Measuring height with a mavic drone

[Ktblack]

New to drones, but have an application at work that they may help resolve my issues.

Was wondering if this would be possible. I am thinking that I could hang a string with a small weight from it. I would then lower until weight rested gently on top of the object.

My questions are as follows..(leaning towards purchasing the Mavic air for this but would consider another if it serves the application better)
1. Can the camera look below the drone enough so that I could view the weight while lowering?
2. How accurate would this be? All objects will be around 30 to 40 feet above take off.
3. Is this even possible?
4. Is there a drone better suited for this?

 

[MAvic_South_Oz]

You want to know the height of an object from it's base ?
Or altitude ASL ?

Not sure how a string from the drone would solve this, unless lowering to a specific length, touching that down, then deduct from drone height ?
(Assuming take off was from dead base of object.)

There was an almost identical thread on this a while ago, hang on . . .

Using a Mavic Pro to measure height of structures and trees?

I have a lot of very tall pine trees in Georgia plus some interesting structures like antenna towers, water tanks, etc. I was wondering what the best and safest method would be to use my Mavic Pro to measure the AGL of these things. Can the camera horizon line or the cross hatch display be...

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Found a few others too searching > Object Height

Measuring Heights

I live in an area with very tall Southern Pine trees and when trying to calculate a mission of Waypoints in Litchi, I have been “gestimating” and holding my breath that I have cleared these trees. Most times I over-estimate just to be sure which often makes me re-work my mission. However, I have...

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Estimating object (buildings, power lines) height

How do you estimate the height of objects, like buildings, power line poles, and signs? You can use trig calculations based on the angle from eye level to the top of the object, but that requires knowing the horizontal distance from where you are standing to the base of the object. It might not...

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[Ktblack]

Thanks for the response. Alot of useful info in those threads.

Yes we have some structures that are being assembled at my place of work. I would be launching right near the base and understand that would be my 0 elevation of the drone. I only need physical height from the ground level for this application. Was thinking of measuring a string approximately 6 feet and hanging from drone. Would then touch it to the highest point on the structure, then deduct the 6 feet from the reading on the drone to get my overall height. Maybe I'm reaching in thinking this will work but had to ask. Thanks again for your response.

 

[MAvic_South_Oz]

No problems, happy to brainstorm.
If you fly up directly next to the structure, keeping the camera at level, there may be a way to line it up in the centre grid (camera settings for camera grid pattern) and get you a fairly accurate height, within a foot perhaps.

On another thread somewhere, someone that flies through things with his mavic uses that grid, said if the centre grid is clear your mavic will go through, so it may be all you need to do.
Maybe test it on lower things to see how accurate it can be, and where to look through the grid to get the right level, something about 10' - 20' height should be easy enough to measure, fly up, and check this ?

It should be reasonable for a reasonable measurement, as the heights are in foot increments anyway, so can't get a more precise reading from the altitude readout regardless of using the string or screen grid method.

 

[sar104]

Thanks for the response. Alot of useful info in those threads.

Yes we have some structures that are being assembled at my place of work. I would be launching right near the base and understand that would be my 0 elevation of the drone. I only need physical height from the ground level for this application. Was thinking of measuring a string approximately 6 feet and hanging from drone. Would then touch it to the highest point on the structure, then deduct the 6 feet from the reading on the drone to get my overall height. Maybe I'm reaching in thinking this will work but had to ask. Thanks again for your response.

Is there a good reason not to use a laser distance measuring device which will be good to less than 1 mm over that distance?

Also - just FYI in case anyone asks - don't forget that this kind of application is not recreational, and requires a Part 107 remote pilot license if you are in the US.

 

[Kilrah]

I only need physical height from the ground level for this application. Was thinking of measuring a string approximately 6 feet and hanging from drone. Would then touch it to the highest point on the structure, then deduct the 6 feet from the reading on the drone to get my overall height. Maybe I'm reaching in thinking this will work but had to ask.

How precise? The aircraft only gives you its height to 1m resolution, and can easily be off by +/- 2m or so...

 

[MAvic_South_Oz]

I think you can set to feet in options @Kilrah ?

The laser thing is another way, not as much fun as doing it with a drone, but can be done if you measure the distance to base and top, know the angle, to get the rise.
Some devices may even be made for this and include inclinometer.
Just had a look as yes there are a few things out there . . .

http://www.haglofcg.com/index.php/se/support-news-2/download/media/en/76-angles-distances-and-heights-2016/file

True about part 107 if in the US, or similar legislation (licensing for commercial work) in pretty much all western countries under their rules to fly.

 

[Ktblack]

Thank you all for sharing. Will definitely look into the 107 license. Will also look into trying a laser to try out. Only reason I was trying to go a different route than the laser is due to the fact that the object I'm measuring has alot of obstructions. The highest parts can easily be obstructed by alot of smaller objects.

Again thanks everyone for helping me get a better understanding.

 

[CC Rider]

What has been mentioned above is accurate and applicable. No DJI drone has a true “altimeter” per se, definitely not one as accurate in a manned aircraft. These altitude devices are barometrically based and are relative at best so, as stated earlier, there will be a substantial discrepancy in whatever readings you obtain.

However, Earth’s gravity is a constant. Although its force varies minutely geographically, any point on Earth has a constant gravitational pull. So, another option for you to use to arrive at a very accurate distance of a structure is to drop an object of a known weight from the top of the structure, time it from the moment it’s released until it hits the Earth, then use the equations provided by Newton’s law of universal gravitation to compute the distance the object fell. Easy, efficient, accurate, no Part 107 required.

But, personally, I’d just use a surveyor’s laser transit to get an accurate distance within less than the breadth of a hair.

First, weigh the transit; toss it off the structure; measure the time until the transit hits the ground; then, apply Newton’s gravitational equations to compute the distance the transit travelled...

Just kiddin.’ A laser transit is the way to go.

 

[Wailuku]

New to drones, but have an application at work that they may help resolve my issues.

Was wondering if this would be possible. I am thinking that I could hang a string with a small weight from it. I would then lower until weight rested gently on top of the object.

My questions are as follows..(leaning towards purchasing the Mavic air for this but would consider another if it serves the application better)
1. Can the camera look below the drone enough so that I could view the weight while lowering?
2. How accurate would this be? All objects will be around 30 to 40 feet above take off.
3. Is this even possible?
4. Is there a drone better suited for this?

There are easier and cheaper ways to do this that do not require a drone (and thus Part 107 cert). Also, using gps-based approaches you have pretty poor accuracy in the Z axis so drone may not solve your problems depending on the precision you need. Others have mentioned a laser based tools, you could also use an inclinometer (such as used in forestry applications for measuring tree height), and maybe even try AirMeasure app.

 

[dsops]

The gps system gets best results for horizontal coordinates and much lower precision for vertical ones. And they are affected by obstruction. Flying is an area with buildings the coordinates may change if some satellites become obstructed or visible by the gps unit. in perfect condition you may expect a horizontal precision of several meters (~3m is maximum you can expect) and the maximum vertical accuracy cannot be better than 10 meters or more in perfect radio propagation conditions which you cannot control. So, the drone is not suitable for precision measurements. I'm using expensive gps trackers for recording travel paths and two records on a similar narrow path in different days may vary horizontal with 10-20 meters and vertical with 20-50 meters. Very expensive specialized gps units can do it better. Please also google "gps precision" or "gps accuracy".

 

[Kilrah]

DJI aircraft use a barometric altimeter as their height sensor though.

 

[dsops]

Yes, most gps devices use multiple gps systems & barometric measurements and also the IPhone has a very high quality such combo. But this not change too much regarding the height measurement precision. The drones also combined them with the gyro sensor, but both the gps signals and the pressure change around buildings, so I would not trust such "measurements".

 

[Kilrah]

You can trust them if the typical error is below your precision requirement... Which is why my first question in this topic was asking what precision he needed, while citing the typical error you'd get from a DJI aircraft over the course of a short flight needed to get that measurement.

 

[AMann]

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