LiDAR vs Photogrammetry Overview
LiDAR vs Photogrammetry -that is the question! This is a question we hear frequently, so let’s set the record straight and give an honest and fair comparison of these technologies. Both acquisition techniques have their strengths, but deciding which one is best for your application can be a daunting task.
Note: For this article we will consider only UAV borne LiDAR vs Photogrammetry systems. This debate becomes significantly more complicated when manned aircraft systems are considered and that is beyond the scope of this discussion.
If you were to read only this opening paragraph, allow us to leave you with this synopsis: LiDAR is more accurate and expensive, while photogrammetry is more visually appealing and cheaper.
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For 90% of applications out there, this is the most important spec in the deliverable. There are the rare occasions where clients want a deliverable for presentation or marketing material, but that really isn’t applicable to what we are talking about today.
So what is accuracy?
Accuracy can be quantified in global (absolute) and relative accuracy. Global accuracy is how accurate the point cloud is in relation to known points in a coordinate system. Relative accuracy is how accurate the point cloud is relative to itself. If you have really good relative accuracy, then two points in your point cloud would be exactly where they are supposed to be in relation to the other points in your project. What this means is that you can have incredibly good relative accuracy (1-2cm), but have absolutely terrible global accuracy. This also means that good global accuracy is impossible without good relative accuracy.
LiDAR is inherently more accurate than photogrammetry. LiDAR (light detection and ranging) shoots outs lasers and measures the time it takes for the lasers to return. Since the speed of light is constant, and time is constant, then its 3D point in space can be determined. This is why LiDAR scanners can deliver high relative accuracy right “out of the box.” Photogrammetry uses the same principle that our human eyes do to establish depth perception and 3D perception. However, without any control added, the software has no idea whether the parking lot you just mapped was the size of your kitchen table, or 5 square miles. New software does a pretty good job of estimating this based of the UAV’s altitude and camera specifications, but still more vague than LiDAR lasers.
With enough control added, photogrammetry can get nearly as accurate as LiDAR. But acquisition and processing time are at least double for photogrammetry.
LiDAR vs Photogrammetry Cost
For many clients accuracy is priority 1, this is usually followed closely by cost. Traditionally, LiDAR was a highly accurate luxury dataset that was beyond most project budgets . Many firms opted for photogrammetry because the acquisition costs were more easily justified. LiDAR prices have been slowly creeping down as the technology matures and devices become cheaper and more capable. Despite the decreasing cost of LiDAR equipment, service prices have remained prohibitive because the cost of manned aircraft operations is up slightly. As a result up to this point photogrammetry has remained substantially cheaper.
Bring on the days of UAV borne LiDAR.
With the recent development of new lightweight LiDAR scanners, it is now possible to mount professional grade LiDAR sensors on UAVs. Initially, this was pioneered with the mindset, “let’s see what’s possible.” Now, the industry has realized how much more practical and efficient this operational model can be. Not only is the acquisition altitude lower for enhanced accuracy (less slant range and less ground ambiguity), but there are no hangars, timing out pilots, commuting, etc. This development finally puts LiDAR at a price point that compares favorably with photogrammetry. Initial site surveys and planning was traditionally outsourced to ground based crews. The process can take weeks and is painfully slow. Now UAVs can go in for a single day and get the same thing done for the same price.
If it is starting to sound like this may be favoring LiDAR a bit over photogrammetry, it’s going to get even worse here.
The real beauty of LiDAR is the information that the sensor gathers. LiDAR scanners with multiple return capability are measuring more than just a single point when a laser is reflected. These multiple reflections can for instance represent; the top of the tree, the middle branches, and then the ground. Processing and modeling software allows the user to analyze certain areas alone or filtered with other data groups. For example, filtering the point cloud data to remove everything except for the ground returns, resulting in a highly accurate bare earth model.
New LiDAR scanners, like our Riegl VUX-1 LR, can get upwards of 4 returns with each laser. Online Waveform Processing is a technology developed by Riegl that takes advantage of Echo Digitalization. The lasers receive echo signals at specific times along its path of travel. These signals are analog-to-digital converted, resulting in a fully digital stream.
Note the figure below.
Photogrammetry can only create points based on what the sensor of the camera can detect illuminated by ambient light. This problem is compounded by commonly used small imaging sensors with poor sensitivity. As a result, flights over vegetated areas, with limited ground visibility and deep shadows, produce few ground points and only treetops are accurately represented.
Photogrammetry on left, LiDAR on the right
The picture above does a great job illustrating this difference. Not only are there points on the ground with LiDAR, but the trees are covered in points along their length. Of course there are situations where vegetation and multiple returns aren’t quite as important. Examples being urban areas, structures, and roadways.
There is one defining difference that photogrammetry will always trump LiDAR with. That being, RGB. Photogrammetry’s point cloud has a RGB value for each point. This means that you’ll have a colorized point cloud. Colorized point clouds are very helpful. They are easier to analyze and interpret, and naturally fit in well with marketing material.
LiDAR point clouds can be colorized with ortho photos, but never looks quite as natural or organic as the photogrammetry point cloud.
Photogrammetry point cloud with 3D triangle mesh
LiDAR point cloud colorized by elevation
Equipment cost is the biggest proponent for photogrammetry. Someone can buy a Phantom 3, get a Pix4D license, and start offering photogrammetry services. However we won’t go in the lengthy legal reasons why this shouldn’t be happening. But it is entirely possible to buy a $6-800 multirotor from Best Buy and start doing it today.
In contrast, UAV LiDAR equipment is very expensive. Not quite as expensive as a 1.5 million dollar MD500 helicopter and $500,000 Leica LiDAR scanner. But Velodyne and Riegl scanners fully equipped with proper IMU/GNSS systems, L1/L2 base stations, required software, and a trustworthy UAV will put you back at least $150-$200,000. This is less than a manned aircraft setup, but significantly more than the Phantom 3. If you’re interested in learning the differences between UAV LiDAR and manned aircraft LiDAR, check out the link here: UAV LiDAR vs Manned Aircraft.
In the LiDAR vs Photogrammetry debate, as far as UAV acquisition goes, LiDAR is hard to beat. The added benefit of vegetation penetration and high accuracy makes it a no brainer when the price is on par with lots of photogrammetrists. However, for the service providers that don’t have $200,000 to drop on a LiDAR setup, then they can still offer a great product with photogrammetry.
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