Question On Altitude When Flying From A Hilltop

[Cyclops2100]

Hi All. New to these forums so I will start with a short intro. I just picked up a Mavic Air for Christmas and am loving it so far. I have registered with the FAA and downloaded all the standard apps the FAA calls for and then some since that is now a requirement it seems.
I used to fly small/mid sized park flyer fixed wing RC aircraft for a few years. I got into tinkering a bit and added a camera to some of those to take areal vids and thought that was pretty cool. However I didn't like the quality of the videos I was getting and the space needed for the take off/landing. The Mavic/Drone world has recently pulled me back into flying. Have been flying over marsh lands and recently from a hilltop over looking a valley. Have been amazed with the results of the video quality taken so far.
However I got a question on my altitude when I launch from a hilltop top and fly over the valleys in front of it. I know my altitude ceiling should be 400ft per FAA. If I launch from a hilltop my drone treats that altitude as 0 but it may actually be 400ft above sea level. I naturally gain 50-100ft on my flights and head out, now over the valley now I may be 500ft even though the reading only 100ft.
In these hills what is the altitude I should be worried about? Is it 400ft above the highest hilltop in the area or 400ft above the ground I am flying over at that moment?

I am assuming it is 400ft above the highest hilltop in the area as I would think that how standard aircraft would treat it.

Thanks in advance.

 

[dirkclod]

Maybe this will help you understand . Welcome to the forum .

 

[Thomas B]

Maybe this will help you understand .
View attachment 89585

Exactly!

 

[Cyclops2100]

Yes, thank you.

 

[Paulf10]

Now I think I’m confused.(not a hard thing to do?)

If I take off from sea level and come to a hill that is 500 ft AGL, I would not be able to make it to the top of the hill.
However if I take off from the top of that hill I could still fly to 400 ft AGL from that point. Now if I were to fly from the top of that hill down 500 ft to sea level, would I then be able to fly back up to top of the hill and even 400 ft higher?
At least that’s how I thought it worked and it’s been my experience. But it wouldn’t be the first time I was wrong.?

 

[Simmo]

That diagram is not quite right up above, but it does give the general idea.
Another way of thinking about it is a 400ft piece of string hanging from your drone.... that string must always touch the ground.

 

[Paulf10]

That diagram is not quite right up above, but it does give the general idea.
Another way of thinking about it is a 400ft piece of string hanging from your drone.... that string must always touch the ground.

Perfectly stated Simmo. That’s what I was trying to say but your string analogy explains so very much better.

 

[WithTheBirds]

Now I think I’m confused.(not a hard thing to do?)

If I take off from sea level and come to a hill that is 500 ft AGL, I would not be able to make it to the top of the hill.
However if I take off from the top of that hill I could still fly to 400 ft AGL from that point. Now if I were to fly from the top of that hill down 500 ft to sea level, would I then be able to fly back up to top of the hill and even 400 ft higher?
At least that’s how I thought it worked and it’s been my experience. But it wouldn’t be the first time I was wrong.?

I think you may have your units messed up. The max altitude the drone can fly to, above take of point, is 500m...... Not feet.

 

[ChrisOutdoors]

Adding to this, DJI UAS' measure your altitude with GPS. So not MSL or AGL but height from launch point. Something to consider when flying around hilly or mountainous terrain.

 

[sar104]

Adding to this, DJI UAS' measure your altitude with GPS. So not MSL or AGL but height from launch point. Something to consider when flying around hilly or mountainous terrain.

It doesn't use GPS for height, except in the EXIF data. It uses barometric pressure.

 

[Thomas B]

That diagram is not quite right up above, but it does give the general idea.
Another way of thinking about it is a 400ft piece of string hanging from your drone.... that string must always touch the ground.

Yep, this will keep you out of trouble and in legal flight.

 

[old man mavic]

the only thing to remember is,its 400 ft maximum height above ground level ,whether you fly from the top or bottom of the hill,the height displayed on the app screen is above the home point, and if you flew from a high point out over a valley then as you descended it would indicate a negative number as you would be flying below the home point,with regards to flying from the bottom of a tall hill as you climb up and if the hill is say 600 ft high you can fly up to the top if in VLOS and you will still be legal as long as you are within the 400 ft AGL zone

 

[dirkclod]

That diagram is not quite right up above, but it does give the general idea.
Another way of thinking about it is a 400ft piece of string hanging from your drone.... that string must always touch the ground.

I’m trying to figure out why you say that. . It’s the same thing you just said. I got that from here seeing it posted hundreds of times with this same question and never have I seen that
posted about it .
Im just curious what’s not quite right about it.

 

[John235]

I’m trying to figure out why you say that. . It’s the same thing you just said. I got that from here seeing it posted hundreds of times with this same question and never have I seen that
posted about it .
Im just curious what’s not quite right about it.

I think he means that the second "height" line in the diagram is not AGL, but rather a measurement of distance from the hill. To measure AGL, you need to always measure straight down as in dropping a plumb line.

 

[dirkclod]

Guess it’s how you interpret it but see what your saying. .
I see it as if the man was at the arrow and the drone was
at the other arrow it would be 400’ AGL .
No matter was just curious.

 

[old man mavic]

actually the graphic is correct if you look at the yellow hill,and then the blue 400 ft zone it follows the same contour as the hill, so what it is saying is you can fly anywhere in that airspace and still be legal but i agree the slanted arrowed line is somewhat confusing

 

[wollins]

Guess it’s how you interpret it but see what your saying. .
I see it as if the man was at the arrow and the drone was
at the other arrow it would be 400’ AGL .
No matter was just curious.

the only thing to remember is,its 400 ft maximum height above ground level ,whether you fly from the top or bottom of the hill,the height displayed on the app screen is above the home point

Ok so how do you know (based on the app) at what height you're really flying at (AGL) if you're flying in hilly country? For instance lets say you're at the top of a 500' hill (your home point) and you take off, climb to 10' and then fly straight out horizontally from that height. Now according to the app you're flying at 10', whereas you're actually flying at 510' AGL! (which now makes you illegal) So, in a case like this (hilly country flying) how would you even know when/if you're flying legally or not ... since you're going by the apps feedback in your flight?

 

[pross]

The drone works out the height based on its take off point, if you walk up a hill or you fly it out into the valley .. it doesn't _know_ its a valley, it's up to you to do the basic maths

 

[wollins]

Ah so the drone does not "adjust" to actual AGL then .... I thought it would show 10' in (my scenario above - post 17) then as we fly straight out (horizontally at the same height) over the valley it would show increasing height in its reading.

 

[Boback]

The FAA is never clear on what it means. For example, in the Ultralight world (Part 103), the definition for "Congested area" might mean a housing development in one case, and 2 houses on a corner in another. Do you best to remain 400' or lower over the ground and/or objects immediately below you. Check out Part 107.41 for specific details on altitude restrictions around structures and objects. Remember is it your decision where you fly, but it is the FAA determination if you broke the rules or not.