[pdal] Calculation of the difference between DSM and DTM

[Connor Manning]

For general information on how pipelines work:
https://pdal.io/en/2.7-maintenance/pipeline.html
And here is the information on the GDAL writer specifically, take a
look at origin_x and origin_y:
https://pdal.io/en/2.7-maintenance/stages/writers.gdal.html#writers-gdal

For your pipelines, the GDAL writer is independently choosing its own
grid each time since you have not specified an origin, so the pixels
don't line up between the two outputs.  You want to explicitly set the
same origin, via the above options, for both of your writers.gdal
outputs to make sure they line up.

- Connor

On Thu, Jun 20, 2024 at 8:36 AM Raúl Casado Barbero via pdal
<pdal at lists.osgeo.org> wrote:
>
> Hi Andrew and thaks for your answer.
>
> I'm pretty new with pdal just now and I'm tryingn to understand the logic of pipelines... What do you mean with set the origin on writers.gdal?
>
>
> Raúl.
>
> El 20/06/2024 a las 13:45, Andrew Bell escribió:
>
> Hi Raul,
>
> You should set the origin on writers.gdal in order to get the consistency you desire. Without this, the origin is set according to the first point processed, which may differ between pipelines.
>
> On Thu, Jun 20, 2024 at 4:24 AM Raúl Casado Barbero via pdal <pdal at lists.osgeo.org> wrote:
>>
>> Hi,
>>
>> my aim is to calculate the fitovolume of a shrub vegetation area. To do that I calculate the DSM, classify a point cloud with ground (2) and low vegetation (3) and then create a DTM.
>>
>> I use 2 different pipelines (see below) to do that modified from this post. But I have 2 issues:
>>
>> When I calculate the difference between DSM and DTM I get negative values
>> The pixels of the DTM are horizontally displaced with respect to the DSM by half a pixel.
>>
>> Does anyone know why I am getting these negative values using this pipelines? Is anything wrong in the pipelines I`m using?
>>
>>
>> Raúl.
>>
>> Pipeline for DSM:
>>
>> [
>>     "nubeRecortadaPdal_1.laz",
>>     {
>>         "type": "filters.range",
>>         "limits":"returnnumber[1:1]"
>>     },
>>     {
>>         "type": "writers.gdal",
>>         "filename": "nubeRecortadaPdal_1_mds.tif",
>>         "output_type": "idw",
>>         "gdaldriver": "GTiff",
>>         "resolution": 0.05,
>>         "nodata": -9999,
>>         "radius": 0.07,
>>         "window_size": 50,
>>         "data_type": "float32"
>>     }
>> ]
>>
>> Pipeline for DTM:
>>
>> [
>>     "nubeRecortadaPdal_1.laz",
>>     {
>>         "type":"filters.overlay",
>>         "dimension":"Classification",
>>         "datasource":"D:/Datos/clasificadasVector.gpkg",
>>         "layer":"ndviRandomForest",
>>         "column":"Id"
>>     },
>>     {
>>         "type":"filters.range",
>>         "limits":"Classification[2:2]"
>>     },
>>     {
>>         "type": "writers.gdal",
>>         "filename": "nubeRecortadaPdal_1_mdt_ndviRf.tif",
>>         "output_type": "idw",
>>         "gdaldriver": "GTiff",
>>         "resolution": 0.05,
>>         "nodata": -9999,
>>         "radius": 0.07,
>>         "window_size": 100,
>>         "data_type": "float32"
>>     }
>> ]
>>
>>
>> _______________________________________________
>> pdal mailing list
>> pdal at lists.osgeo.org
>> https://lists.osgeo.org/mailman/listinfo/pdal
>
>
>
> --
> Andrew Bell
> andrew.bell.ia at gmail.com
>
> --
> Raúl Casado Barbero
> Servicio de Evaluación, Restauración y Protección de Agrosistemas Mediterráneos
> Estacion Experimental del Zaidin, CSIC
> Calle Profesor Albareda 1, 18008 Granada, Spain
> Phone: +34 958181600 VolP CSIC 439186
>
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