request4advice: power lines and depth of field

[Gorlash]

I have an area in Newark, CA, near the bay, where I've been flying my new drone around. There are two high-voltage power lines which run through the area and then run out across the bay; one is roughly 100' high at the wires, the other is possibly twice that. I've been finding it *very* difficult to tell where the drone is relative to the wires, because my depth perception really doesn't give me much useful information about their relative positions.

I was out yesterday, and wanted to fly out over a field and take some pictures, but I needed to get past those lines... it is *really* challenging!! The drone was about 200' from me, about 100' up, with high, hazy clouds but nothing near the ground. I could see the drone and lines clearly, but because I myself was well below the lines, I had major parallax issues!! I tried setting the camera to facing level, then try to visually detect the lines (i.e., trying to estimate the altitude of the lines), but cannot really see them in many cases.

I eventually set the drone to 120' feet, then slooooowly moved over the lines... I got away with this, but never *did* get obstruction warning from the drone... interestingly, though, when I was coming back again, I *did* get obstruction warnings, saying the lines were about 18' "below", which was only partially useful info because the drone was still some horizontal distance away from them, so this was a 'hypotenuse' measurement.

info: Mavic 2 Pro, in normal P mode. Using a 10' tablet for DJI app.

Anyway, is there some more predictable, consistent way to interpret proximity to power lines?? This is a location where I would like to practice at regularly, as there are no houses or people or anything to object, and other drone fliers operate in the area as well.

 

[Thomas B]

Not that I know of. I have a line of 100’ transmission lines cross my practice area. I set RTH to 150 feet when flying there and just rise to higher altitude when approaching the poles.

 

[MavicMan17]

I would say if your out their not near anyone use more of your FPV from the camera to judge where the lines are, I have several at my normal flying location as well and just stay well over them.
Setting a RTH height that you know is 20ft or so above the lines it a great idea, that way if you question your sight and the relative location of the drone to the lines and you, that you can just hit RTH and save yourself the heartache of crashing a $2000 drone into a high voltage powerline. again set a proper RTH height. When in doubt RTH!

 

[Mrmund]

Try flying towards the power lines but not to close because the EMF could cause interference issues.
Make sure you're camera is pointed directly forward (not tilted up or down). Ascend until the wire is centered on your screen, note altitude. Repeat for second wire.
Make sure your RTH is set at least 50' above.
Now you will know that when crossing the wires you will have to be at an altitude above or below the wire heights noted earlier, just make sure you double check your height before crossing.
Note: I have had interference issues getting too close to high voltage lines resulting in complete loss of control of my aircraft so be aware of how close you get.

 

[Gorlash]

Try flying towards the power lines but not to close because the EMF could cause interference issues.
Make sure you're camera is pointed directly forward (not tilted up or down). Ascend until the wire is centered on your screen, note altitude. Repeat for second wire.
Make sure your RTH is set at least 50' above.
Now you will know that when crossing the wires you will have to be at an altitude above or below the wire heights noted earlier, just make sure you double check your height before crossing.
Note: I have had interference issues getting too close to high voltage lines resulting in complete loss of control of my aircraft so be aware of how close you get.

Yeah, that was what I was trying yesterday, but as I commented, I had trouble seeing the wires with the camera. I'll try again next time I go out.

The second, taller power poles are more of a challenge, because the wires sag substantially when running between the towers, which are roughly 1500' feet apart (measured on Google Earth). The maximum sag at center is something like 1/4 of their total height... so the target altitude varies much over the area.

I'm enclosing a link to a spot where I was flying recently, out at the levees in Newark, CA. You can see both power lines and towers here, though it doesn't really show the difference between the shorter towers on the right and the taller ones on the left, but you can somewhat see the sag in the lines.

paths and poles, at Newark levees

 

[dronerdave]

Even on flat ground it's important to understand each tower or pole can be a different height above ground level plus the wire sags between each structure. If you cross a wire mid span and know you have a few feet above the wire, it doesn't mean that you'll clear the structure or wire on either side of your crossing point. If the RTH takes a different path than what you flew, you might be in trouble. Best to know that you can safely clear the top of the tallest structure nearby your flight path and set your return to home several feet above that. I usually fly toward the structure that appears to be highest point of the line and (as noted in the above post), while keeping the camera level ascend until you know you're at the top of the structure and note the height. No reason to get closer than around 100 feet of any part of the line or structures so, I'd set RTH 100' (or more) above the top. On a 150 foot tower my RTH setting would be at least 250 Feet. I always make sure I know that I have enough clearance over anything that can get in the way of an abrupt RTH path. Depending on your orientation to the line, the towers to the right or left of the one you height checked might visually look similar but may be much taller (or shorter) and it's a good idea to know that. I also don't suggest flying under a transmission line. It would be a long shot for something to go wrong but I still don't recommend it.

That said, communication towers make me nervous and I stay clear of them. With their higher than normal heights and numerous down guy wires they're like a spider web of trouble.

 

[Gorlash]

That said, communication towers make me nervous and I stay clear of them. With their higher than normal heights and numerous down guy wires they're like a spider web of trouble.

Yeah, they make me nervous too... but there are a few places that I've really wanted to investigate once I got a drone (yesterday's spot being one of them), and I'm stuck with the power poles at some of them... I'll try the suggestions mentioned here, and hope those keep me clear.

I note your comment about not flying underneath the wires... that was actually something I was thinking of doing, once I could figure out how high they were. What particular hazards does the drone face when flying down there??

 

[dronerdave]

Probably nothing but if it were to RTH for some reason while you're below the lines, your quad might end up hitting them. It kind of depends on the line height and sometimes the line voltages. The higher the voltage, the more chance it can cause issues but as long as you keep a safe distance you'd probably be OK. Another thing worth noting is during windy days static or EMF's can affect things further away than days where winds are lighter. We've canceled days working on a de-energized transmission circuit due to the circuit next to us being up wind & energized. Still it's surprising how close you can get with a drone and not have a problem. But I consider lines like you're flying around to be critical infrastructure and getting too close is highly discouraged. Looking at your picture, those towers look like the Hetch Hetchy 230 kV lines near the Dumbarton Bridge that feed SF.
I'd like to fly over those salt fields around there... talk about some colorful high altitude shots, but I stay away from the more populated places...just me.
Fly safe.

 

[MavicMan17]

PRO TIP: RTH set Altitude - if below setting then the AC will ascend until RTH selected altitude is achieved. If you are already at or above the Set RTH Altitude your AC will not ascend or descend. It will simply RTH at its current height.

 

[Forensics_Pilot]

If you really want an accurate measurement, why not try tying a string (non-metallic like kite string) to a helium balloon and have the string marked in 5 ft. increments? This can be done on a calm day observing when the balloon reaches the height of the tower structure. Take a friend along to help you observe. Good luck.

 

[Forensics_Pilot]

As an alternative you could triangulate with a protractor set on a pole and measure the angle from horizontal to top of structure. Then measure horizontal distance from where you had set up protractor to the tower structure. Draw your right triangle and calculate height above where your protractor was set as you now have a right triangle with 3 angles known and one leg known. Remember to add height above ground of where you had protractor. Good luck.

 

[LivinLarge]

I have some tall trees in my neighborhood that I needed to know the height of to make sure I always cleared them on RTH, Here's how I measured them. First I flew up to an altitude that I was sure would be well above them. Then I pointed the camera straight down and flew forward until I was directly above them. Next, I backed up until I could see that I was still close but a safe enough distance to descend. Then I set the camera level and descended beside the tallest tree until I could see that the top of it was in the center of my screen. I noted the altitude and set my RTH to always be 30 feet above this.
I have found that any time I am flying towards a distant object visually, not using the screen, when I think I am close, I am actually still far away.

 

[Gorlash]

All of these ideas are excellent... however, the ones which require physical access - using triangulation or other measurement devices, will not be useful in many new situations that I encounter, as such access often isn't available - part of the reason for using the drone!!

It sounds like the most practical option is going to be the one that I first tried - trying to level the camera, and then flying up level with the wire, and hope I can see it... though, as @LivinLarge noted, it's often much further than it seems, and of course, I already have the problem that I started this thread with; the fact that visually I don't have any accurate frame of reference (for distance to an object) myself.

I also am leaning toward an earlier suggestion, which is to try to find the lowest point on the long, sagging lines, and set RTH to somewhere *below* that... otherwise, the drone might pick an RTH elevation that is right where the wire is at a given time...

 

[LivinLarge]

All of these ideas are excellent... however, the ones which require physical access - using triangulation or other measurement devices, will not be useful in many new situations that I encounter, as such access often isn't available - part of the reason for using the drone!!

It sounds like the most practical option is going to be the one that I first tried - trying to level the camera, and then flying up level with the wire, and hope I can see it... though, as @LivinLarge noted, it's often much further than it seems, and of course, I already have the problem that I started this thread with; the fact that visually I don't have any accurate frame of reference (for distance to an object) myself.

I also am leaning toward an earlier suggestion, which is to try to find the lowest point on the long, sagging lines, and set RTH to somewhere *below* that... otherwise, the drone might pick an RTH elevation that is right where the wire is at a given time...

Some other ideas to help you see the wires:
Fly very high with the camera pointed down. You should be able to see the path of trees that are cut along the powerline.
Look for the towers first and then fly along between them following the lines.
Use a satellite map rather than road map to see your location relative to the lines.