LAStile removing reversible

[Steven Anderson]

Hello,

I am attempting to create tiles using -core 8 argument. 

When I use -cores 8 I receive the warning message "turning off reversible. not possible in single tile mode".

Can anyone explain why this is happening?

Thanks,

Steve

[Martin Isenburg]

Hello Steve,

first - just for anyone on the list who may not know - let me explain the '-reversible' option again and when it needs to be used. Most people will probably not need to ever use it: When you have one long flightline with 253 million points (or so) that you want to, for example, ground-classify ***while*** preserving their current order in the file then (and only then) the '-reversible' option of lastile is useful. Because lasground / lasground_new cannot be used directly on 253 million points we need to tile the flightline (with buffers) to get to around 10 - 15 million points per file. In order to reassemble the ground classified tiles into the original flightline (and only for that) the '-reversible' option of lastile is used.

What does this option do? It stores with every point its original position in the flightline file (e.g. its index in the array of all points) using an unsigned 4 byte integer that is stored as extra bytes. For all points that are buffers points it stores a special number. This information allows to read the points (after classifying them) from the set of tiles and store them back into their original order into a single flightline.

http://rapidlasso.com/lastile

http://lastools.org/download/lastile_README.txt

This does not currently work with the '-cores n' option because when doing spatially accelerated seeks in the file (note: *only* use use lastile to tile flightlines on multiple cores after spatially indexing the LiDAR with lasindex - aka creating a LAX file for every flightline) the original point index (the one you get when doing a sequential read) is currently not exposed in the LASlib API and that is needed to store the point position.

http://rapidlasso.com/lasindex

http://lastools.org/download/lasindex_README.txt

Regards,

Martin @rapidlasso

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[Steeve Brissette]

If that can help people, I use -reversible option to classify gound in a flightline.. for alignment purposes. I beleive I had a better classification algorithm using Lastools than Terra Scan.. also, I have a different approach when come to classifying my ground, which dynamically set some parameters in the workflow using generic ground slopes.. this is why I use -reversible option.. I want my flightlines to be ground classified.

I run a loop over every flightline, tiles them with buffer and reversible option, than classify my ground, than reverse the tilling... a for loop will help you with that.

Sincerely Yours

Steeve Brissette 

On May 17, 2018, at 9:29 AM, Martin Isenburg <martin....@gmail.com> wro

[Martin Isenburg]

Hello Steeve,

excellent use case. However, only if you really need your points to be back in whatever order they *originally* were in the flightline file that you started from is that '-reversible' option needed. If you are happy with other orders you could simply merge the tiles (after removing the buffers or - given that '-flag_as_withheld' was used - when filtering using '-drop_withheld' while merging). Other orders could be better than the original flightline order - for example for spatial indexing. So you could run lasoptimize on your tiles before lasmerging them.

http://lastools.org/download/lasoptimize_README.txt

Regards,

Martin