[GRASS-user] Problem with Lidar Tools

[Markus Neteler]

Hi John,

On Sat, Oct 31, 2009 at 12:39 PM, John Tate <john.tate at ntlworld.com> wrote:
> Hi Kaipi,
>
> I've only been doing rural LiDAR processing, so can't help regards building
> removal for the DTM.
>
> One thing that isn't mentioned in the Brovelli, 2004 article or the online
> documention on the wiki, is that spline 'step' relates to metres (1 step =
> 1m). Obvious I know, but wasn't to me when I started fiddling with the
> parametersand the LiDAR data for the first time. Therefore:
> -see and sen parameter in v.lidar.edgedetection to 400
> is 400m! and hence:
> -the detected edges are now so wide, that they cover complete buildings
>
> The spline step parameters should be double the average distance between
> points. So for 50pts/m² the spline steps chould be 0.04 (2*(1/50)) for
> example. Play with these either side of 0.04 in the parameters until you get
> a good result. These modules were designed a few years ago now, and not
> really designed for such high resolution data, though should still work.
> Though I am not entirely sure what you are attempting to do (e.g. purpose of
> data: remove building for DTM/ or create 3D model of the building DSM).
>
> You will find that v.outlier is the biggest restriction on how many points
> you can process at one time. I was working with roughly 1.5pts/m² and found
> that I had to work in 1km tiles due to the v.outlier restriction. Something
> to be wary of when you decrease the spline step incase you start getting
> errors.
>
> For reference, I was using:
> GRASS GIS (v.6.4.0svn, library Revision: 37101 (2009-05-10)) (windows
> version)

These comments are very intesting!

(Un)related: I have started a small, incomplete, Lidar tutorial:
http://grass.osgeo.org/wiki/LIDAR#Micro-tutorial_for_LIDAR_data_analysis

It would be great if you could integrate your comments in this Wiki
page (Wiki syntax is pretty easy) for the benefit of other users. In
the mailing list archive things get easily "lost".

thanks
Markus