LiDAR vs photogrammetry. Which is best, which is more accurate, which tech will give the best results? 

Over the last few years, one of the most commonly searched terms (when it comes to surveying equipment) has been LiDAR Vs Photogrammetry. Today, we have debunked the myths and will talk you through the pros and cons of both.

The fact of the matter is, there is no easy answer as to which is best. It depends on so many different factors. What kind of project are you working on? Is your budget part of ongoing capital investment in a digital strategy or are you just dipping your toe in the water of this kind of technology? This is why we apply a consultative approach when it comes to finding the best, more appropriate tech for our customers and why we continually revisit this subject. The technology moves on so quickly so it’s time for a reboot – a sequel if you will.

So, what is photogrammetry?

Photogrammetry is perhaps the old stalwart of the geospatial technology family. If total stations were the Grandfather of LiDAR, Photogrammetry would be the easy-going Uncle from the other side of the family. More experienced tried and tested, stable and reliable. Not directly related, yet inextricably linked.

Photogrammetry was born around the turn of the last century, which makes it pretty ancient. Each year, newer, more advanced systems and processes are invented which makes tried and tested tech obsolete. Yet photogrammetry is still one of the most powerful surveying tools out there.

LiDAR is the young upstart. Born in the 1960s and originally used to measure clouds, LiDAR is still an emerging and developing technology. Up until around 3 years ago, the lightest LiDAR systems weighed in at around 10kg and had to be set up multiple times to cover a site over 600m. Nowadays, LiDAR is a trim, lightweight and highly efficient tool in the progressive surveyors’ toolkit.

But, which is best? Both LiDAR and Photogrammetry have their place. It all depends on your intended outcomes, on your subject matter, on your budget and equally importantly, the accuracy required. See…. the plot thickens!

Photogrammetry Accuracy

Photogrammetry uses photos to calculate measurements, whereas LiDAR uses lasers. At this early stage, you can already guess which technology is more expensive and more complex to manage. This is perhaps why LiDAR hasn’t succeeded in its evil plot to unseat photogrammetry at the head of the surveying technology table.

If your budget is limited but you still want accurate and ultimately fast results, you can’t go far wrong with photogrammetry if you’ve got the right setup.

You’ll need:

· An entry-level commercial drone with a decent payload capacity such as the DJI Matrice 300 RTK

· A good quality drone camera such as the PHASEONE iXM-100 or iXM-50, Share UAV PSDK102S, P1

· Photogrammetry software, such as PIX4DMAPPER, DJI Terra, Context Capture

· A trained drone pilot and….

· Someone with a degree of understanding of the outcomes you’d like to achieve. The software will do the rest.

Thanks to advancements in digital imagery, photogrammetry is more accurate than ever before. At this stage, it’s important that we quickly cover the difference between Relative and Absolute Accuracy.

Relative Accuracy – Defined by the accuracy of individual features on a map in relation to each other, not their physical location within a coordinate system.

Absolute Accuracy – Defined by the accuracy of the measured features on a map, orthomosaic or other location-based measurement system.

Photogrammetry is great when it comes to relative accuracy providing the overlap between images is high, the quality of the images is good, and whether ground control points (GCPs) are used.  Meanwhile, Absolute Accuracy is harder to achieve with photogrammetry without using ground control points or an RTK or PPK-enabled drone.

Thanks to DJI M300 RTK, when RTK is fixed solution, Share UAV PSDK102S 5 lens oblique camera, the professional drone camera will get high accurate POS for each perspective image, and achieve GCPs-free, which means you don’t have to use any GCP, improve your working efficiency greatly and reduce manpower, more cost beneficial for users.

The relative accuracy of a photogrammetric survey can be roughly estimated to be between one and three times the ground sampling distance (GSD). GSD is the distance between two pixels as measured on the ground, from the center of each pixel. As an example, if your GSD is 10cm, 1 pixel in your photograph will represent 10cm of coverage on the ground. The lower the GSD, the clearer your imagery, therefore the more ‘accurate’ your results will be. This is a very important factor when doing aerial photography. You can get GSD from 1.5cm to 10 cm when using PSDK102S. Below is a table for your reference:

To determine the best GSD for your project, you’ll need to consider the accuracy you require. The lower the GSD, the more data collected meaning more processing time. You’ll have to fly lower which not only means more battery changes but may also make your flight planning more complex. If you’re limited to a minimum flying height such as in urban infrastructure surveys, Share Data Manager to help determine the GSD before you set off.

This may all sound very complicated, but actually, photogrammetry surveys are quite simple and cost-effective.

You can get lessons from the official Share UAV YouTube account.

LiDAR Accuracy

LiDAR systems represent the ultimate in surveying technology. The platinum-plated systems know no bounds when it comes to collecting accurate and detailed data. Where once you needed a small team and a fair few hours to carry out even the smallest survey, LiDAR systems are now so light and stable that they can be mounted to a drone with great results.

LiDAR works by firing pulses of light from a laser towards the earth’s surface. The LiDAR system then calculates how long it takes for the light to be reflected back to the unit to measure distance. This method of distance calculation is known as the Time of Flight principle (ToF) and is similar to that used in Sonar systems. Many LiDAR systems are capable of emitting hundreds of thousands of pulses of light per second. When you’re comparing the performance of LiDAR systems, the laser pulse rate will be something you’ll want to consider. This ultimately dictates how many measurements you’ll get per second and how detailed your results will be without having to make multiple passes of your target area.

Where photogrammetry uses the pixels from a series of stitched-together photos (orthomosaics) to create maps and models, LiDAR collects and collates ‘points’ which are used to create a 3D representation. Known as a ‘point cloud’, this high-resolution data output can comprise tens of millions of measurements and multiple terabytes of data.

The result is not always a pretty picture. Unlike photogrammetry, a LiDAR-derived point cloud doesn’t rely on imagery to provide measurements. depending on your set-up, payload capacity, and software, it is possible to combine traditional colorized imagery with a LiDAR dataset for enhanced visualization.

But what about the accuracy? LiDAR has long been known for its accuracy – both relative and absolute, with some terrestrial LiDAR systems providers claiming accuracy down to sub-millimeter level. But, this is where it gets complicated

With UAV LiDAR, the accuracy of your results will depend on multiple factors outside of just ground control points and your GSDs. It will depend on your sensor, your GNSS system, IMU, your subject matter, and the intended output.

We like to share the good work of others and this great Case Study from Terra Drone compares the accuracy of photogrammetry with that of LiDAR for the purpose of creating a digital terrain model. Although the photogrammetric survey came out the clear winner when it came to the accuracy of the project, it’s important to note that, in the conclusion, the author, like us, sits on the fence. LiDAR can be used in any survey where photogrammetry can be used but not the other way around. Because of its passive nature, photogrammetry is ineffective if the target area is covered by dense vegetation or the subject of the survey has narrow features which need to be measured or monitored.

The Bottom Line

More than often, the bottom line is budget. LiDAR systems haven’t just become smaller and lighter, they’ve also become cheaper. Once upon a time, a standard LiDAR system would cost in the region of £250,000. As with all technology, cheaper does sometimes mean settling for a reduction in quality. Yet, the evolution of LiDAR has meant that exceptional results can now be obtained by a system at a fraction of the price. Read the Top 5 LiDAR Drone Sensors for Your Business here.

 

Despite huge reductions in the cost of laser scanning systems, photogrammetry set-ups are still far more budget-friendly. If you’re not sure whether LiDAR is the right investment at this stage in your drone strategy, then this may be the way to go. Our UAV surveying expert, Vanson Choo specializes in helping companies to incorporate a drone strategy into their standard workflow. From UAV platforms to sensors and software, we’re here to help you create a sustainable solution for your business.

Find your perfect drone solution with a complimentary 30-minute strategy consultation with Vaosn by submitting the form on our website

Zenmuse P1 & PSDK102S & L1 Payloads

The latest Payloads for the DJI Matrice 300 RTK are set to change the face of Aerial Surveying.

The Zenmuse P1 integrates a full-frame sensor with interchangeable fixed-focus lenses on a 3-axis stabilized gimbal. Designed for photogrammetry flight missions, it takes efficiency and accuracy to a whole new level.

The Share PSDK 102S, an exclusive official 3^rd party payload, integrated 120 million effective pixels, super configuration to deal with urban, rural, and other projects, with the DJI M 300 RTK flying platform, played an important role in the wave of real estate integration and cadastral survey work. The best industry mapping camera, drone camera

Highly integrated GCPs-free technology, deep access to the RTK communication, can achieve microsecond time synchronization with the drone, during the data acquisition process on-site, it can avoid the complicated GCP marking work in the early stage of the project, which greatly saves work cost and time, improves work efficiency. 

You can take a look on YouTube:

 

The Zenmuse L1 integrates a Livox Lidar module, a high-accuracy IMU, and a camera with a 1-inch CMOS on a 3-axis stabilized gimbal. When used with Matrice 300 RTK and DJI Terra, the L1 forms a complete solution that gives you real-time 3D data throughout the day, efficiently capturing the details of complex structures and delivering highly accurate reconstructed models.

For more information plz send us an email: info@shareuavtec.com

Source: www.coptrz.com