What if you’ve got a tree in your yard that looks like it may come down in the next big storm, and you want to know whether it will hit your house if it falls. You want to know if your drone can be of any use to measure the height of the tree so you don’t have to wrack your brains trying to remember something from high school geometry about triangles and shadows to figure out how to calculate the height. So, can drones measure height?
Drones can easily be used to measure height. The simplest way to use a drone to measure the height of an object or structure above the ground is to fly the drone to the level you wish to measure, and look at the altimeter reading on the screen of your controller.
Perhaps you want to use your drone to measure other things such as distance, area, or volume. It is certainly possible to use a drone for those measurements as well. Distance is also a fairly simple measurement, but for area and volume, the methods for measuring become a little more complicated, and may require other software programs to aid in the calculations.
Your typical consumer drone such as a Mavic or Phantom comes with a built in altimeter. One of the primary purposes of the altimeter is to help you keep your drone under the 400 ft Above Ground Level (AGL) height limit. As you fly, you should be able to see an altitude reading on the screen of your controller (often on the bottom of the screen), usually measured in feet in the US. Depending on the drone, you may be able to set the reading to be measured in meters as well.
This altitude reading of the drone is measured from the take-off point of the drone, and does not measure directly to the ground underneath its position at any given moment. The reason for this is that the altitude is usually measured with a barometer to detect changes in air pressure with increases or decreases in elevation, relative to the drone’s initial position at take-off. Many drones also provide a VPS height measurement that is measured with downward vision sensors. This measurement will tell the distance to the closest object below it, whether it’s the ground, a tree, or some other obstacle. GPS is not used as part of the altitude measurement function.
So what does it all mean? This means that if, after take-off, you fly out over a cliff or the edge of a building, your altimeter will give you a negative reading. Or if you fly up the grade of a hill, your altitude measurement may read 50 feet, when you’re in reality only 20 feet above the level of the ground as you’re flying up the grade.
Going back to the example of measuring the height of a tree, in order to get an accurate measurement, you would need to take off from directly at the base of the tree, and fly straight up to the top to get the most accurate altitude reading. You couldn’t, for example, take off from your second floor patio, fly over to the tree, and read the altitude measurement and get the right height – it would be missing all the distance from the ground to the level of your patio.
In depth studies have been done to determine the accuracy of this method of measurement using a drone, and the findings are pretty much that it gives a “close-enough” reading. If you’re concerned about precise accuracy, one method to improve the accuracy of the altitude reading is to land your drone and turn it off between each flight to measure a different object. So, if you want to measure the height of each tree in a row behind your house, fly each one separately, land the drone and turn it off before moving on to the next one.
If you’re really concerned about measuring height with pinpoint precision accuracy, this can also be done with a drone, but perhaps not with your standard Mavic. There are a number of Professional and Enterprise level drones that can be equipped with laser range finders and LiDAR sensors that would be able to give much more precise height measurements.
Similar to the method for measuring height, a typical drone can also be used to give a “close-enough” distance measurement. On the screen of the controller, there is usually a reading for distance, and this would mean the distance of the drone from the controller, and presumably from where you are standing.
Let’s take another for-instance. You want to know how far it is from your house to the end of your driveway. If you stand at the door of your house, and have your drone take off from right in front of you, then when you fly to the end of your driveway, the distance reading on your controller would tell you how far away in feet the end of your driveway is from you.
There are a few important limitations to this method of measuring distance that you need to keep in mind. The first is that you are required to keep your drone in physical line of sight from where you are operating it. This would limit your ability to measure distance in places that have a lot of obstacles that would obstruct your view of the drone.
Also you need to keep in mind that the drone cannot travel beyond the range of your controller. For a standard consumer drone, this is probably somewhere around 500 feet or so. If your drone gets beyond the range of your controller, you risk a flyaway drone at worst, or at best, the drone will automatically return to home when the radio control signal is lost. These two limitations mean that an average drone can only be used for measuring relatively short distances, at least using the simple, no extra software or knowledge approach.
Drones are commonly being used in industries such as surveying, mining, building construction, agriculture, and more, to measure all kinds of things, including area and volume. This is typically done with a method called photogrammetry. In basic terms, photogrammetry involves taking one or more photos, making measurements of the objects in the photos, and generating a three dimensional reconstruction based on those measurements.
This process would be quite complicated and inefficient to try to achieve on your own, but if you want to get accurate area or volume measurements from images that you take with your standard drone, there are a number of software options (Pix4D, DroneDeploy, DJITerra, SimActive) that would allow you to upload your photo files, and would turn around and deliver you measurable maps or 3D reconstructions based on your images. You could use these maps to find measurements of things such as land area or even the volume of objects or structures.
For absolute, centimeter level accuracy, you would need to go beyond the capabilities of a standard drone, and venture into the realm of enterprise drones that would offer LiDAR (Light Detection And Range) sensors that can get through vegetation and other obstacles to give accurate ground level readings. This professional level of drones and camera sensors also offer georeferenced imaging to accurately connect images to geolocations.
If you want to use your drone to perform a few simple measurements of height or distance, it’s a very simple and straightforward process to take a few readings right from your controller screen. These readings can give you a more-or-less accurate measurement of the height of your tree or roof, or the distance from your porch swing to your mailbox. For more complex measurements, or where a greater degree of precision is required, you may need to call in reinforcements from other software programs.
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