[GRASS-user] Problems with lidar tools

[Markus Metz]

On Mon, May 30, 2011 at 2:57 PM, LeeDaniel <Lee.Daniel.1986 at gmail.com> wrote:
> Hello,
>
> I'm having major problems with the Lidar tools. I've got a dataset with
> 5.5-7.0 points per m² and have run v.edgedetection on the original point
> cloud containing all points with a the spline step parameters set to 0.6 and
> the other parameters set as outlined in "Inverse calibration of lidar
> filterimg parameters" by Brovelli & Lucca. Nonetheless, the edges detected
> don't seem to have a whole lot to do with actual edges in the scene,
> buildings look like they're completly ignored. Another fact is that I can't
> see the data in nviz, it claims that there are no points in the region, even
> though I've set the region settings to match the LiDAR data set.

If I understand the Brovelli & Lucca procedure correctly, there are no
hard rules to determine the settings for v.lidar.egedetection, instead
it is trial and error (iterative calibration against a reference). You
may have to experiment with different settings until the edges of
buildings are properly detected.

About nviz, I guess nviz needs vectors with topology for display, and
the output of v.lidar.edgedetection does not have topology, you may
need to run v.build manually.
>
> More confusing, I can tell GRASS to show the points with random colors
> according to their classes, but I can't see the classes themselves in the
> attribute table, so I can't extract them thematically or control how they're
> shown. If I build topology again for that classes, I can open the attribute
> table, but it only has two fields: ID and Interp. Can't figure out what they
> mean, and v.lidar.growing doesn't change the map at all. What's going on??
>
In the output of v.lidar.edgedetection, the classes TERRAIN, EDGE,
UNKNOWN are not in an attribute table, they are only coded by cats.
You can add a table to layer 1 with columns cat and label and populate
the table with label = TERRAIN for cat = 1, label = EDGE for cat = 2
and label = UNKNOWN for cat = 3.
The table with the fields ID and Interp is attached to layer 3.

v.lidar.growing adds categories to layer 2, but no attribute table.
Again, you would need to add a table manually. And there is probably a
bug in v.lidar.growing, needs (time for) testing.

HTH,

Markus M