Decoding the "Edge of Flight Line" Flag in LiDAR Data: Is It Still Relevant?

The world of LiDAR (Light Detection and Ranging) data is filled with technical details, and one that often sparks questions is the "Edge of Flight Line" flag. This flag, embedded within LAS files (a common format for storing LiDAR data), is intended to mark points located at the end of a scan line. But is this flag still relevant in modern LiDAR processing? Let's delve into the history, intended use, and current practicality of this often-misunderstood attribute.

What is the "Edge of Flight Line" Flag?

According to the LAS specification, the "Edge of Flight Line" data bit has a value of 1 only when a point is at the end of a scan line. It signifies the last point recorded before the scanning direction changes. In simpler terms, it's meant to identify the perimeter of each flight line in your LiDAR dataset.

• Official Definition: The last point on a given scan line before it changes direction.

The Original Intent: Quick Coverage Maps

As Paul Galla from Leica Geosystems notes, the initial idea behind the "Edge of Flight Line" flag was to enable the rapid creation of coverage maps. By simply reading and displaying the points flagged as "Edge of Flight Line," users could quickly visualize the boundaries of each flight line without processing the entire dataset.

• Primary Goal: Facilitate quick creation of flight line coverage maps.

Real-World Applications: Use Cases and Challenges

While the initial intent was clear, the actual usage of the "Edge of Flight Line" flag has been varied and, at times, inconsistent.

• Identifying Micro Gaps: Ramesh Sridharan mentions that some customers use this flag to create crisp boundaries of strips and identify tiny gaps between flight lines. This can be crucial for ensuring complete coverage and data quality.
• Creating Quick Extents: Ty Bowdoin suggests that filtering points by the "Edge of Flight Line" flag could expedite the creation of a rough extent using tools like lasboundary.
• Inconsistent Implementation: Martin Isenburg points out that different LiDAR system manufacturers (Leica, RIEGL, Optech) implement the flag differently. Some even mark entire scan lines across the flight line, leading to potential confusion.

The Problem of Acquisition Order and Alternative Solutions

One of the significant questions raised is what use the "Edge of Flight Line" flag has if the points aren't in acquisition order. If the data is unsorted, the flag's usefulness diminishes significantly.

Fortunately, alternative solutions exist:

• Scan Direction Flag: The "Scan Direction Flag" indicates the direction the scanner mirror was moving when the pulse was emitted (1 for positive, 0 for negative).
• Mirror Facet Index: For scanners with multiple mirror facets (like some RIEGL models), this index helps distinguish pulses from different scan lines.

These alternatives raise the question of whether the "Edge of Flight Line" flag is truly necessary, especially if the data is time-stamped and properly sorted.

Expert Opinions and Practical Considerations

The discussion among LiDAR experts reveals a range of perspectives:

• Evon Silvia (Watershed Sciences Inc): Suggests the flag could mark returns during mirror edges or reversals, which might be of lower quality. However, they are unaware of hardware using the flag this way, with most people using the scan angle for clipping.
• Karl Heidemann (USGS): Emphasizes that the USGS requires this flag to be set and that all points be included in LiDAR deliveries, even if flagged as "Withheld" due to spatial unreliability.

Conclusion: Is the "Edge of Flight Line" Flag Still Relevant?

The answer is nuanced. While the "Edge of Flight Line" flag was initially intended for creating quick coverage maps, its inconsistent implementation and the availability of alternative solutions have complicated its usefulness.

Key Takeaways:

• The "Edge of Flight Line" flag marks the end of a scan line in LiDAR data.
• Its original purpose was to facilitate rapid coverage map creation.
• Inconsistent implementation across manufacturers has led to challenges.
• Alternative solutions like the "Scan Direction Flag" and "Mirror Facet Index" exist.
• Its relevance depends on the specific application and data quality requirements.

For users aiming to create coverage maps or identify micro-gaps, the "Edge of Flight Line" flag can still be a helpful tool, especially when dealing with data from manufacturers like Leica and RIEGL. However, users should be aware of its potential inconsistencies and consider alternative methods for achieving similar results. Ultimately, a thorough understanding of your data and the specific sensors used to collect it is crucial for effectively utilizing this flag.