does anyone use the "edge of flight line" flag ???

[Martin Isenburg]

Hello,

i am trying to get some practical insight on the "edge of flight line" flags. In there anyone who is producing or consuming LAS files and is either concerned  with correctly setting the "edge of flight line" flag when producing LAS files or with actually exploiting these "edge of flight line" flag for some purpose when processing the LAS data. The latest LAS specification describes this attribute as follows:

"Edge of Flight Line: The Edge of Flight Line data bit has a value of 1 only when the point is at the end of a scan. It is the last point on a given scan line before it changes direction."

It seems that Leica marks all points that are returns from the last pulse of flight line with this flag, So does RIEGL for the Q-line but the V-line seems to use these flags for something else (see samples at http://lastools.org/las14) and Optech does it similar as Leica (but sometimes seems to have entire scan lines marked across the flight line?!?).

Does anyone know about the intended use for this flag or does one have any good examples for when this information may be valuable? There is also the "scan direction flag" from which the "edge of flight line" flag could be deduced. You may argue that for a scanner using multiple mirror facets like the RIEGL scanners there is no change in scan direction so the scan direction flag should always have the same value and the the "edge of flight line" could not be deduced. Again I quote the latest LAS specification:

"Scan Direction Flag: The Scan Direction Flag denotes the direction at which the scanner mirror was traveling at the time of the output pulse. A bit value of 1 is a positive scan direction, and a bit value of 0 is a negative scan direction (where positive scan direction is a scan moving from the left side of the in-track direction to the right side and negative the opposite)."

No worries. I am not asking because I am planning the LAS 1.5 format (God forbid (-;) but because of the PulseWaves format. I have an "edge of flight line" flag in the current specification but I really see no point in keeping it. We have both a "scan direction flag" as well as a "mirror facet index" to distinguish pulses from different scan-lines ... assuming they are sorted in time stamp order. But for files - LAS files and PulseWaves files alike - what use would an "edge of flight line" flag have if the points / pulses are not in acquisition order? I could see a use - namely outlining the scanned area and the flight line overlap - if *both* edges of a scan line were to be marked with the "edge of flight line" flag. But that is not how the current LAS specification describes it.

Can anyone provide some insight on the history, original intention, and today's use of the "edge of flight line" flag?

Regards,

Martin @rapidlasso

PS: Please only reply to pulse...@googlegroups.com or las...@googlegroups.com ... all replies to last...@googlegroups.com will get withheld but  will get redirected to the appropriate forum eventually.

[Paul....@leicaus.com]

Hi Martin,

The original intent was to give the ability to quickly create a coverage map for a flight by only reading / displaying the perimeter of the flight lines.  I am not aware of applications that are using this flag and as you suggest, there is enough information in the file to do this without this flag.

Paul


Paul Galla

Senior Software Engineer
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[Ramesh Sridharan]

Hi,

Some of our customers use this information to create crisp bounds of strips and hence identify micro gaps in the strips. There may be other ways to derive this but this one bit helps a lot without relying on scan direction and so on (which by the way can reset in the middle of scans for some reason).  

Regards,
Ramesh Sridharan
Ph: 505-463-3960
http://www.linkedin.com/in/rameshs

Its only weird if it doesn't work.     

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From: "Paul....@leicaus.com" <Paul....@leicaus.com>
To: pulse...@googlegroups.com
Cc: las...@googlegroups.com; LAStools - efficient command line tools for LIDAR processing <last...@googlegroups.com>
Sent: Monday, March 4, 2013 11:23 AM
Subject: Re: [PulseWaves] does anyone use the "edge of flight line" flag ???

[Martin Isenburg]

Hello Paul.

thanks for clearing this up. That is what I had been suspecting but the wording in LAS specification does not really make this clear. Maybe that could be reworded slightly to get the intent better across. The current definition does not provide coverage maps for scanners with rotating facets mirrors because the scan line direction never changes and even if you are "generous" and count the jump when you are missing one edge.

"Edge of Flight Line: The Edge of Flight Line data bit has a value of 1 only when the point is at the end of a scan. It is the last point on a given scan line before it changes direction."

But how to change it? The returns from the first and last pulse of a scanline would double the amount of points for a zig-zag scan pattern. Maybe it could be formulated "less precise" to allow for any style of flagging of points that deliminate the flight line.

Martin

I received a few other replies that went to the wrong list.

Evon Silvia (Watershedsciences Inc) wrote:

"This could be totally off base, but my impression of the edge-of-flightline flag was that it's to flag returns during acquisition that occur during mirror edges (in the case of a multi-face mirror like Riegl's) or during  mirror direction reversal (in the case of a single-face mirror). Points  recorded during those events are often of unpredictable or poor quality and  edges of flightlines are frequently clipped as a result. An end product  from a provider, therefore, will probably not contain any points with that  flag.

All that said, I'm not aware of any hardware that uses this flag in this  manner and AFAIK most people use scan angle to do the aforementioned  clipping.

Evon"

Ty Bowdoin (CAO Stanford) wrote:

"I don't have insight into the history or original intention of the edge of flight line flag, however, I can think of one important use for it. It would be handy to create a quick extent (perhaps using lasboundary). You  could filter the points to just edge points and avoid calculating which  ones are on the edges. Seeing as I have an optech system and our edge of flight line flag is often messy, I have not been able to use it in this way, but I could see that other users might."

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[Ramesh Sridharan]

I have seen Leica and Reigl Airborne data which carries this edge bit filled and serves the intented job pretty well.

Regards,
Ramesh Sridharan
Ph: 505-463-3960
http://www.linkedin.com/in/rameshs

Its only weird if it doesn't work.     

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From: Martin Isenburg <martin....@gmail.com>
To: las...@googlegroups.com
Cc: pulse...@googlegroups.com
Sent: Monday, March 4, 2013 11:56 AM
Subject: Re: [The LAS room] Re: [PulseWaves] does anyone use the "edge of flight line" flag ???

[Martin Isenburg]

I forward Karl's message (see below) here to take some load of the LAStools group.

Oups ... looks like we messed up.. I certainly meant "edge of scan line" but I just copied & pasted the LAS spec where it has always stated: "Edge of Flight Line: The Edge of Flight Line data bit has a value of 1 only when the point is at the end of a scan. It is the last point on a given scan line before it changes direction."

http://www.asprs.org/a/society/committees/standards/LAS_1_4_r11.pdf

That's why my variable name in LASlib is called "edge_of_flight_line" and not "edge of_scan_line" ... )-:

---------- Forwarded message ----------
From: Heidemann, Hans <kheid...@usgs.gov>
Date: Mon, Mar 4, 2013 at 9:49 AM
Subject: Re: [LAStools] does anyone use the "edge of flight line" flag ???
To: last...@googlegroups.com

Martin, 

First, I am presuming that you mean the "edge of scan" flag ... the one that demarks the turn-around point of the mirror in oscillating systems, and the first and last point of each cross-track scanline in rotating prism systems. As you note, the wording as it exists could be interpreted such that the first point of the scanline in a rotating prism system might not be flagged, however, this has not been my experience with the data I've seen from Riegl instruments. The wording is as it is because it was written back when most all US lidar systems were oscillating mirror. 

The USGS NGP requires that this flag be set for those points, for all data, and that ALL points be included in ALL lidar point deliveries. If the data producer feels that the points on the extreme side edges of the swath are not spatially reliable, they are to be tagged as "Withheld" -- but NOT REMOVED FROM THE FILES.

The intent was an is, in my view, precisely what you describe: Having these points flagged allows users to precisely define the boundary of a swath -- quickly and efficiently.

Unfortunately, manufactures of many popular instruments never bothered to implement this in their pre-processing software, and then many data vendors chose to strip off the extreme edges (or in some cases ALL of the overage points), rendering the flag moot sine the flagged (or would-be-flagged) points were never output into a LAS file anyway. Thus, software developers never felt any need to make their applications aware of the flag. OOPS!!!    And so, many a poor sod has had to manually (and only roughly) digitize swath boundaries, or derive them from rasters, or spend uncounted hours attempting to develop algorithms the recreate something that was required by specification in the first place.  Nothing like being penny-wise this quarter, and pound-foolish in the long run, huh?

Karl

Karl

H. Karl Heidemann, GISP

Physical Scientist, Lidar Science

U.S. Geological Survey

Mundt Federal Building

47914 252nd Street

Sioux Falls, SD  57110

605-594-2861

kheid...@usgs.gov

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