Manual for performing roof inspections with drones

In the US, the rise of large commercial buildings is huge. This has significantly increased the demand for safe and effective inspection of roofs.

This is because every roof is unique and has multiple inspection needs. Leaks and holes can cost thousands of dollars and cause other problems in a building. Moreover, most large commercial buildings have installed HVAC systems and solar panels, all of which require routine inspections.

The rise of drones over the past decade has changed the way we inspect roofs. For example, craftsmen no longer need to fetch ladders and – literally – get off the ground. During a simple drone flight, they gather valuable information that can be easily fed back to stakeholders and decision-makers.

In this article, we go through the steps to take when using drones for a roof inspection.

Collecting data

HOW DO YOU UNDERSTAND ASSETS?

Roofs come in all shapes and sizes and from residential to commercial. When inspecting roofs, it is important to look at the size of the project so you can understand how best to address the site.

A first factor to consider is the size of the roof. With small roofs, additional details can be captured in minutes (or even seconds). When it comes to large commercial roofs, a longer escape time may be required.

In addition, the height of the building is an important factor when planning a mission. A quick flight to the top of the building can give you an idea of its height. This will help you plan your mission better.

A final point is the building’s surroundings. After all, we always want to fly safely. If you are inspecting the roof of a building and there is a car park next to it, make sure you follow the relevant guidelines for flying above people. When planning a mission, it’s also best to make sure the green flight line doesn’t fall too far outside the perimeter of the building, if that’s a factor.

For these types of missions, the Mavic 3 Enterprise uses O3 Enterprise Transmission for a stable connection to the drone. Together with Omnidirectional Obstacle Avoidance and APAS 5.0, this keeps the drone safe when flying in tight environments. Together, they ensure the drone gets home safely when the mission is complete.

SET YOUR MISSION GOAL

On a roof, there may be different assets that need inspection. Each requires a different data source (visual, thermal, etc.) or different accuracy/resolution requirements.

Some of the most important things to inspect on a roof are the following:

• Crack or leak detection
  For leak detections, try not to fly immediately after rain. Wait at least 24 hours to a week after it rains to gain insight into drainage/leakage. Thermal analysis will also be challenging if flown too soon after a rain shower. Especially if there is standing water hiding a problem.
• HVAC inspections
• Solar panel inspections
• Exhaust inspections
• Measurement needs

In situations where you need a thermal sensor (solar panel inspection, leak detection, HVAC inspection, etc.), the flight should often be performed just after sunset. This way, you avoid thermal stress from direct sunlight, even though the roof or solar panels are often still warm from daytime. In turn, finding cracks in a roof is almost impossible with a visual sensor at dusk. Therefore, it is better to fly over the same roof twice: once before and once after sunset.

It is also important to estimate the size of the building. Take your time to fly over the surface. The Mavic 3 Enterprise – with RTK module – has about 42 minutes of flight time. This makes large missions possible. Plan your missions accordingly.  

Another aspect best kept in mind is the accuracy of the data for your rooftop inspection. It can often be difficult to measure targets with a base station on top of the roof. With the Mavic 3 Enterprise and   RTK-module  however, you can measure accurately down to centimeter level without the need for ground control points. However, control points are still needed to validate accuracy. In roof inspections, accuracy is often not of primary importance because many of these applications are inspection oriented. But if the data needs to be aligned with other data on a construction site, RTK is a great option. Thanks to RTK, PPK and Cloud PPK technologies, you get a high level of accuracy for your projects.  

CHECK THE SENSOR SETTINGS

There are a few different factors to consider when choosing your camera/sensor settings. The automatic setting will usually be enough to gather good data. But if you are looking for some guidelines for setting the visual sensor, we’ve penned down our recommendations below.

• Shutter speed of 1/1000 or higher during a day flight. During night flights, motion blur will always be an important factor. Try to set the shutter speed as quickly as possible while you can still see the roof clearly.
• Use ISO to compensate for the shutter speed. During the day it is best to keep the ISO at ‘Auto’ but during night flights you can use it to make the image ‘brighter’ if you need faster shutter speeds.
• Image format: JPG
• Aspect ratio: 4:3
• Mechanical shutter: ON
• Sensors to record (with thermal recording): ALL

Are you performing a thermal inspection? Then we usually suggest setting the color palette to IronRed. Otherwise, there is a big color difference between the temperatures in the camera’s image.

We also recommend taking the time to do a quick flight over the roof at the start. This can help you find the best camera settings even before the flight. A roof can be much brighter than you expect. If you fix camera settings manually during the first waypoint, the images are often ‘blown out’.

PLAN YOUR FLIGHT

The most common method of inspecting a roof is to collect enough overlapping photos. This is how you create a high-resolution map and 3D model of the roof. When flying with the Mavic 3 Enterprise, you can do this using the DJI Pilot 2 App.

When planning missions, it’s then again best to choose the ‘Mapping Mission’ option. We’ll give you some settings below that we specifically recommend for roof inspections:

• Use the default overlap settings of 70% front overlap and 80% front overlap. This should be sufficient for a high-quality reconstruction of the 3D model for the visual sensor. Are thermals required? Then we recommend 80% side and front overlap.
• When choosing an altitude, you will want to use both the gliders Flight Route Altitude and Target Surface to Takeoff Point. The optimum flight altitude above a roof for residential buildings is 25-50 feet above the roof. For larger commercial buildings, that resolution may not be feasible. 50 to 100 feet above the roof should be sufficient here. By scheduling a quick flight to check the height of the building, you can then set the mission altitude. For example, if you are checking the height of a residential roof and the roof is 25 feet, set the target area to 25 feet and the mission height to 50-75 feet. If you are checking a roof 50 feet high for commercial use, set the Target Surface to Takeoff Point to 50 feet and the Flight Route Altitude to 100-150 feet.
• By using the Target Surface to Takeoff Point slider, you will still achieve the correct overlap settings. Even when the drone is launched from the ground. The Mavic 3 Enterprise’s 4/3″ sensor lets you capture incredible detail with a wide dynamic range.

Is 3D reconstruction the goal of your mission? When using the Mavic 3 Enterprise series to do so, the Smart Oblique function may come in handy. It helps you take control of the gimbal during the flight to automatically capture oblique images instead of just NADIR.

IMPORTANT: if you need to perform a solar panel inspection on a roof that uses heat, Smart Oblique is not recommended for accurate temperature measurements.

Other aspects to consider are flight time and speed. The Mavic 3 Enterprise uses a 4/3″ mechanical shutter that allows fast recording while maintaining image accuracy and minimal image distortion. With a recording time of 0.7 seconds, the drone can map much faster than previous versions. Flight speed is not that important for the Mavic 3E. However, if you are performing a thermal inspection with the   M3T , we recommend limiting the speed to less than 10 mph (~4.4 m/s) to minimize image blur and incorrect image readings from the thermal sensor.

Bij het plannen van de vliegrichting en het vastleggen van alleen visuele beelden, is het aanbevolen om in de meest efficiënte richting te vliegen. Voor thermische inspecties van zonnepanelen op daken is het dan weer aangeraden om parallel aan de panelen te vliegen. Zo krijg je de beste resultaten bij het verwerken van de gegevens. 

CAPTURE YOUR DATA

Now that you understand the building, have determined the scope of the project, and have prepared your mapping mission, you should be ready to capture the location.

• Make sure you can always maintain a visual line of sight with your drone. This can be difficult when capturing rooftops.
• Keep a close eye on the drone’s flight plan and FPV camera to make sure you are not flying over people.
• Is your mission complete? Then the drone will come back or stay put. This depends on the end-of-mission settings.

MANUAL INSPECTION

After your automated mission is complete, you can (optionally) capture additional data from the location. Through manual capture, you have many features to help you get the most out of your manual inspection. The Mavic 3 Enterprise and the Mavic 3 Thermal both use a 56x hybrid telephoto zoom sensor. By using the right scroll wheel, you adjust the zoom level of the sensor.

To better understand your target during this manual inspection with the Mavic 3T, DJI offers Side by Side view. By clicking the SBS button on the screen, you can choose to display both views at the same time. And see both the zoom and thermal camera side by side.

Do you use the zoom sensor of the M3T? Then we also recommend using the Link Zoom function. That way you keep the zoom and thermal sensor locked at the same zoom level.

Process data

THERMAL AND VISUAL DATASETS

Once you’ve captured the location, it’s time to convert the data into a high-quality 2D orthomosaic and 3D model. DJI Terra makes it easy to create great datasets.

Quick steps for processing data with DJI Terra are:

• Import photos/folders into DJI Terra.
  Are you processing both visual and thermal datasets? Then it is recommended to process the datasets separately.
• Select the desired output types (2D map, 3D models) and file extensions (TIFF, Obj, etc.) and define the coordinate system (if using an NTRIP service).
• Perform the aerotriangulation.

Optionally, you can change the boundary of the reconstruction at this point. This can help speed up processing times and output data size when focusing only on the object to be stitched.

• Optional: import Ground Control Point data and select the appropriate EPSG code for region.
• Perform the reconstruction steps for the 2D map and 3D model.

Note that DJI Terra does not guarantee radiometrically stitched output, only the raw images.

When done, you can view the Accuracy Report to understand the accuracy of the map. Your data is now ready to be viewed and exported.

DISPLAYING DATA

DJI Terra has several features that allow you to view and analyze your data. Thanks to the annotation tools, you measure cracks and leaks. You can then use the mouse to navigate through the 3D model. And for prolonged viewing, DJI Terra even has a tool to put the 3D model into orbit indefinitely.

But let’s look at the typical outputs for roof inspections.

When looking for leaks, cracks, and thermal irregularities, it is common to analyze a 2D orthomosaic rather than a 3D model. The latter helps you put the location in perspective. But often external thermal inspection analysis tools analyze the raw images, rather than the 3D model. If a customer requests a dataset, there are some supporting outputs from DJI Terra. All exported data is georeferenced and can be imported into an external analysis tool of your choice (DroneDeploy, Raptor Maps, etc.).

DJI also has a Thermal Analysis Tool. Within this application, you can analyze raw images and processed datasets to fully understand temperature measurements. There is also a publicly available tool from Eric Olsen to convert your thermal data to RJPG. This way, you can import them into Flir’s Thermal Analysis tools.

THIRD-PARTY ANALYSIS PROVIDERS

There are many specialized solutions for automating inspection analyses. Do you want to automate crack or leak detection or solar panel inspection or more? Then the below providers can help you automate the workflow with their solutions.

DroneDeploy is a cloud processing provider that has mapped and processed more than 500 million acres worldwide. Their tools cover various sectors (construction, agriculture, oil, and gas, solar, etc.). DroneDeploy has a few specialized tools and reports specifically for roof inspections.

With their Roof Report, for example, you obtain dimensions of roofs from a processed 3D model. Its use is more for solar roof planning and understanding roof dimensions and has no automatic damage detection.   

In addition, DroneDeploy’s radiometric thermal analysis can help determine problems within a thermal map. In doing so, use the histogram on the left to change the temperature range. They also have a side-by-side tool to understand the differences between multiple flight data.

If the mission goal is more focused on damage detection, Loveland Innovations and Eagleview are two great options for automated damage detection. They have several tools that can detect not only hairline cracks, but also small holes/pits of hail and tree damage.

Then again, is the mission objective thermal? Then there is Raptor Maps, which is known for analyzing thermal images. It has so far analyzed more than 50 GW of solar panels. Their tools are therefore the go-to for solar panel analysis.

Want to know more about the Mavic 3 Enterprise series’ capabilities for roof inspections? Or are you curious which of these tools can help you with this?  Contact us for advice tailored to your project(s).