[pdal] "resolution" keyword in

[Matt Beckley]

Hi - thanks for the responses.  Basically, I'm trying to develop a workflow
to calculate the number of points per class in a given ept dataset. My
experience is that the `pdal info --stats` command will read all of the
data into memory and will not work on large ept files (like the usgs lidar
datasets).  Conor helped me out and suggested that I could use the
"resolution" flag to estimate the number of points at a given level in the
ept structure, and I've done that with a resolution = 50. So, for example,
I have run a query like:

pdal info --stats --filters.stats.dimensions=Classification
--filters.stats.count=Classification ept://
https://s3-us-west-2.amazonaws.com/usgs-lidar-public/USGS_LPC_MD_VA_Sandy_NCR_2014_LAS_2015
--readers.ept.resolution=50
--readers.ept.threads=12

I'm trying to understand if the counts I get per classification is
representative of a 50 m^2 area?  It seems like from the documentation (and
Jim's answer), that it is trying to take data at the octree level that is
closest to the specified resolution.  So, is there a way I can "back out"
the point totals per classification to get a representative count per
classification for the entire dataset?  Is there a way to calculate what
the actual returned resolution level is?  Currently I am just taking a
ratio between total count of the entire dataset to returned count from the
subset dataset using the "resolution" flag.  I then apply that ratio to the
different class counts to get a rough estimate of points per classification
for the entire dataset.  This is certainly not the best method as it will
over-estimate some of the classification totals.

Thanks for your help.

matt.
---------------------------
Matthew Beckley
Data Engineer
UNAVCO/OpenTopography
beckley at unavco.org
303-381-7487


On Mon, Mar 25, 2019 at 1:30 PM Andrew Bell <andrew.bell.ia at gmail.com>
wrote:

>
>
> On Mon, Mar 25, 2019 at 2:49 PM Matt Beckley <beckley at unavco.org> wrote:
>
>> Is there more information available regarding how the 'resolution'
>> keyword works for the latest version of readers.ept?  Is it "gridding" the
>> data at a specified cell size?
>>
>
> The EPT format is essentially an octree of point data.  The resolution
> options essentially tells the reader how far to descend the tree when
> fetching data.
>
>
>>   If so, how does it handle nominal class data such as lidar
>> classifications?  Is it taking the first, last, or a random sample to
>> assign as the classification for each cell(assuming a scenario where there
>> are multiple classifications that fall within a given cell).  In other
>> words, how is the defining classification assigned for each cell given a
>> sample of points with varied classification values?
>>
>
> The resolution option is unrelated to point classification, so I'm not
> sure that I understand your question.  Perhaps you can provide some more
> detail or an example.
>
> --
> Andrew Bell
> andrew.bell.ia at gmail.com
>
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