400' AGL... off the top of a hill

[Thomas B]

Yes! I checked it out before. Just north of me is a 2000’ tall granit escarpment, nearly a sheer cliff, about 200 miles long called the Mogollon (mo - gee - yon) Rim. I checked it out as I wanted to fly off the top... which would put me at 2000’ altitude. As above maintains proximity is the key here to legal flight.

 

[Starz]

This one has turned complicated to some.
Above ground level is just that.
Drop a four hundred and one foot string straight down from your aircraft.
If it doesn’t touch the ground, your not in compliance.

 

[AMann]

Yes! I checked it out before. Just north of me is a 2000’ tall granit escarpment, nearly a sheer cliff, about 200 miles long called the Mogollon (mo - gee - yon) Rim. I checked it out as I wanted to fly off the top... which would put me at 2000’ altitude. As above maintains proximity is the key here to legal flight.

I did the same with a hill about 540’ above where I stood at homepoint, had to accept the warning that I set maximum altitude too high to do it, but as we have a busy airport in our valley, I stayed within 20’ or less from the top as I couldn’t see the other side if a plane or helicopter wanted to fly right over it from the other side.

 

[AMann]

This one has turned complicated to some.
Above ground level is just that.
Drop a four hundred and one foot string straight down from your aircraft.
If it doesn’t touch the ground, your not in compliance.

Not true if you are within 400’ of a sheer 5000’ cliff...your string would not touch bottom but you would be legal. Sorry, you are right for most terrain, had to poke fun at it a bit.

 

[Thomas B]

The rim is the edge of a plateau that extends hundreds of miles north behind it.

All those little green dots below are 150-200’ ponderosa pine trees.

 

[sar104]

You must remain within 400 ft of the ground when moving away from the hill. If the hill is more like a cliff, 400 ft from the side of the cliff is acceptable.

If you mean that 400 ft from the cliff is reasonable then I'd agree. But if you actually mean acceptable then acceptable to whom, and what is your reference for that?

 

[AMann]

If you mean that 400 ft from the cliff is reasonable then I'd agree. But if you actually mean acceptable then acceptable to whom, and what is your reference for that?

If there’s boulders and ledges and such below the drone, like in Thomas’s wonderful sunset cliff picture above, wouldn’t those count as terrain? I wouldn't fly out further than 400’ from that though, nor as I mentioned before, would I fly as high as 400’ or even over 50’ or so above it even though it would be permitted if I couldn’t see what was approaching the summit from the other side. I’d do the same for structures like buildings where I couldn’t see behind them as well.

 

[sar104]

If there’s boulders and ledges and such below the drone, like in Thomas’s wonderful sunset cliff picture above, wouldn’t those count as terrain? I wouldn't fly out further than 400’ from that though, nor as I mentioned before, would I fly as high as 400’ or even over 50’ or so above it even though it would be permitted if I couldn’t see what was approaching the summit from the other side. I’d do the same for structures like buildings where I couldn’t see behind them as well.

Agreed. The interesting question is regarding the hypothetical vertical cliff. Even Part 107, which refers to staying within 400 ft laterally of structures, says nothing about terrain, and the recreational rules purely refer to AGL - i.e. the ground below the aircraft. I think it's reasonable to assume that there is some way to operate in the vicinity of vertical terrain, but there is no formal 400 ft allowance.

 

[JamesD71]

I live in flat-land USA these days so I haven't a convenient way to test this.

Given I am on top of a hill at least 400' above the surrounding terrain and I lift off straight up... my Mavic 2 will top out just under 400' AGL from my launch point. I then move the drone in any direction. And my AGL will increase in direct relation to the decreasing elevation of the hill.

Does the software/internal GPS firmware take this slope into account and cause the drone to descend or otherwise alert the user the drone is above 400' AGL?

Very good question, and the reason why I study topo maps for my flight path before flying. I doubt it, but maybe someone else knows otherwise.

 

[kadras]

I live in flat-land USA these days so I haven't a convenient way to test this.

Given I am on top of a hill at least 400' above the surrounding terrain and I lift off straight up... my Mavic 2 will top out just under 400' AGL from my launch point. I then move the drone in any direction. And my AGL will increase in direct relation to the decreasing elevation of the hill.

Does the software/internal GPS firmware take this slope into account and cause the drone to descend or otherwise alert the user the drone is above 400' AGL?

No, the drone has no idea what the terrain below it unless it's within range of any downward sensor. Even then it doesn't cause the vehicle to change altitude. There is no terrain following capability.

 

[JamesD71]

If you said that, it would be incorrect as well.
If he launches from a hilltop that is 400 ft above sea level, he can climb another 400 feet and still be within regulations.
How do you think people fly at Denver?

If he is in fact at any hill top and climbs to be 400 above that hill top, then he had better be good at hovering in that specific position, not flying in any direction horizontally. If he flys north, south, east, or west the ground directly below the drone descends and his defined altitude increases above 400 ft AGL. He must carefully descend as he flys out of that launch site hover, according to the change of topography directly below...not to exceed 400 ft AGL as he leaves the launch point reference.

 

[dronerdave]

I'd like to fly here. The canyon is deep and although my dimensions are not to scale I'd like to fly off a shelf road where it drops vertically several hundred feet. I thought that (as a rec flier) as long as I'm w/i 400' away from my launch point horizontally the AGL doesn't matter. Wrong?

 

[kadras]

I thought that (as a rec flier) as long as I'm w/i 400' away from my launch point horizontally the AGL doesn't matter. Wrong?

Wrong. There is nothing in the rules for rec or Part 107 flyers about horizontal distance from launch point for any aspect of flying.

 

[AMann]

I'd like to fly here. The canyon is deep and although my dimensions are not to scale I'd like to fly off a shelf road where it drops vertically several hundred feet. I thought that (as a rec flier) as long as I'm w/i 400' away from my launch point horizontally the AGL doesn't matter. Wrong?

View attachment 76561

As Sar104 mentioned in Post #48 above, it’s terrain, not an structure you are flying near, so you’d be 400’ too high flying out to that 800’ AGL part.

Impressive place, where is that road?

 

[AMann]

I'd like to fly here. The canyon is deep and although my dimensions are not to scale I'd like to fly off a shelf road where it drops vertically several hundred feet. I thought that (as a rec flier) as long as I'm w/i 400' away from my launch point horizontally the AGL doesn't matter. Wrong?

View attachment 76561

kadras is right, the 400ft horizontal rule pertains to flying near structures: “The maximum allowable altitude is 400 feet above the ground, higher if your drone remains within 400 feet of a structure.”

 

[kensteele]

bottom line is recreational users aren't going to care about this particular aspect. if this question is on the recreational flyer's test, most everyone who isn't part 107 (if they have to take the test) will fail the question and if we pass the test overall, we will fail those details in the field as well.

we don't care; we are here to fly for fun and such a complex situation is not application to recreational flying. same as when you are driving, some of the rules you don't bother with especially the ones that don't always make sense (like lane-spitting on a motorcycle or car length). no need to get bent out of shape because 100% of the rules aren't being followed at all times. like i said, some of the rules just don't make sense and this one really confused me at first. but after reading all the posts (and seeing that others are confused as well about different part of it), i've decided that i won't be bothered with applying it....probably because i'm in flat land and it's not going to really impact me anyway. land, structures, horizontal...don't care.

 

[wsalopek]

Although it is more accurate in measuring latitude and longitude, it is more difficult to get an accurate reading of altitude with a consumer grade GPS, and a barometer is much more accurate and precise than the type of GPS that is in the drone for measuring altitude. One would have to use a much more expensive and larger survey grade GPS and do differential correction to get an accurate altitude reading.

GPS altitude, 10+ years ago(?), when the gov't introduce error on purpose, was indeed rather unreliable...esp with the GPS receivers of those days...but these days, GPS altitude is quite accurate.

The best idea would be to use GPS altitude above, oh 30 feet AGL, and then use the barometer for precision landing.

 

[sar104]

GPS altitude, 10+ years ago(?), when the gov't introduce error on purpose, was indeed rather unreliable...esp with the GPS receivers of those days...but these days, GPS altitude is quite accurate.

The best idea would be to use GPS altitude above, oh 30 feet AGL, and then use the barometer for precision landing.

It doesn't need the barometer or GPS for precision landing - it uses VPS which is far more accurate than either of them.

 

[wsalopek]

Does anyone know why the M2 series uses barometric altimeter readings instead of GPS, which is capable of determining altitude?

And if you then compared the GPS altitude with a topo map, everyone would be happy. You’d have accurate AGL values everywhere you fly.

Just FYI,

Ardupilot software has had the capability to use digital elevation models for many years...the APM and Pixhawk flight controllers can do true terrain following based on uploaded/onboard terrain data.

It is kinda weird, what with DJI trying so hard to keep us from flying in certain areas and altitudes, that they would have on board terrain data to some degree, which would esp useful for those of us who live in mountainous terrain, and want to fly up the side of the mountain, never going above 400 AGL, but going higher than 400 feet (or even 500 meters) above the takeoff point.

I know LOS issues are involved, but just saying...

 

[wsalopek]

It doesn't need the barometer or GPS for precision landing - it uses VPS which is far more accurate than either of them.

Right, exactly, I was just making the point between GPS and barometric altitude.