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How does law measure the height above ground?

skan

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If they say you can fly up to 150 meters above ground...
Does it mean above the highest obstacle (tree, building, hill...) or just some official mean above sea or what?
It makes difference on steep and rough areas or areas with high trees or a tower or power lines.
 

neggy

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in the USA? It is above ground level , not sea level.

Lets use KGHG airport as an example, the runway is 9 feet above sea level, the pattern altitude is 1000' above the airport, if your barometer is accurate/set properly, you want to be at 1009 feet in the pattern.

Now how does this apply to the DJI drones?

The altitude is calculated by (among other things) a on board barometer that detects pressure and measures the difference from when it was launched to where it is now.

theoretical Barometric pressure at sea level is 29.92 " of mercury, at 500' ASL it is 29.38

If you were 1500' asl, 28.33, go 500' agl to 2000ASL, and you are at 27.82.

PS if you want to know where the "ceiling" is, or at what altitude the clouds are at so you have legal vertical visibility and are not flying in clouds ( a huge no no) try this: Find the difference between the surface temperature and the dew point. This value is known as the "spread". Divide the spread by 4.4 (if temperatures are in °F) or 2.5 (if temperatures are in °C), then multiply by 1000. This will give you cloud base in feet above ground level.
 

skan

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in the USA? It is above ground level , not sea level.

Lets use KGHG airport as an example, the runway is 9 feet above sea level, the pattern altitude is 1000' above the airport, if your barometer is accurate/set properly, you want to be at 1009 feet in the pattern.

Now how does this apply to the DJI drones?

The altitude is calculated by (among other things) a on board barometer that detects pressure and measures the difference from when it was launched to where it is now.

theoretical Barometric pressure at sea level is 29.92 " of mercury, at 500' ASL it is 29.38

If you were 1500' asl, 28.33, go 500' agl to 2000ASL, and you are at 27.82.

PS if you want to know where the "ceiling" is, or at what altitude the clouds are at so you have legal vertical visibility and are not flying in clouds ( a huge no no) try this: Find the difference between the surface temperature and the dew point. This value is known as the "spread". Divide the spread by 4.4 (if temperatures are in °F) or 2.5 (if temperatures are in °C), then multiply by 1000. This will give you cloud base in feet above ground level.


I guess this is the way the drone calculates the height, or maybe with the GPS, but I'm almost sure the official height is not calculated with a barometer but it's the true height above the ground.

Anyway that doesn't reply to my question, what happens if there are trees or hills?
 

lisadoc

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Anyway that doesn't reply to my question, what happens if there are trees or hills?

In the US, altitude limits (for Part 107, not for recreational fliers) are calculated above the ground or "structure" that is higher than ground level. From the FAA: "The maximum allowable altitude is 400 feet above the ground, and higher if your drone remains within 400 feet of a structure." So as a hill rises, your 400-foot limit also rises, to keep 400 feet above the increased ground level. If you were flying over the Empire State Building (just an example, not a suggestion), you would be good for the height of the building + 400 feet.

Over trees, it depends on what you and the FAA define as a "structure". Technically, the FAA has not defined "structure" as it relates to their guidelines for UAS. For manned aircraft, they do not use the term "structure" but rather "obstacle" (manned aircraft must maintain "an altitude of 1,000 feet above the highest obstacle within a horizontal radius of 2,000 feet of the aircraft") and structure is restricted to usage concerning marking and warning lights requirements for buildings, antennas, and the like.

So, if you're flying 450 feet high over a forest of 50-foot high trees, are you within that restriction? I can't answer that for you, as the FAA hasn't answered that themselves. But just remember, if you're flying recreationally in the US, there's no technical altitude limit for your flight.

Outside the US, all of this is moot and dependent upon your own country's laws. Your mileage (or altitude) may vary.
 

JSKCKNIT

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If they say you can fly up to 150 meters above ground...
Does it mean above the highest obstacle (tree, building, hill...) or just some official mean above sea or what?
It makes difference on steep and rough areas or areas with high trees or a tower or power lines.

It means exactly as it says, above ground level. Not above tree level etc. So you have to adjust your altitude depending on the surrounding ground elevation.
 

skan

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In the US, altitude limits (for Part 107, not for recreational fliers) are calculated above the ground or "structure" that is higher than ground level. From the FAA: "The maximum allowable altitude is 400 feet above the ground, and higher if your drone remains within 400 feet of a structure." So as a hill rises, your 400-foot limit also rises, to keep 400 feet above the increased ground level. If you were flying over the Empire State Building (just an example, not a suggestion), you would be good for the height of the building + 400 feet.

Over trees, it depends on what you and the FAA define as a "structure". Technically, the FAA has not defined "structure" as it relates to their guidelines for UAS. For manned aircraft, they do not use the term "structure" but rather "obstacle" (manned aircraft must maintain "an altitude of 1,000 feet above the highest obstacle within a horizontal radius of 2,000 feet of the aircraft") and structure is restricted to usage concerning marking and warning lights requirements for buildings, antennas, and the like.

So, if you're flying 450 feet high over a forest of 50-foot high trees, are you within that restriction? I can't answer that for you, as the FAA hasn't answered that themselves. But just remember, if you're flying recreationally in the US, there's no technical altitude limit for your flight.

Outside the US, all of this is moot and dependent upon your own country's laws. Your mileage (or altitude) may vary.

Finally an answer that makes sense.
 

lisadoc

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It means exactly as it says, above ground level. Not above tree level etc. So you have to adjust your altitude depending on the surrounding ground elevation.

As a lawyer would object: "Assumes facts not in evidence."

The FAA hasn't really come out and been very clear about this. When they mixed the verbiage of structure and obstruction across airspace guidelines and regulations, many people conflate the two and could assume that they are equivalent. Others might claim just the opposite - that they are distinct and meaningfully different declarations. Playing Devil's Advocate, it would be weird to observe the rules for manned aircraft related to obstructions, and yet, at the same time, only utilize physical structures for UAS pilots. The intention was to de-conflict the two (with 100-foot minimal separation), so it's unclear that trees and other natural obstructions would/wouldn't qualify as "structures". That's why I didn't deem it one way or the other.

To support your declaration, I would point to 72 Fed. Reg. 6689 ("Unmanned Aircraft Operations in the National Airspace System"), where the FAA said: "Model aircraft should be flown below 400 feet above the surface to avoid other aircraft in flight." In that case they mention yet another term - "surface" (meaning ground level), but no mention is made of structures, obstructions, or any such other objects. Of course, this policy notice was created in 2007, a mere lifetime ago with respect to drones and UAVs, when it was mostly the military and other such agencies using them and recreational quads were just coming on board. (The Phantom 1, for example, didn't come out until Jan 2013).

But ultimately, until a court, or Congress, or the FAA, or someone of authority makes it clear what flying over a "structure" is, it will remain an open question.
 

Thwyllo

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And such confusion,open to interpretation, is the problem. I'd argue that you wouldn't and couldn't treat a giant redwood, for example, as any different from a communications tower - what would be the logic in doing so?
 

tcope

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Your guesting is WAY to vague. What rule are you referring to? What country? Who made this "rule"? Are you flying commercial or hobby? The list goes on and on. As such, you can (and will) get several different, but perhaps still correct, answers. As those answers will be different they will simply lead to more confusion.