Call a "cat" a "cat" - Altitude vs height vs AGL vs MSL ...

[bobville]

Hello, There is a lot of pilot being confused about the difference between: Altitude vs height vs AGL vs MSL ... It's very important to use the RIGHT words when talking about that subject and there is so many mistake around the different forums...
So, it's time to be careful and to call a "Cat" a "Cat" ... for the safety of everyone and to try to be a better pilot ...
My english can be incorrect as I'm from Belgium and speaking french (please do not confuse Belgium vs France ;-) so I will not try to make a long explanation but instead you will find hereafter a link giving you the right definition;

What is the difference between AGL and MSL? - Drone Pilot Ground School

Above Ground Level, or AGL, describes the literal height above the ground over which you’re flying. Mean Sea Level, or MSL, is your true altitude or elevation. It’s the average height above standard sea level where the atmospheric pressure is measured in order to calibrate altitude. On a...

www.dronepilotgroundschool.com

And if you want to be "open minded" play the game and make the calculation of those "measure" in... meters ! Ok, ok,ok; it's a joke !
Bob

 

[sar104]

Hello, There is a lot of pilot being confused about the difference between: Altitude vs height vs AGL vs MSL ... It's very important to use the RIGHT words when talking about that subject and there is so many mistake around the different forums...
So, it's time to be careful and to call a "Cat" a "Cat" ... for the safety of everyone and to try to be a better pilot ...
My english can be incorrect as I'm from Belgium and speaking french (please do not confuse Belgium vs France ;-) so I will not try to make a long explanation but instead you will find hereafter a link giving you the right definition;

What is the difference between AGL and MSL? - Drone Pilot Ground School

Above Ground Level, or AGL, describes the literal height above the ground over which you’re flying. Mean Sea Level, or MSL, is your true altitude or elevation. It’s the average height above standard sea level where the atmospheric pressure is measured in order to calibrate altitude. On a...

www.dronepilotgroundschool.com

And if you want to be "open minded" play the game and make the calculation of those "measure" in... meters ! Ok, ok,ok; it's a joke !
Bob

Unfortunately DJI doesn't help here, referring to height above home point as altitude. On the plus side, these drones never fly high enough to care about altitude MSL, so everything is actually height.

 

[PhantomFandom]

Hello, There is a lot of pilot being confused about the difference between: Altitude vs height vs AGL vs MSL ... It's very important to use the RIGHT words when talking about that subject and there is so many mistake around the different forums...
So, it's time to be careful and to call a "Cat" a "Cat" ... for the safety of everyone and to try to be a better pilot ...
My english can be incorrect as I'm from Belgium and speaking french (please do not confuse Belgium vs France ;-) so I will not try to make a long explanation but instead you will find hereafter a link giving you the right definition;

What is the difference between AGL and MSL? - Drone Pilot Ground School

Above Ground Level, or AGL, describes the literal height above the ground over which you’re flying. Mean Sea Level, or MSL, is your true altitude or elevation. It’s the average height above standard sea level where the atmospheric pressure is measured in order to calibrate altitude. On a...

www.dronepilotgroundschool.com

And if you want to be "open minded" play the game and make the calculation of those "measure" in... meters ! Ok, ok,ok; it's a joke !
Bob

If you are going to ask other to be precise then perhaps use the proper terminology?

Yes there is a BIG difference between AGL and MSL. However, I contend that there is no difference between height and altitude. Those terms both need context to be meaningful. The "height" above the ground is AGL. The "height" above the mean seal level is MSL. You could interchangeably use altitude and height. In fact, I think you would be hard pressed to define the word altitude without using the word height.

In the world of aviation we basically always use altitude. So yes we should always be clear about AGL vs MSL, but I see no point in differentiating between height and altitude.

Also the article you pointed to is a bit misleading. I won't say it is factually wrong, but it is not written well. It seems to say (in my reading of it) that in an airplane the altimeter somehow magically switches between MSL and AGL depending on the airplane's proximity to ground, as shown in the snippet below...

"Pilots use altimeters, which measure the AGL, when the aircraft is flying at relatively low heights landing at an airport. But as the plane increases in altitude, the altimeter reading becomes less accurate. Once the plane reaches transition altitude, the aircraft uses the MSL along with air pressure readings to maintain level flight. "

In actuality the instrument called the altimeter (which is the only thing the article mentions) always reads MSL, which depends on the correct barometric pressure being properly set. In light GA aircraft that is generally the only instrument you have to determine your altitude. In larger planes and commercial aircraft you will commonly also have a radio altimeter (totally separate instrument), which can measure actual AGL altitude within a certain proximity to the ground.

 

[sar104]

If you are going to ask other to be precise then perhaps use the proper terminology?

Yes there is a BIG difference between AGL and MSL. However, I contend that there is no difference between height and altitude. Those terms both need context to be meaningful. The "height" above the ground is AGL. The "height" above the mean seal level is MSL. You could interchangeably use altitude and height. In fact, I think you would be hard pressed to define the word altitude without using the word height.

In the world of aviation we basically always use altitude. So yes we should always be clear about AGL vs MSL, but I see no point in differentiating between height and altitude.

Also the article you pointed to is a bit misleading. I won't say it is factually wrong, but it is not written well. It seems to say (in my reading of it) that in an airplane the altimeter somehow magically switches between MSL and AGL depending on the airplane's proximity to ground, as shown in the snippet below...

"Pilots use altimeters, which measure the AGL, when the aircraft is flying at relatively low heights landing at an airport. But as the plane increases in altitude, the altimeter reading becomes less accurate. Once the plane reaches transition altitude, the aircraft uses the MSL along with air pressure readings to maintain level flight. "

In actuality the instrument called the altimeter (which is the only thing the article mentions) always reads MSL, which depends on the correct barometric pressure being properly set. In light GA aircraft that is generally the only instrument you have to determine your altitude. In larger planes and commercial aircraft you will commonly also have a radio altimeter (totally separate instrument), which can measure actual AGL altitude within a certain proximity to the ground.

Technically I think that the formal difference between altitude and height is that altitude has a fixed datum (MSL) whereas height can be relative to anything, although usually taken to be the local ground level.

 

[PhantomFandom]

Technically I think that the formal difference between altitude and height is that altitude has a fixed datum (MSL) whereas height can be relative to anything, although usually taken to be the local ground level.

I see what you're saying but again I think that either term is meaningless without a frame of reference (or datum).

Even altitude is not absolutely fixed above a given datum. It always depends on which type of altitude reading you are looking at. In aviation we have at least 3 and maybe more:
AGL is a "floating datum" since the ground below you is constantly changing. Thus resembling your height description above.

MSL is indeed fixed to a reference point that does not change (well it changes slowly over long periods of time).

Pressure altitude (commonly referenced as flight level) is also fixed to a reference point, but it does not directly refer to AGL or MSL.

 

[sar104]

I see what you're saying but again I think that either term is meaningless without a frame of reference (or datum).

Even altitude is not absolutely fixed above a given datum. It always depends on which type of altitude reading you are looking at. In aviation we have at least 3 and maybe more:
AGL is a "floating datum" since the ground below you is constantly changing. Thus resembling your height description above.

MSL is indeed fixed to a reference point that does not change (well it changes slowly over long periods of time).

Pressure altitude (commonly referenced as flight level) is also fixed to a reference point, but it does not directly refer to AGL or MSL.

Exactly - the altitude datum reference doesn't change - it's always MSL. The measurement method is all that changes. Pressure altitude is still a measurement relative to MSL, but simply assumes a standard atmospheric relationship between height above MSL and pressure.

 

[gnirtS]

Unfortunately DJI doesn't help here, referring to height above home point as altitude. On the plus side, these drones never fly high enough to care about altitude MSL, so everything is actually height.

Just to be picky there are some rare circumstances where it does... Singapore for example has a rule of not above 200ft MSL not takeoff/ground!
Commercial drone operation wise its used to deconflict with real aircraft as well (especially operating in proximity to an airfield and so on). You might be over 400ft then legally.

But i agree, in real terms, the standard recreational drone operator has no need at all. Height above ground is the only useful thing needed.

 

[NonoT]

What do drones measure with, MSL or AGL?

 

[gnirtS]

What do drones measure with, MSL or AGL?

Barometric height above takeoff location.
The metadata also records absolute GPS (WGS84) height.

 

[Crusader14]

Altitude is the vertical distance between a point in space and sea level. Pilots in the U.K. flying an altitude will usually set a regional pressure setting on an altimeter which is based on the lowest forecast pressure for a period of time. This means that the pilot will Always be at or above the actual altitude shown on the altimeter (QNH). Not relevant to drone pilots.

Height or elevation is the vertical distance above a specified point on the surface (usually the runway touchdown point). This is achieved by setting the altimeter to the barometric pressure at that Point (QFE). Not strictly relevant to drone pilots.

AGL is what it says. Height above the ground. Highly relevant to drone pilots as that’s what we are supposed to use in terms of maximum “height”.

 

[gnirtS]

. Highly relevant to drone pilots as that’s what we are supposed to use in terms of maximum “height”.

Possibly better described as 400ft from "an object or terrain" to count for hillsides, cliffs etc.

 

[theDRONEranger]

Altitude vs height
- - - I can fly at 400 feet maximum altitude, but I can legally exceed that by 400 feet more when I fly over a tower of 700 feet height!

So, for me I mostly interpret height as distance of measure from the ground to top of an object, whereas I interpret altitude as the distance between the ground to an object suspended in air above or below my level of the ground. The cliff I am on is 1000 feet high and my drone flies off and down, I am still ar 1000 feet height but the drone now gains negative altitude.

My 2 cents only, not trying to establish definitions, but rather how I use them.

 

[bobville]

Altitude is the vertical distance between a point in space and sea level. Pilots in the U.K. flying an altitude will usually set a regional pressure setting on an altimeter which is based on the lowest forecast pressure for a period of time. This means that the pilot will Always be at or above the actual altitude shown on the altimeter (QNH). Not relevant to drone pilots.

Height or elevation is the vertical distance above a specified point on the surface (usually the runway touchdown point). This is achieved by setting the altimeter to the barometric pressure at that Point (QFE). Not strictly relevant to drone pilots.

AGL is what it says. Height above the ground. Highly relevant to drone pilots as that’s what we are supposed to use in terms of maximum “height”.

_______________________________________________________________________________________
Hello Crusader,
You won ! You made the shortest, very accurate and realistic definition that everyone can understand.
Let's hope that every one is on the same page now (removed by moderator).
Happy flight at "safe height/elevation"
Bob

 

[TA147]

AGL is what it says. Height above the ground. Highly relevant to drone pilots as that’s what we are supposed to use in terms of maximum “height”.

This is the bit that confuses me - let's say you take off on top of a 400ft cliff. You ascend to 20ft above the home point, so according to DJI your altitude is 20ft. Then you fly over the side of the cliff. Now according to the AGL rules stated here, your altitude is 420ft and suddenly you are flying illegally. Which one is correct? And if the latter, how would I know I was flying illegally, since the telemetry from the drone will only tell me height above the takeoff point, not the height from the ground 420ft below the drone? Bit hypothetical maybe since it's not a common scenario, but would be interesting to know the answer.

 

[sar104]

This is the bit that confuses me - let's say you take off on top of a 400ft cliff. You ascend to 20ft above the home point, so according to DJI your altitude is 20ft. Then you fly over the side of the cliff. Now according to the AGL rules stated here, your altitude is 420ft and suddenly you are flying illegally. Which one is correct? And if the latter, how would I know I was flying illegally, since the telemetry from the drone will only tell me height above the takeoff point, not the height from the ground 420ft below the drone? Bit hypothetical maybe since it's not a common scenario, but would be interesting to know the answer.

AGL is height above the ground below. So if you are 420 ft above the ground then of course you have exceeded the 400 ft limit. The DJI telemetry doesn't tell you height AGL - as you clearly know perfectly well, it only tells you height above the takeoff point, so how can you possibly be confused?

As for how you would know that you are flying illegally - you would know if you had put any planning into your flight and checked the height of the cliff. That said - provided that you remain near the cliff face then you are very unlikely to pose a hazard to aviation and so, while a technical breach of regulations, no one is likely to care about it.

 

[TA147]

AGL is height above the ground below. So if you are 420 ft above the ground then of course you have exceeded the 400 ft limit. The DJI telemetry doesn't tell you height AGL - as you clearly know perfectly well, it only tells you height above the takeoff point, so how can you possibly be confused?

As for how you would know that you are flying illegally - you would know if you had put any planning into your flight and checked the height of the cliff. That said - provided that you remain near the cliff face then you are very unlikely to pose a hazard to aviation and so, while a technical breach of regulations, no one is likely to care about it.

Not confused about the height, just the legality. Basically from what you are saying, if something happened and someone wanted to make something of it, then the pilot would be in the wrong.

 

[sar104]

Not confused about the height, just the legality. Basically from what you are saying, if something happened and someone wanted to make something of it, then the pilot would be in the wrong.

The legal requirement is the height limit AGL. From Section 349 (recreational exemption):

49 USC 44809: Exception for limited recreational operations of unmanned aircraft​

​

(a) (6) In Class G airspace, the aircraft is flown from the​

surface to not more than 400 feet above ground level and complies​

with all airspace restrictions and prohibitions.​

From Part 107 (non-recreational flights):

§107.51: Operating limitations for small unmanned aircraft.​

A remote pilot in command and the person manipulating the flight controls of the small unmanned aircraft system must comply with all of the following operating limitations when operating a small unmanned aircraft system:​

​

(a) The groundspeed of the small unmanned aircraft may not exceed 87 knots (100 miles per hour).​

​

(b) The altitude of the small unmanned aircraft cannot be higher than 400 feet above ground level, unless the small unmanned aircraft:​

​

(1) Is flown within a 400-foot radius of a structure; and​

​

(2) Does not fly higher than 400 feet above the structure's immediate uppermost limit.​

​

(c) The minimum flight visibility, as observed from the location of the control station must be no less than 3 statute miles. For purposes of this section, flight visibility means the average slant distance from the control station at which prominent unlighted objects may be seen and identified by day and prominent lighted objects may be seen and identified by night.​

​

(d) The minimum distance of the small unmanned aircraft from clouds must be no less than:​

​

(1) 500 feet below the cloud; and​

​

(2) 2,000 feet horizontally from the cloud.​

 

[gnirtS]

This is the bit that confuses me - let's say you take off on top of a 400ft cliff. You ascend to 20ft above the home point, so according to DJI your altitude is 20ft. Then you fly over the side of the cliff. Now according to the AGL rules stated here, your altitude is 420ft and suddenly you are flying illegally. Which one is correct? And if the latter, how would I know I was flying illegally, since the telemetry from the drone will only tell me height above the takeoff point, not the height from the ground 420ft below the drone? Bit hypothetical maybe since it's not a common scenario, but would be interesting to know the answer.

Might depend on country....

In the UK it helps to think of the drone being in the middle of a 400ft bubble. the edge of the bubble must always touch something.

 

[sar104]

Might depend on country....

In the UK it helps to think of the drone being in the middle of a 400ft bubble. the edge of the bubble must always touch something.

I think that the UK version might be unique - I've not seen that anywhere else. Australia seems to have the same simple AGL rule as the US.

 

[Crusader14]

Possibly better described as 400ft from "an object or terrain" to count for hillsides, cliffs etc.

Objects don’t count I’m afraid. Just the “terrain I’m afraid.