace): 29.92 is used; pressure altitude is the reference; altitudes are stated as flight levels (e.g., FL180, FL250)
Why the distinction matters:
Below 18,000: All aircraft in the same general area use the same altimeter setting from local sources. Vertical separation works because everyone has the same reference.
Above 18,000: Atmospheric pressure variations are less significant operationally. Using 29.92 globally provides consistent reference for high-altitude traffic without requiring continuous altimeter setting changes.
The transition at 18,000 feet:
Climbing through 18,000: Set 29.92, transition to flight levels
Descending through 18,000: Set local altimeter setting, transition to indicated altitude
Cold Temperature Altimetry
In cold temperatures, true altitude can be significantly less than indicated altitude. For IFR operations, this can compromise terrain clearance.
The standard correction: For temperatures below -15°C, FAA publishes specific cold-temperature altitude corrections. These are added to:
Minimum published altitudes (MEA, MOCA)
Minimum descent altitudes (MDA)
Decision heights/altitudes (DH/DA)
Approach segment altitudes
Application:
Look up the airport in the chart supplement (formerly A/FD)
Check if cold-temperature operations require specific corrections
Apply the correction to the minimum altitude before flight
Practical example: In Alaska in January, an instrument approach with a published MDA of 2,500 feet may require a cold-temperature correction of 200-400 feet. The pilot must add this correction (approach at 2,700-2,900 feet rather than 2,500) to ensure adequate terrain clearance.
FOR GA pilots:
Most VFR operations don't require these corrections
Some IFR operations in cold climates require them
The chart supplement specifies which airports require corrections
AC 91-79A provides the methodology
On the Written Test and Checkride
Altitude types appear consistently on tests and oral exams. The most commonly tested topics:
Definition of each altitude type
Indicated altitude vs. true altitude (when do they differ?)
"High to low, hot to cold, look out below"
Density altitude calculation
Flight levels vs. altitudes (what the transition altitude means)
Cold temperature altimetry
Quick Reference
The Five Altitudes:
Altitude Type | Definition | Reference |
Indicated | What the altimeter reads | Local altimeter setting |
True | Actual height above MSL | Mean sea level |
Absolute | Height above ground | Ground/terrain |
Pressure | Altitude in ISA matching current pressure | 29.92 inHg / ISA |
Density | Pressure altitude corrected for temperature | ISA conditions |
Conversion formulas:
Pressure altitude = field elevation + [(29.92 - altimeter setting) × 1,000]
Density altitude = pressure altitude + (120 × ISA deviation in °C)
Absolute altitude = true altitude − terrain elevation
True altitude ≈ indicated altitude (in standard conditions)
Cold weather true altitude = indicated altitude − cold weather correction
ISA values:
Sea level pressure: 29.92 inHg
Sea level temperature: 15°C
Lapse rate: 2°C per 1,000 feet
Density at sea level: 1.225 kg/m³
Key memory aids:
"High to low, hot to cold, look out below" — altimeter reads high in unfavorable conditions
"Density altitude is the performance altitude" — what the airplane "feels"
"Pressure altitude is for performance charts" — the reference for POH calculations
"True altitude clears terrain" — the actual height above MSL
When each is used:
ATC clearances: Indicated altitude
Charts and obstacles: True altitude
Pattern operations: Absolute altitude (AGL)
Performance calculations: Pressure altitude (with temperature)
Aircraft performance: Density altitude
Transition altitude (US): 18,000 feet MSL
Below: Local altimeter setting (indicated altitude)
Above: 29.92 inHg (flight levels = pressure altitude)
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Author: Nathan Hodell
CFI, CFII, MEI, ATP, Creator and CEO
Nathan is an aviation enthusiast with thousands of hours of flying and dual instruction over the past 15+ years. Through his aviation career he has been able to earn his ATP, fly as an airline pilot, own/operate flight schools, and create and host wifiCFI.