The seventh of Blue Marble’s GeoTalks Express online webinar series entitled Using Lidar for Archaeological Research, was conducted on June 10th, 2020. During the live session, numerous questions were submitted to the presenter, Forrest Briggs from LiDARUSA. The following is a list of these questions and the answers.
What kind of ASPRS points do you use to build DTM with buildings?
Ground classified points for the surface (DSM) and first returns for the DTM.
For the aerial mission did you employ ground control points? If so, how many targets on a typical project?
LiDAR is an active sensor meaning each point already has an XYZ, Photogrammetry does not provide that natively meaning control points are required to determine the position of the data. We always recommend using control points for survey-grade or construction-grade accuracy projects, however, for more archeology projects, it is not necessary. We recommend control, but it’s not required. It depends on the project.
What about bidirectional nadir satellite imagery?
Satellite and UAV don’t go hand in hand. Geo-referenced data (lidar and/or imagery) can be used together regardless of the source.
How is the laser aligned to the photogrammetry? Do you combine the reconstruction data from the photogrammetry with the laser scan data?
All of the instruments are bore-sighted, calibrated, and then geo-referenced to a common coordinate system.
If you scan it for free who owns the data you collect?
We own the data, along with the sponsor of the project and normally the country or landowner.
Can this be used to get bathymetry in a shallow lake?
We don’t manufacture green laser systems for bathymetric surveys, they are one very expensive and have a very limited application. They work well in clear water, zero turbidity, and Secchi depth.
Are your drones equipped with PPK/RTK module or do you need a typical base/rover setup?
We utilize PPK systems for LiDAR systems, RTK is far too limiting for LiDAR projects, you will either need a local base station near the project or take advantage of satellite or CORS networks base stations or utilize the advanced PPP algorithms that are created during an extended static aliment.
For the post processing of the data, do you exclusively ues GM?
We utilize our propriety software to create and fuse the lidar data with the IMU and GNSS to create a las or laz file format, that is what you import into global mapper for classification and feature extraction.
How do you separate the data from the different channels? Are there distinctions in the wavelength?
It varies with the scanner. The LAS files allow for annotating every point per channel, which we do.
What would you say are the main advantages of Lidar against photogrammetry and vice-versa?
With lidar, you can get points on the ground in most cases whereas photogrammetry is a “first return” system (so tops of grass and trees). Lidar is also better at sharp linear features such as transmission lines and railroads. Water boundaries are much easier with lidar as well. It is not a matter of accuracy on a hard surface as you can always get a better camera for this. Images are easier for a normal person to interpret.
How long did it take to fly the Uxmal area and what was the point density?
The 25 acres was collected in a 12-minute flight with 20% battery left. We were using our Revolution 120 lidar system mounted to the V1 DJI M200.
Do channels equate to returns?
A channel typically refers to the laser number and orientation. Each laser or channel has either 2 or 3 returns depending on the system. For instance, a VLP-16 is a 16 channel system, capable of 2 returns per channel.
Are the flights automated or are you manually flying the drone?
Generally, flights are automated in fixed-wing mode, you can fly manually in a pinch.
Is the output proprietary or .las/.laz format?
We export into las / laz / e57 / txt / XYZ / and several other formats. The most common are las and laz.
Did you have to file a flight plan with the FAA to be able to fly this mission?
Generally speaking, no flight plan is required to fly a UAV in the USA.
How much detail can you get on vegetation?
Can you identify individual species? Generally, the detail on the trees would need to be combined with on our LiDAR systems that integrates a LiDAR and camera to ensure you were to determine leaf and color and design.
Do you encounter a lot of high/low noise with your systems and do you find the Global Mapper auto noise classifier is sufficient in cleaning the data or do you need to couple it with manual noise cleaning?
Yes, all lidar systems create noise, regardless of what presented data is shown in marketing material. We use the GM automatic feature for the majority but almost always you will need to clean the data using manual methods as well.
What kind of drone you used for the lidar survey?
We use lots of different drones, multirotor, RC helicopters, gasoline, battery-powered. Your application and budget will determine what type of drone to use.
For water applications, how deep does lidar penetrate below the water surface?
We do not offer bathymetric lidar.
Do you use full waveform?
All LiDAR is waveform but only a few companies actually store it in raw form (it’s expensive to store and process). Most systems store discrete returns, which has proven sufficient for 20 years. We offer both for some systems.
Can you tilt the equipment when flying it on a helicopter, for looking sideward?
We offer 360-degree lidar systems as well as nadir lidar systems. Yes, you can mount the lidar whichever way you want and we of course will provide our expertise to ensure you are satisfied with the end result.
Flying time is the one limitation, but the other one surely is the data volume. So what is the max storage capacity onboard?
Flight time is a function of the drone, we offer a long-range drone with a 2-hour flight time, the data volume is also not an issue we provide with the system and external drive that can store 14 hours of scan data. We started building lidar systems for mobile ground-based scanning and normal scanning days can easily reach 8-10 hours, we have carried that technology forward to our UAV LiDAR systems to ensure you can scan all day without stopping.
Has LiDAR been used to find things that are underground?
LiDAR does not penetrate the ground, you will need GPR for that type of work.