[GRASS-dev] ] LiDAR LAS import - filter on import?

[Markus Metz]

On Fri, Jun 3, 2011 at 3:30 PM, <Doug_Newcomb at fws.gov> wrote:

>
> Markus,
>         Were you planning on adding filtering options by return number on
> v.in.lidar?  It occurs to me that you could speed things up by only
> processing the subset of the data that you want to use.   I could see it
> being a useful thing to create a vector layer composed only of 1st returns (
> top of canopy/buildings)  Selecting only last returns is useful, as well as
> selecting points that are neither first nor last returns .
>
> Yes, I was thinking of such filter options, they would be very easy and
straightforward to implement, something like v.in.lidar
returns=[all,first,last,middle]. Currently there is only spatial filtering
available.

I was also thinking about an option to select columns to be imported as
attributes, equivalent to the las2txt --parse option, but could not yet come
up with a user-friendly solution: flags don't work because too many and
conflicting with existing flags, a parse option very much like the one of
las2txt is too cryptic. I think I will settle for something like v.in.lidar
attributes=coords,classes,color,egde,angle,return,nreturns,... where the
attributes option takes none, one or multiple answers

Markus M



>         I've been primarily working with aggregate las files that I have
> dumped to text ( via liblas las2txt )  and then parse via python for first,
> middle, and last  returns. I have not taken the time to learn C yet, but
> perhaps the logic of these simple python programs would be useful .
>
> The following is the python script I use to separate the last returns.
>
> ---------------------------------------------------------------------------------------------------------------------------------
> import struct,os,string,re,binascii,glob
> infile = raw_input("Enter the aggregate lidar filename:  ")
> outfil = raw_input("Enter the ASCII output filename for the Lidar Data:  ")
> intxt=open(infile,'r')
> outtxt=open(outfil,'w')
> while 1:
>     lasline=intxt.readline()
>     lasinfo=lasline.split(',')
>     if (len(lasinfo))< 5:break
>     numreturns=int(lasinfo[4])
>     returnnum=int(lasinfo[5])
>     # In the data input file for this instance, the number of returns
>     # is the fifth column, and the return number is the sixth column (
> x=1,y=2,z=3,intensity=4).
>     # If the value of these colums is equal, it should be the last return.
>     if ( numreturns==returnnum):
>         outtxt.write(lasline)
>
>
> intxt.close()
> outtxt.close()
>
> --------------------------------------------------------------------------------------------------------------------------------
> For parsing the first returns,  substitute if (returnnum==1):
>
> Here is the python script I use to separate out the "middle" returns.
>
>
> -------------------------------------------------------------------------------------------------------------------------------
>
> import struct,os,string,re,binascii,glob
> infile = raw_input("Enter the aggregate lidar filename:  ")
> outfil = raw_input("Enter the ASCII output filename for the Lidar Data:  ")
> intxt=open(infile,'r')
> outtxt=open(outfil,'w')
> while 1:
>     lasline=intxt.readline()
>     lasinfo=lasline.split(',')
>     if (len(lasinfo))< 5:break
>     numreturns=int(lasinfo[4])
>     returnnum=int(lasinfo[5])
>     # In the data input file for this instance, the number of returns
>     # is the fifth column, and the return number is the sixth column (
> x=1,y=2,z=3,intensity=4).
>     # If the value of these colums is equal, it should be the last return,
> so skip that entry
>     # If the return number is 1 , skip that value.  All other values are
> middle canopy values,which is what we want.
>     if ( numreturns==returnnum): continue
>     if (returnnum==1):continue
>     outtxt.write(lasline)
> intxt.close()
> outtxt.close()
>
> ------------------------------------------------------------------------------------------------------------------------------
>
> I really appreciate your adding lidar data classifications from the
> standard into the program.  I haven't actually seen any lidar data with
> classifications yet, but I have hope for the future :-).
>
> As a bit of background , I'm taking the last returns and then running
> r.in.xyz to create a raster with the intensities as the z values to get
> relative soil moistures, and looking at points that are neither first nor
> last returns as a possible measure of vegetation density.  It works great
> for large datasets( > 4 billion points) , but I'm feeling the need for more
> point analysis.
>
> Doug
>
>
> Doug Newcomb
> USFWS
> Raleigh, NC
> 919-856-4520 ext. 14 doug_newcomb at fws.gov
>
> ---------------------------------------------------------------------------------------------------------
> The opinions I express are my own and are not representative of the
> official policy of the U.S.Fish and Wildlife Service or Dept. of the
> Interior.   Life is too short for undocumented, proprietary data formats.
>
>
>  *Markus Metz <markus.metz.giswork at googlemail.com>*
> Sent by: grass-dev-bounces at lists.osgeo.org
>
> 06/03/2011 02:20 AM
>   To
> Hamish <hamish_b at yahoo.com>
> cc
> GRASS developers list <grass-dev at lists.osgeo.org>
> Subject
> [GRASS-dev] Re: [GRASS-user] LiDAR LAS import
>
>
>
>
> Hamish wrote:
> > Markus Metz wrote:
> [snip]
> >
> >> v.in.lidar is a notch faster than las2txt | v.in.ascii. And
> >> easier to use...
> >
> > I'm not too surprised the speed difference is not so huge, as
> > unix pipes are very efficient. but the easier to use thing is
> > very important.. both las2txt and v.in.ascii are a bundle of
> > command line switches to get right.
> >
> >
> >> Speed comparisons:
> >>
> >> # sample las file with 1,287,775 points
> >>
> >> # with table and topology
> > ...
> >> real    6m34.430s
> > ...
> >> real    6m13.823s
> > ...
> >> # without table, with topology
> > ...
> >> real    1m53.578s
> > ...
> >> real    1m44.876s
> >
> >
> > I take it that without topology it runs in just seconds?
>
> Update: no attribute table, no topology
>
> time las2txt -i points.las --stdout --parse xyz --delimiter "|" |
> v.in.ascii in=- out=points_ascii -ztb x=1 y=2 z=3
>
> real                 0m20.932s
> user                 0m18.424s
> sys                 0m6.869s
>
>
> time v.in.lidar in=points.las out=points_las -obt
>
> real                 0m9.117s
> user                 0m2.946s
> sys                 0m5.985s
>
>
> Markus M
> _______________________________________________
> grass-dev mailing list
> grass-dev at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/grass-dev
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.osgeo.org/pipermail/grass-dev/attachments/20110603/80ea92dd/attachment-0001.html