Drone-based building inspections have come a long way in recent years. It’s no longer a case of taking hundreds of random images and sending them to the client. We’re now creating 3D models and sometimes even 4D models of buildings to understand more about their true condition.
But drone building inspections aren’t without their challenges. Even using remote sensors and UAVs can be tricky when a building is dozens or hundreds of feet high.
Here we look at the workflow for inspecting tall and supertall buildings using drones.
Getting Started: Understanding the Brief
The first thing you should do when planning any drone mission is to check with your client about deliverables. What are they expecting? Do they just want a facade inspection? Maybe they’re just interested in the roof condition. Or do they require a full building inspection report? Whether your building is one story high or a hundred, understanding your client or stakeholder’s needs is key to collecting the best data and getting the right outcome.
Option 1: A Roof Inspection
Now you understand your client’s needs it’s time to plan your mission. You’ve been asked to conduct a roof inspection of a super-tall building. This is where it’s super-important to take the ground offset into account. When you’re doing a roof inspection, in most cases you’ll want to make sure that your overlap calculations are done with respect to the roof, not the ground.
We explain more about Ground Offset and more in this video:
At this stage, you’ll also want to look at the polygon you’re using to capture the roof. If it’s a really tall building survey you might not be able to fly the drone to the top just to check all the boundaries for the polygon. Look at your map and give yourself a healthy margin outside of what you see.
Option 2: A Facade Inspection
If you’re just carrying out a facade inspection, you’ll want a vertical flight plan which is right in front of the facade that you want to capture. Remember to maintain a healthy distance - approximately five to 10 meters from the facade so that you can easily identify issues such as cracks, spalling, damaged wiring etc.
If you're surveying multiple facades it can be best to cover one facade at a time, depending on the clearance you’ve got on site. If you have more room to manoeuvre and want to cover more than one, make sure you’ve got a good GPS signal and good connectivity with the remote controller - safety always comes first!
Option 3: Full Drone Building Inspection
If your client requires a full building inspection of both the roof and facades, that’s where things get a little more interesting, especially when you are inspecting super-tall buildings. To create an accurate 3D model you've got to get both the roof and the facades in one picture. If you don't have an overlap between the roof and the facade images it can be difficult to then deliver your inspection images as part of an entire project.
Solution:
One solution is to fly the mission as you normally would for a roof inspection and then also to fly an orbit or a lateral capture which will capture the boundary of the polygon. You’d want to be looking straight down with an oblique angle of the building. That will allow you to take images of the roof and the oblique images taken in an orbit (at the same altitude as the roof) and create a 3d model from those images. Because the images are going to have a high overlap, your 3D model will be very detailed.
Once you have your initial 3D model you can then add your facade images to the same dataset and create your overall picture for your super tall building.
To recap, you’d follow these steps:
Capture your roof images
Capture an orbit or lateral capture around the tall building
Then add your facade images
You can now build a complete, comprehensive picture of the entire building, allowing a structural engineer, architect or any other stakeholder to see the building and any issues in 3D, from the safety and comfort of their office.
A Word of Warning
Remember the client and the brief. A random approach where you try to merge some roof image with some facade images will result in a disjointed project where the client can’t accurately tell where the images are from. Following the steps above will help you to make sure you have all of the data you need to build one comprehensive project.
Be Prepared…
Super-tall buildings can block your GPS signal. If you’re flying an automated flight plan you’ve got to be prepared to take control of your drone at any time. Even if you’re flying manually, you need to remember that losing GPS will mean that your flight conditions will change,
One way to prevent this is if you’re capturing a roof you can take off and fly to the height of the building, or potentially even higher, and acquire more GPS signal before you start your automated mission. If you’re using a DJI drone, we typically see increased chances of success when you have 15 satellites or more before you start your mission.
Go Big & Go Home
Another important consideration when surveying super-tall buildings is that you have your go-home height and emergency sequence in place. This should apply to almost every drone mission you do but it's even more important when you’re surveying a large building. Make sure you've got your go-home sequence nailed and multiple landing points designated in case an emergency occurs.
Drone Distances
Unlike capturing something on the ground, you do have something in your favour surveying super-tall buildings. As the ground is so far away from the drone and the building is so close, your GSD or Ground Sampling Distance will be very high. This means you’re going to capture a lot of detail.
If you want to know more about GSD and drones, watch this:
But what do we mean by ‘close’? How close should you get to the building with your drone? Ideally, you should be aiming for around 10 meters away but if you need to get closer then remember that most drones have obstacle avoidance sensors which will kick in when you get too close. Check with your drone manufacturer for more details.
Thermal Building Inspections
If you’re really going to go the extra mile for your client you may consider adding a thermal inspection to your workflow but don’t overcomplicate the process. Thermal building inspections look really great but they’re only useful for specific applications. If you are looking for moisture, water ingress or insulation-related issues then it makes sense to use a thermal camera. But if you can, try to plan your thermal flight alongside your visual flight to save time and money by repeating the whole process a second time.
If you're capturing thermal data make sure you plan your flight with your RGB camera as if you’ve got a good GSD on that you’ll automatically get a good GSD on your thermal camera.
The Final Detail…
Do not forget your inspection images. Capturing tall buildings is an art form and you want to capture one dataset that allows you to create your 3D model but you also want another dataset which includes all of the finer detail. You could use a high-megapixel camera with a high focal length to be able to capture from a distance but typically you’ll need a much larger, heavier and more expensive drone - often a no-no when you’re surveying in a built-up environment.
To provide the most meaningful results to your client, your workflow should ideally prioritize your inspection images over your 3D model. Make sure your inspection images are sharp and clear taken close to the building and present them to your client to supplement your 3D model.
Conclusion
There’s a lot to keep in mind when you’re conducting inspections of tall and supertall buildings but safety always comes first. Drone building inspections are already much safer than traditional surveys but there are still risks which should be considered from the outset. But as with any drone mission, a well-oiled workflow will give you repeatable success project after project.
Drone building inspections are changing the way modern cities manage, monitor and maintain their structural assets. To find out more about using drones for structural inspections download our free eBook.