What format is the Mobile Mapping Data delivered in?
For those purchasing professional LiDAR services, you are well advised to know your provider and “trust but verify” that their deliverables meet your specifications.
Riegl’s spec sheet for the system says that the accuracy for the system is 2.5 cm (1 sigma at 150m above ground). Previously, radar was used to acquire these speeds but the accuracy of the system was always in doubt. A better understanding of the accuracy of LiDAR is perhaps best described on the speed guns often used by cops. 6315 Chancellor Dr. Cedar Falls, IA 50613The final accuracy of a point cloud (or the bare earth elevation model derived from the point cloud) is a product of the LiDAR “system” and a “production process”, and not simply the ranging from the laser.
Second, these accuracy specifications are usually quite conservative. How well the system is calibrated plays an important role. A modern LiDAR system has many other components (like a GPS sensor, inertial system, mirror, aircraft, etc.)
Riegl’s spec sheet for the system says that the accuracy for the system is 2.5 cm (1 sigma at 150m above ground). Really interesting article about bench-marking accuracy, but it doesn’t really address LIDAR hardware itself. This can be quite difficult and expensive. Radar shoots out a short, high-intensity burst of high-frequency radio waves in a … "Anyone relying on lidar is doomed."
Second, these accuracy specifications are usually quite conservative. Operating a lidar through a windshield will diffract the infrared beam introducing an angle alignment error. that when integrated, calibrated, and operated together all contribute error to the ultimate accuracy of the point cloud.
Bluesky LiDAR has a vertical accuracy of up to 10cm +/- RMSE and a horizontal accuracy of up to 5cm +/- RMSE; Latest Support Articles. The degree of interference depends on the intensity and wavelengths of the light. So to get to the bottom of the question of error we have to approach it from a systems and operational level and not only from an overly simplistic “laser” perspective.In conversations with experts at Riegl, I learned two important things. Second, they have LiDAR sensor operators that do deliver bare earth point clouds with vertical accuracies that exceed the printed specifications.
A two-aperture system must align both apertures to thousandths of a degree, and this combination must then be aligned to the optical aiming device. A 3 milliradian beamwidth edge occurs 0.086° off center, a 4 milliradian beamwidth edge occurs 0.115° from beam center.Bright lights, such as halogens, beaming directly into the aperture will desensitize or entirely mask reflections. Increasing range short to long produces a receding speed, sweeping long to short range produces an approaching speed.The narrow laser beam and aim crosshairs must be closely aligned to very tight tolerances. For these reasons there is no way to really know how accurate the deliverable is unless resources are spent that are necessary to statistically measure (estimate) its accuracy in the different point classifications. Pulse widths vary from 5 to 35 nanoseconds.The operator must aim the beam to an accuracy of within tenths of a degree, the aperture must be aligned within hundredths of a degree to the optical aiming device.
If the specification was written using NSSDA guidelines it would be expressed at 2 sigma’s or “5.0 cm with a 95% confidence level”.When talking with your professional services provider, clarify these things:
"Lidar is really a shortcut," added Tesla AI guru Andrej Karpathy. Further, project deliverables are usually delivered with “absolute” vertical accuracies that meet certain ASPRS or NSSDA thresholds. One fundamental problem with the current crop of commercial SLAM scanning solutions is that they rely on hardware not designed for survey applications in the first place.
All of these factors (and others) in total contribute to the actual measurable error in the final deliverable.On top of these factors, we have the practical consideration as to how (or if) the vertical accuracy of the point cloud will be assessed.
But it is incumbent upon those purchasing these professional services that they understand the fundamentals and exercise due diligence to ensure qualified professionals are capable of delivering the needed information in a quality and timely manner.Raw LiDAR data after it has been properly aligned and inaccuracies due the IMU have been removed.As it turns out, the accuracy of the laser ranging is actually one of the most accurate components in the whole system.
How much less? The first problem one runs into when reading LiDAR manufacturer “accuracy specifications” is that they are normally reporting “relative accuracies” not “absolute accuracies” of their systems.
The distance of the GNSS base station(s) to the LiDAR system during acquisition has a major effect on final accuracy.