Introduction
All LiDAR data is NOT created equal. This is the “elephant in the room” of airborne LiDAR data collection that very few people want to talk about but everyone should be made aware of. You would be surprised how many in the industry do not fully understand this and often settle for (sometimes unknowingly) low quality data deliverables. It is easy and cheap to make poor quality LiDAR data but usually difficult and often more costly to produce high quality data…dare I say even just acceptable LiDAR data. Some focus heavily on this concept while others only see the cost per square mile.
Merrick & Company has always focused on meeting or exceeding LiDAR project requirements. With the clearly defined USGS LiDAR Base Specification (LBS), we all now have an industry-wide minimum requirement to reference. I say industry-wide because all USGS related projects must meet these specifications and most non-USGS related projects usually reference this specification document to some degree. I am a firm believer, speaking from experience, that not all LIDAR sensors are able to meet all of these requirements.
New Sensor Acquisition Announcement
Merrick is pleased to announce that after substantial and rigorous testing, we have added the Teledyne Optech Galaxy LiDAR system to our arsenal. This next generation sensor was designed to meet all USGS LiDAR Base Specification requirements while providing us with technical capabilities that allow for extremely efficient data collection. These new features help us keep collection and processing costs down while maximizing the area collected per flight mission. This promotes shorter project turnaround schedules.
System Specifications and USGS LBS Adherence
In real world project applications meeting USGS QL2 point density requirements, we have witnessed the Galaxy repeatedly produce the following data quality measurement capabilities:
- Absolute accuracy: typically around 5 cm RMSEz for NVA check points
- Relative accuracy: typically around 4-8 cm RMSD per overlap region (Note: Merrick’s use of Galaxy’s POS AP60 and the new beta version of LMS 4.0 [Optech’s LiDAR processing software] shows probable improvement even beyond this)
- Smooth Surface Repeatability: typically around 2-5 cm absolute due to its laser ranging precision of 8 mm at 1 σ
- Maximum range error due to highly reflective targets (such as paint stripes): 0 cm
- Maximum number of range measurements per pulse: 8 returns
- Minimum pulse separation (distance between subsequent multiple recorded returns): ≤ 0.7 m
Obviously acquired point density is customizable by flight planning parameters but the above results are achievable for USGS QL1 and QL2 data density requirements (≥ 8 pls/m2 and ≥ 2 pls/m2 respectively). With careful flight planning and appropriate processing techniques, the Galaxy can achieve even USGS QL0 specifications.
Maintaining High Effective Pulse Rates
Galaxy’s new PulseTRAK™ technology breaks through a more than decade long industry barrier that has historically limited Multiple Pulse in Air (MPiA) use. Until PulseTRAK™, the use of MPiA meant LiDARs could emit laser pulses before the preceding incoming reflected pulse returns but only within limited above ground level (AGL) operations. For this reason, most systems offering MPiA have been limited to only two pulses in air. Some newer systems offer more pulses in air but this limits high pulse repetition frequency (PRF) use to very flat terrain. More importantly, this MPiA pulse “juggling” has always suffered from receiver “blind” zones, which are AGL regions where the sensor cannot operate at a fixed PRF. This again limits high PRF use to very flat terrain.
Galaxy’s PulseTRAK™ removes these limiting blind zones by sophisticated in air pulse tracking technology found only on this sensor allowing for a continuously operational envelope with consistent point density collection even across receiver blind zones. PulseTRAK™ also allows for up to six pulses in air exploiting its very high 550 kHz maximum PRF. This translates to a sustained use of much higher PRFs at higher AGLs allowing us to dramatically optimize collection efficiency. This benefit is greatly increased in areas of mild to substantial terrain relief.
All graphics courtesy of Teledyne Optech
Dynamically Adjustable Swath Width
Galaxy’s new SwathTRAK™ technology dynamically adjusts the scan field of view during collection to maintain fixed swath widths and even point distribution, even in variable terrain. This is an industry first allowing for yet again a significant increase in collection efficiency. Working in conjunction with PulseTRAK™, SwathTRAK™ provides collection of predictable and consistent point density from mountain peaks to valley floors, all in the same collection swath.
Optech advertises this feature alone can provide up to a 40% collection efficiency improvement but this is a Galaxy-to-Galaxy comparison, with and without SwathTRAK™. We found that when compared to systems that are only a few years old and are commonly used today for data collection, a much more significant efficiency can be realized. Our real world flight planning and data collection experience shows up to a 75% efficiency improvement over systems only a few years old when analyzed over areas of significant terrain relief. Stated another way, these older systems would have to fly up to four times the amount of flight lines to match Galaxy’s collection capabilities using PulseTRAK™ and SwathTRAK™.
If you would like to learn more about the benefits of Merrick’s high quality LiDAR data collection using the Galaxy, please contact me, Matt Bethel, Director of Operations and Technology for Merrick & Company at matt.bethel@merrick.com or 303-353-3662.
