Multipurpose Scanning Systems - A Good Idea?
Published On: July 01, 2020
This time last year, we introduced the True View 410 3D Imaging System (3DISâ„¢) for unmanned aerial vehicle (UAV) mapping applications. A 3DIS is a laser scanner, one or more cameras and the math/software to make it all work. A 3DIS is designed from the ground-up as a colorized point cloud generator. In June, we introduced the True View 615 and 620 systems. These are 3DIS that use the new RIEGL miniVUX-2UAV laser scanner, bringing survey quality data to the GeoCue 3DIS family of sensors. Below we will answer some questions that have arisen with the release of our new systems.
Can a GeoCue 3DIS be mounted on a ground-based vehicle for mobile laser scanning (MLS)?
The answer is - "yes, in theory though the geometry of the laser and cameras is not very good for this application - you would be disappointed in the results." Of course, I understand the motivation for wanting a dual use system - you cannot always fly a project due to drone flight restrictions, you can't easily fly under overhead obstructions and so forth.
I will say that we have a tremendous amount of experience with both airborne and mobile laser scanning (ALS/MLS) systems from the work we do in the ALS/MLS software and training part of our business. Our experience with assisting customers who have dual use systems has been rather spotty. An MLS requires a different sort of camera system than an ALS. An ALS being used for aerial mapping needs imagery over the useful cross-track field of view of the laser scanner (typically 90° or less). An MLS, on the other hand, needs image data for at least a 180° field of view. A typical MLS camera is shown in Figure 1.
Figure 1 : FLIR Ladybug 360° Camera System
A further handicap is that a truly professional MLS will have dual laser scanners. This is necessary to enable forward-looking scans. Right off the bat, a dual use system is going to be a compromise since it will have a single laser scanner. This means a lot more drive passes to collect data. Now that is not a big deal - we're all willing to make compromises to save money. The big deal is merging all of the data lines collected in these multiple passes. It is tough to drive north on a road to collect the north face of assets (assuming your system is mounted in the conventional rear facing position) then turn round some minutes (sometimes hours) later to collect the south-facing data. It is very finicky to get these data to merge at survey-grade accuracy. It typically takes a lot of ground/wall control and a bit of dubious data warping.
So are dual use systems just a bad idea altogether?
No, I don't think so. They make good asset collection systems (often termed "mapping-grade"). Asset grade generally means I will be happy with network accuracies of 10 cm or worse, Root Mean Square Error (RMSE). Even if you have a survey grade (say 3 CM RMSE network accuracy) on your UAV, do not expect to remove it, configure it as an MLS and get survey grade mobile scanning. You are going to have to do a pretty detailed calibration after the move. So asset-grade work is the sweet spot for dual use scanners. Thus if you are using a scanner with fairly poor network accuracy (say a Velodyne VLP-16, for example), this would be a good candidate for a dual use asset grade system. A second scenario is very long duration operating modes. For example, if you are going to move your drone-based laser scanner to a land-based platform and keep it there for a long campaign, it might be worth the days of work to get the new configuration calibrated and fully functional. One final thought on this - do not purchase a dual use system that does not have a special camera configuration for MLS operations. It is just too difficult to do data extraction in an MLS scenario without properly calibrated imagery.
Will GeoCue build a dual use system?
I would say we are in the evaluation mode. We won't proceed until we solve the problem of keeping the system in calibration between moves. I would want to introduce a system that changes the market, similar to the True View 410 and True View 615/620 3DIS. If you have to have a dual use system now, by all means proceed. Just be very conservative in your expectation of system performance - you'll have an OK airborne system and a rather marginal mobile system. If you must perform survey grade mobile mapping and do not want to invest in a purpose built MLS, consider subcontracting that portion of your work.