Author: Shahryar P. Monajjemi, Parisa Mansourian
Date: 09-05-2019
Hello Everyone!
Hope you all had a great Memorial Day weekend.
In this post, we want to share more information about laser scanning technologies, focusing on Aerial and Mobile LiDAR and Photogrammetry.
We received some relative questions through our blog email (blog@bimnyc.com) which we are going to address some of them in this post and discuss in more detail about the technology behind each answer. Again, if you have any questions or if you would like us to discuss about any particular topic, please feel free to email us your request.
Q1- Is LiDAR same as photogrammetry?
A1: No. They are completely different technologies. If we want to make it simple, we should say the accuracy of 3D Laser Scanning (or LiDAR, Laser Surveying, Laser Scanning, 3D Scanning) is very much higher than photogrammetry. Photogrammetry is a series of overlapping images, matched by each other’s pixels to create an overall 3D image of target area. It can be done very quickly, the implementation is cheaper than LiDAR, but is not a reliable tool to be used as an engineering aid tool. If you have a drone with a normal camera (around $1k), you will be able to perform photogrammetry from low elevation, but of course, for a better quality you need to spend more money on your equipment.
Until 2017, Google was using the same technology to generate the street views. After 2017 in some locations they added LiDAR scanners on their vehicles too.
Let’s talk about a simple example: you need data for façade renovation of a historical building. A detailed as-built drawing is needed. There are several details in the façade and dimensions must be precise. Photogrammetry can be done in less than 10 minutes for this building, but you will not have the dimensions of the bumps, details, window frames, etc. in your image data. You will have an overall 3D image of the entire façade. But with LiDAR, you will be able to acquire all that information.
Another example: a mine operator wants to know the approximate amount of excavation in a high elevation area. A photogrammetry drone can capture all required data in less than a day, and the approximate volume of excavation can be calculated in various software programs like Revit. If they want to utilize traditional terrestrial LiDAR, it can take days with a massive amount of point cloud data, not so easy to use.
Q2- Can LiDAR be performed by drones?
A2: Very good question! And YES it can. It is called Aerial LiDAR. Until a couple years ago, it could be done through airplanes and was very expensive. Also, there was issue of very high altitude and vibration, which needed to be resolved in a series of filtering and optimization software programs. Please note that, for performing laser scanning, the device has to be in a stable position with minimum vibration. Recently some companies developed drones, compatible with industrial laser scanners (e.g. some series of FARO scanners).
That means if you already have a laser scanner, you just need to buy that drone. But what is specific about this? Why this is a different thing comparing to the past? Because laser scanners are heavy and needs more powerful drones to lift it. In the meantime, it needs to minimize the vibration. But more important than drone itself, is the registration and processing workflow of aerial LiDAR. Why? Because however you capture the data from the air, there will be some amount of vibration due to wind and motors vibrations, which generates noisy and wavy point cloud data. The processing programs with some specific algorithms fix the vibration issue. Also, the drone moves in the air and is not stable. Please remember, laser scanners calculate each point’s coordination by light detection and ranging, which is a distance dependent logic. So when the device moves, an algorithm needs to figure out the distance parameter to generate a clean point cloud data.
But do we really need aerial LiDAR? For sure! It can save time and money significantly.
I would not say we definitely need this technology, but imagine you need to laser scan Golden Gate Bridge. Do you really want to set up your scanner on several positions on a barge on the water? Or do constant lane closures? How many positions you need to capture? I would say more than 200 positions from top and bottom of the deck! Now imagine how easy it can be done by aerial and mobile LiDAR. Mobile LiDAR? What is this one?! Well, if you assemble a scanner on your car, this will be a mobile LiDAR. Yes, this easy, but this also needs special equipment. Like aerial LiDAR, the equipment for mobile LiDAR was also very expensive in the past. It was a vehicle itself, equipped with LiDAR devices. Now there are platforms which are compatible with industrial laser scanners. Devices can be installed on the platform, and the platform can be assembled on a car. An advantage of mobile scanner is, for both geo-referencing and registration process, mobile scanner platforms uses GPS devices. Which means there is no need for post processing and geo-referencing point cloud.
Q3- What are the advantages of mobile scanning?
A3: One example of mobile LiDAR was the bridge scanning example which could save time and money. The other one is roadway and railway scanning. Let say New York City Department of Transportation wants to evaluate the quality of asphalts. With photogrammetry technology, it is not possible to detect cracks, crack depths, bumps, manholes dimensions, etc. on a street. But with mobile LiDAR technology a comprehensive information of above mentioned factors can be acquired.
In our next post, we are going to answer more questions, along with topics as follows:
Noise and reasons of noise in point cloud
360 HD photos, and color point cloud
Point cloud conversion and semi-automated object detection
At BIMNYC, we are using cutting edge technologies to suit our clients needs and budget. Our team of engineers and designers are trained, educated and experienced in Building Information Modeling (BIM), Virtual Design and Construction (VDC), 3D Laser Scanning, and Digital Construction Management techniques, and their related technologies such as remote sensing, computational design, and asset management technologies.
Our goal is to share what we know and help you, as the client, to achieve your goals. If you are interested in collaborating with BIMNYC, give us a call at 212-433-3553 or shoot us an email at info@bimnyc.com
Cheers,
BIMNYC Team