What do Geiger Mode and Single Photon Mean for Linear LiDAR?
Next-generation LiDAR technologies could greatly impact aerial surveys.
Innovation is no stranger to geospatial technology and the surveyors who use it, but recent LiDAR advancements have left many within the profession wondering what it all means for them.
While a range of specialized LiDAR technologies for various applications has been available to surveyors for years, linear mode LiDAR has generally been accepted as the most conventional LiDAR approach. As word continues to spread about newer options like Geiger-mode, single photon and flash LiDAR, members of the surveying community have mixed emotions.
“The overall feedback I’m getting is excitement. People are so excited about it because they’ve been hearing pieces here and there in the last year or two,” says Qassim Abdullah, CP, PLS. The senior geospatial scientist with Woolpert and adjunct professor of geospatial-related topics at Pennsylvania State University and University of Maryland Baltimore County covered this topic as a panel moderator at the International LiDAR Mapping Forum (ILMF) in February. “There is a lot of anxiety too, because people who already invested, for example, in the linear LiDAR — some of them made huge investments — they are anxious about what’s going to happen.”
With so much new stuff on the horizon so often, it can be hard to know when to take what seriously. Abdullah says this is a time to pay close attention. “It is really a big deal because this new technology brings to our industry a new paradigm.”
The key difference from linear LiDAR that Geiger-mode, single photon and flash all share is a new sensor design. That key difference results in higher data density and, in many cases, takes aerial surveys to new heights.
As Abdullah explains, the new sensor design is based on a focal plane array. He compares it to digital camera CCDs. Whereas linear LiDAR involves one pixel, the new technologies bring an array of pixels. So instead of using each laser pulse as one point cloud, they are divided into hundreds or thousands of sub-pulses, which is where the higher density comes from. This characteristic is where the similarity between the three new LiDAR technologies ends. From there, Abdullah says it’s most appropriate to categorically group Geiger-mode and single photon together, and leave flash separate.
Geiger-Mode and Single Photon
In addition to a more complex sensor system, Geiger-mode and single-photon LiDAR share a lower laser energy requirement. With linear LiDAR, Abdullah says the pulse has to return to the airplane in an aerial survey with at least 500 to 1,000 photons in order for the signal to be sensed. With Geiger-mode and single photon, only a few photons are necessary.
That lower energy feature combined with higher data density capabilities means a lot for surveying from the sky. For example, on a project that calls for a density of 10 to 15 points per square meter, Abdullah says you generally cannot fly to more than 3,000 or 4,000 feet utilizing linear LiDAR. Stretching linear to its limit, flying to around 12,000 feet, chances are you won’t get more than a few points per square meter. With Geiger-mode and single photon, he says you could potentially fly as high as 30,000 feet and acquire data as dense as about 25 points per square meter.
With respect to the attributes that differ the most from linear — higher density and altitude potential — Geiger-mode and single photon have a great deal in common. Much of what is different about the two is very technical, complicated and doesn’t make a great deal of difference to actual users, Abdullah says. Key differences between them worth considering are the multiple returns capability that is associated only with single photon, the laser color they utilize and solar noise sensitivity.
The current manufacturer of single photon LiDAR, Sigma Space, utilizes a green laser in their LiDAR sensors. One of the advantages of the green laser is its ability to penetrate shallow water in a way that the infrared laser used in Geiger-mode cannot.
As for solar noise, Abdullah says a slight difference between the two exists. Differing sensitivities to solar noise mean that current Geiger-mode LiDAR sensors are more effective when data is acquired during nighttime flights while single photon sensors can be flown during the day or at night. It is important to note that the different kind of noise users see is removable with the right algorithm and processing, he says.
While flash LiDAR offers higher density data and more efficiency than linear for lower altitudes, it isn’t exactly in the same category as Geiger-mode and single photon, according to Abdullah. Because it is close to linear in its need for higher laser energy, flash technology cannot be flown at the altitudes that the other two can and still collect highly dense data.
“I think for certain applications, it will be a great system, Abdullah says. “Certain applications, for example, where you don’t require to fly 10,000 feet or 20,000 feet. So, for low altitudes, for helicopter flying, for the driverless car, for UAV for example.”
He says it could be useful on a drone or helicopter for corridor mapping, but airplane altitudes aren’t its forte. He says the market is much more focused on Geiger-mode and single photon right now because the capabilities they bring to surveyors so much farther surpass linear than the capabilities of flash do.
Being able to fly so much higher than currently possible using Geiger-mode or single photon without losing density greatly changes the way aerial surveying can be carried out. In thinking about who stands to benefit most from Geiger-mode and single photon, Abdullah says it is important to understand that the advantage of the technology gets exaggerated in projects that involve wide area coverage.
The higher a survey is conducted, the larger the footprint of the flight plan and the wider the swath of coverage. In addition, the lower laser energy need allows service providers to fly much faster. “So instead of covering the project with hundreds of flight lines, you might reduce it to 30 flight lines or something like that. That’s the advantage of flying it higher,” Abdullah says. “So with this technology you can easily fly 180 knots or maybe 200 knots, and it’s impossible to do that with linear LiDAR and maintain high density points.”
He says Geiger-mode and single photon LiDAR are most useful for surveyors working on largescale jobs that are countywide, statewide or nationwide. In these cases it is most economical because it so immensely reduces acquisition efforts and increases efficiency.
“I think the promise, at least for us, to collect large areas at lower costs is a big deal. I don’t think we’re quite there yet, but I think the promise of them definitely is exciting,” says Jason Stoker, U.S. Geological Survey (USGS) elevation product service lead for the National Geospatial Program and assistant director of the American Society for Photogrammetry and Remote Sensing (ASPRS) LiDAR division.
Stoker is involved in a country-wide USGS project called 3D Elevation Program (3DEP), which aims to collect and share consistent and systematic data of the entire U.S. and its territories. The goal is to create an elevation map, largely from LiDAR so far, at at least two points per square meter. Being that the eight-year project is so largescale, higher flying and resolution could prove to be extremely beneficial. So far they have mainly used linear LiDAR, but a recent test done utilizing Geiger-mode and single-photon proved promising.
Amar Nayegandhi, vice president of geospatial and technological services at Dewberry Consultants, was a part of the Dewberry team involved with overseeing 3DEP geospatial products and services. He says testing on the new LiDAR technology, including the flying of a Harris Corporation Geiger-mode sensor and a Sigma Space single photon sensor, has already taken place. While some kinks needed to be worked out and more enhancements will likely be made before full-fledged commercialization, he says the new LiDAR technology is extremely relevant to the end goals of 3DEP.
“It’s definitely going to help the 3DEP program if the requirement for 3DEP is to get a high-density data; such as for QL1 products at eight points per square meter and higher, this technology is much more viable for doing it,” Nayegandhi says. “It would help in advancing the ability to quickly map large areas. It sort of comes down to if there is funding available for this program, but Geiger mode and single photon can definitely map these areas more quickly and at a lower cost.
For smaller aerial projects like corridor mapping, it doesn’t make much sense for service providers to invest in the new technology if they are already invested in the current technology, Abdullah says. This means that for traditional land surveyors using stationary or terrestrial LiDAR, the new stuff is especially irrelevant. “I think it is much more relevant to the aerial [operation] because of the advantage of flying higher and covering a wider area. The higher you fly, the more efficient the acquisition becomes, and that’s the main advantage.”
What about Linear?
With so much promise in Geiger-mode and single photon LiDAR, many members of the surveying community are left wondering if and how they will co-exist with linear. So do the new developments threaten the existing? Abdullah says they didn’t come to replace linear LiDAR — at least not right away.
“I think they’re going to coexist for a while because each of them has strengths and weaknesses, he says. “Linear is still good for low altitude areas, for corridor mapping, for a lot of applications where it doesn’t make sense to use the new technology when you consider the cost of the new systems. So they complement each other in their capabilities and weaknesses.”
He says he can see commercialization of Geiger-mode and single photon taking place in as soon as the next two or three years, and then users’ minds should be thinking about whether or not they should upgrade their systems. He says linear has come a long way since its onset in the ‘90s, but that he doesn’t think many more advancements will be made to it with regard to density. “The more they see the advantage of the new technology, it is going to go toward the new technology for sure.”
For those who choose to upgrade, Abdullah says he expects that Geiger-mode and single photon sensor cost will be two to three times more expensive than linear. However, the upfront cost will likely be accounted for quickly because so much cost will end up being saved during the data acquisition step with largescale projects down the road.
As for challenges, higher density makes for bigger data, which calls for bigger storage and stronger processing power. So adoption of Gieger-mode and single photon will likely call for upgrades to complementary hardware and/or software as well.
It’s no secret that even linear LiDAR is something that many surveyors have yet to embrace. For those surveyors across specializations who still have not invested in LiDAR technology, Abdullah says they should know that, “It’s a big deal.” He says he thinks that in many ways LiDAR surveying will replace the current practices of field surveying indoors and outdoors.
“I think they still have time. They have a great opportunity because there is so much offered in these LiDAR tools for their surveying technique and practices definitely. They’re behind definitely, but they still have a great chance to go and buy terrestrial LiDAR or mobile LiDAR or LiDAR on UAS/UAV and all of that.”