Lidar vs photogrammetry. Which is best, which is more accurate, which tech will give the best results?
Last year, we wrote a blog to address the material differences between LiDAR and Photogrammetry. So why are we writing another one? Over the last few years, one of the most commonly searched terms (when it comes to surveying equipment) has been Lidar Vs Photogrammetry. The fact of the matter is, there is no easy answer as 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.
A Quick Recap…
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 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 set-up.
- An entry-level commercial drone with a decent payload capacity such as the DJI Matrice 600 Pro
- A good quality drone camera such as the PHASEONE iXM-100 or iXM-50
- Photogrammetry software, such as PIX4DMAPPER
- 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.
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 centre 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.
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, PIX4D have created a handy little calculator 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.
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 of 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. That said, depending on your set-up, payload capacity and software, it is possible to combine traditional colourised imagery with a LiDAR dataset for enhanced visualisation.
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-millimetre 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, James Pick specialises 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.
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