Last week we produced a free report on the future of aerial lidar. We asked a number of industry experts to answer: What is the future of single-photon and Geiger-mode technologies? What’s hype and what’s not? In that article, many of the experts made the case that these new technologies are going to have a powerful effect on the landscape of aerial data acquisition.
But it’s important to note that “classical” and “conventional” lidar technologies still have significant benefits. Since the report only briefly covered these benefits, we have decided to publish the entirety of our interview with RIEGL’s CTO, Dr. Andreas Ullrich. In his answers, he lays out a comprehensive case for the value of full-waveform lidar, as well as the reasons RIEGL is continuing to develop this technology.
Sean Higgins: What are the major strengths of these technologies as compared to “linear” LiDAR?
Dr. Andreas Ullrich: The use of the term “linear” relates to an older analog signal processing version of lidar, not the advanced digital regime of RIEGL Waveform-Lidar. The proprietary RIEGL Waveform-Lidar varies significantly from the so-called “linear” lidar. The technical difference lies in how to analyze the signals from the receiving element, the photodetector, to derive precise, highly accurate, and attribute-rich lidar data. Waveform lidar data is extremely precise because it this technology provides high ranging accuracy and low noise, offers high multi-target resolution, gives pulse shape information on every point, and allows for straightforward radiometric calibration.
Geiger-mode and single-photon lidar have the potential to collect more points but at a loss of precision and the intensity information. The dramatic increase in the number of points with Geiger-mode or single photon lidar seems an overwhelming positive. However the lack of precision and attribute information takes the data users back in time to an inferior result. All of us want our surveys and maps more precise not less precise. The growth of the lidar industry has been based upon more and more informative data at a lower cost.
Lower cost of acquisition and thus high efficiency of land mapping are typical strengths that are touted. The theoretical gain in speed crashes into reality when planning for clear days and weather conditions needed for Geiger-mode lidar. Acquiring the data is one aspect in the efficiency picture – it also must be processed and delivered. These and other limitations of the “new” technologies eliminate much of the speed advantage.
What are the major weaknesses of these new technologies when compared to Waveform-Lidar?
The points that are created by Geiger-mode Lidar are synthetic with an unknown heritage. It is not identified if they have been derived from just a few or many raw points, or whether these measurements have been created by resampling and interpolation. This is a critical weakness for Geiger-mode. There are other weaknesses, such as the lack of estimated intensity, poor dense vegetation penetration and the inability to efficiently capture multiple returns.
Single-photon lidar is, compared to Geiger-mode, more sensitive to multiple target returns. Theoretically it can measure the whole scene ranging from wires to ground beneath the canopy. However, as demonstrated by experiments carried out by USGS and presented at the ILMF in 2016, the ranging accuracy of current systems is much inferior compared to Waveform-LiDAR due to the lack of appropriate signal processing. Ranging errors of more than half a meter can clearly be seen in the data on objects with varying reflectance and size.
Classical lidar are available at various wavelengths ranging from green 532 nm, to 1 µm, 1.55 µm and beyond in the infrared, thus enabling multi-wavelength lidar to provide information on forest health for instance. Single-photon lidar is only available at visible wavelengths for the time being, and most likely also for the foreseeable future.
Will these technologies someday replace conventional technologies like Waveform-LiDAR? Why or why not?
This is the most pressing question on people’s minds regarding lidar technologies – will these technologies replace what they have come to understand as lidar? My answer is no. The two “new” lidar variants are powerful tools when applied to the correct application. Geiger-mode lidar is already the best choice when it comes to quickly derive DTMs of desert-like areas or war zones where you must stay outside the reach of surface-to-air missiles, especially when you have a cloudless atmosphere and no ground cover by vegetation.
RIEGL Waveform-Lidar provides superior results through vegetation. Waveform-Lidar provides data to see a great variety of objects and features, not just points for deriving digital surface models but also poles, wires, meshes and other details. In the broad range of applications, Waveform-Lidar is the best technology and will stay so, with continuous evolution and higher acquisition speed while maintaining the outstanding accuracy and precision. RIEGL Waveform-Lidar provides near realtime availability of processed point clouds. This opens new applications such as rapid response or rescue operations. RIEGL has demonstrated all of these things with the latest VQ-1560i system – it can collect 450 square kilometers per hour at 8 points per square meter; all this coupled with industry leading data processing speeds and the quality of data that the data users have come to expect.
Are single-photon and Geiger-mode Lidar mature technologies? Do you believe they need to be developed further before they reach their full potential?
Both single-photon Lidar, which is essentially a linear Lidar based on a photo-multiplier-tube, and Geiger-mode Lidar, essentially non-linear, are not new technologies at all. However, applying these techniques for commercial surveying and mapping is new. As announced by the originator of the single-photon LiDAR at ILMF 2016, the technology has to be improved and will be improved by adding means for estimating the amplitude of the received echo signal, which has been done in the industry ages ago when 8-bit intensity has been introduced as the first attribute to the points of the point clouds. So, in this respect, single-photon Lidar is not mature at all.
Geiger-mode LiDAR has a long history in the military segment and a myriad of optimizations have already been applied. However, you cannot trick physics. One of the key issues is the lack of a Geiger-mode detector to recover from triggering fast enough to catch further details on the targets. Another is that it will never provide an intrinsic estimate of signal strength – alias intensity – which would be the key to compensate for trigger walk and thus improve the accuracy.
Does RIEGL have any plans to produce a LiDAR unit based on the single-photon or Geiger-mode technologies?
As stated previously, we believe RIEGL Waveform-LiDAR is the superior technology. Waveform LiDAR is extremely precise because it provides high ranging accuracy, low noise, offers high multitarget resolution, gives valuable pulse shape information, and allows for radiometric calibration on every point. Our customers have built their business models relying on the data quality and valuable attributes they are used to derive from RIEGL Waveform-LiDAR. Other discussed technologies cannot provide all these features. We will continue to even further advance and sophisticate RIEGL Waveform-LiDAR technology.
