When the Federal Aviation Authority in the United States first implemented its mandatory drone registration program (which is no longer mandatory at the moment), there were a lot of questions. In fact, the FAA kept a list of the most frequently asked ones – and some are hilarious. Slate Magazine preserved them for posterity. A favourite: How high is 400 feet ?

The FAA suggests flying no higher without special permission. But many amateur pilots are right to wonder just how high they’re flying. If you have a drone with sophisticated telemetry like the DJI Mavic Pro or even the new Spark, you’ll already know exactly how high you are. But many inexpensive hobby drones don’t have that capacity.

So what’s up?

There are a lot of ways to determine height, but here’s an inexpensive solution that is reasonably accurate and should give you a benchmark for future flights.

Here’s what you need:

  • A mobile phone
  • A 20 foot long length of string. Even longer for greater precision, but it’s important to measure the length accurately
  • A binder clip, a really big one that can hold an inch thickness of paper
  • A glue gun
  • A 12 inch long tube. I used a length I cut off from an old steel mop handle, but you could use a paper one from a role of tinfoil.

Heat up the glue gun and fasten the binder clip halfway up the tube. My tube is actually longer than 12 inches which will be more accurate, but more unwieldy. Find an app for your mobile phone that acts like a carpenter’s level.

handy Level

I use Handy Level, but there are many others that use the accelerometer in your device.

Clip the phone carefully in the binder clip. I glued little pieces of leather to the clip’s jaws for extra protection.



Now tilt the tube back and forth to make sure the level is giving you a reading. You can use radians or degrees, but I suggest radians for reasons that will become clear.

Set that piece of 20 foot string on the ground and stretch it out taut. Three inch nails can help anchor it. Then set your drone about 25 paces or 100 feet from where you plan to fly, along the same line as the string. March back to the start of the string and get that drone airborne. When you reach a height you’d like to measure, pull out that tube and spot the drone.

Dave spotting

At this point it would be helpful to have a friend read the angle or radian on your cellphone and write it down as AngleA. Then, walk the twenty feet to the end of the string and spot your drone again. Have your friend read that angle and call it AngleB.

Here’s what you do with those angles:

Height = tan AngleA × ((tan AngleB × StringLength) ÷ (tan AngleA − tan AngleB))

Height = tan (AngleA degrees) × ((tan (AngleB degrees) × StringLength) ÷ (tan (AngleA degrees) − tan (AngleB degrees))

Yes, it’s ugly, but don’t despair. You can just fill in your StringLength and radians and cut and paste it into the Google search window.

So with AngleA = 0.959931 and AngleB = 0.837758 (radians) and StringLength = 20,

You get this:

google screenshot

These angles suggest a height of just under 100 feet. Remember you’re measuring from eye height, so add another five or six feet.
You see why I recommend radians. The Degrees Formula is much longer than the Google window. It still works, though. And precision counts when you measure those angles.

Here’s a little video to give you a clearer idea of the set-up and a rough idea of the kind of angles you’ll measure. Your numbers will be different since ours is an ideal example based on a drone placed exactly 100 feet from the pilot.

Maximum flight height varies from county to country and even from municipality to municipality. Here’s a few recent maximums culled from the internet. But remember, they can change anytime.

France: 150m (500 feet)
Australia, UK: 400 feet
Hong Kong: 300 feet
Canada: 90m (295 feet)

Chances are a 400 foot high flight will really test your resolve. You’ll get better photos closer to the ground. In case you’re wondering what height is absolutely too bloody high for a hobby drone, a Dutch flier known as Tollymaster seems to have set that dubious mark for all time – reaching 11,000 feet.

Such flights are obviously illegal. Thankfully, they are very few and far between – and the drone community has become excellent at self-policing.

You’ll get better images, have more fun – and stay within the law – by flying at lower altitudes. Plus, you can (with a bit of help) calculate pretty much exactly how high you’re flying.

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