[pdal] filters.normal giving odd results

[Tom van Tilburg]

Hi,

This question is also on stackexchange:
https://gis.stackexchange.com/questions/250810/erratic-results-from-filters-normal-in-pdal
---

Given this roof structure:

[image: roofpointcloud]

I get the following output for the normalx, after using

    {
      "type":"filters.range",
      "limits":"Classification[6:6],ReturnNumber[1:1]"
    },
    {
        "type":"filters.eigenvalues",
        "knn":8
    },
     {
         "type":"filters.normal",
         "knn":8
     },

[image: point normalx]
(orange to purple : -1 to 1)

Comparable situation for normaly and normalz

Where I was expecting homogeneous colored roofplanes, I get a mixed-up
situation, where the values appear to be sometimes flipped to the negative.

It seems to me there are some artifacts here based on the laser scan-line,
considering the flipping of the normal on the same roof plane.

I cannot wrap my head around what happens here. Anybody ideas? The purpose
of this work is to get the slope and aspect from the roofplanes. I can get
the slope correct by taking the absolute values of normalz (slope will
never pass the 90 degrees) but this is not an option for calculating the
aspect it seems. The resulting pointset can be downloaded here
<http://research.geodan.nl/sites/tomt/output.las>

Best,

 Tom
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