[pdal] [PDAL] PGPointCloud: Updating point info

[Rémi Cura]

Hey,
currently updating points inside patches is little bit of a pain,
you need to explode the patch to points, convert the point to array,
change the value for every point, then reform the patches.
I'll suppose the classification attribute class_id is in position 7.

WITH my_patch AS (-- spatial join. Should be very fast, both table indexed
    SELECT patch_id, patch
    FROM patch_table, polygon_table
    WHERE ST_Intersects(patch::geometry,geom)=TRUE
)
, my_points AS ( --converting patch to point. Note that could have been
done directly in the previous CTE
    SELECT patch_id, point[1:6]|| my_new_class_id||point[8:n_dims] AS
updated_array
    FROM my_patch, pc_explode(patch) as pt, pc_get(pt) as point
)
, updated_patch AS (
    SELECT patch_id, pc_patch(updated_array,XX) AS n_patch
    FROM my_points
    GROUP BY patch_id
)
UPDATE ...

Actually, I would do this in python outside the database (if necessary
using shapely).
Why? Because it would be much faster (basically, when you use pc-explode
you are screwed)
.
If doing this in base, I would probably embed the query in a plpgsql
function to force the index use.

We could discuss this more but there are many solutions and they strongly
depend on your requirement.
For instance Pgpointcloud would be easy to extend (in C) to update a whole
dimension at a time (setting all point class_id for instance).

In another direction, it may be a better idea to not store the
classification per point, but per patch (all points in one patch would have
the same class).
To enforce this you would need to split/merge your patches when necessary,
which is fairly easy.
It would be much much faster, and would probably makes more sense for your
application (especially if you plan to access your point by tree_id).

If you need python ressources (like read write patches in binary, and
examples):
https://github.com/Remi-C/PPPP_utilities/tree/master/pointcloud

You may be interested by my working paper on classification at patch level (
http://arxiv.org/abs/1602.06920), although only a part of the paper is not
about classification,
and I'm affraid this paper is a pain to understand (working on this).

Cheers,

Rémi C

2016-05-11 18:43 GMT+02:00 Jean-Francois Prieur <jfprieur at gmail.com>:

> Wow thank you for your quick reply.
>
> Yes, you have the gist of it, although we do classification outside of the
> database (using random forest), so it is really just assigining the crown
> id (from the polygon shapefile) to all the points in the patch(es) that
> fall under it. That way, we can easily pull all the points from a specific
> crown by ID and not spatial join. The part I am having an issue with (the
> shapefiles and point cloud is already in PostGIS/pgPointCloud) is:
>
> "then write SQL to update the points inside the crown polygons."
>
> I am not seeing the SQL needed to do this, it is probably trivial.
>
> Thanks a lot for that ressource as well, it is similar to what we do and
> seems to be full of examples. We are trying to identfy species at the
> individual tree level using ALS and multispectral imagery or multispectral
> lidar.
>
> Thanks,
> JF
>
> On Wed, May 11, 2016 at 12:12 PM Howard Butler <howard at hobu.co> wrote:
>
>>
>> > On May 11, 2016, at 11:03 AM, Jean-Francois Prieur <jfprieur at gmail.com>
>> wrote:
>> >
>> > Hello,
>> >
>> > Newbie to PostGIS/PGPointCLoud, used to work with MySQL many moons ago
>> so know just enough to be dangerous!
>> >
>> > I am working in remote sensing research , precision forestry to be
>> specific.
>> >
>> > We use ALS data and tree crown segmentation on the resulting canopy
>> height models to segment individual trees. We then use the shapefile to
>> extract the points from the las files. This is done using Python, it works
>> but it is a slow process and we have to redo it when we want to update the
>> info on our points.
>> >
>> > We are looking into doing a lot of this in PostGIS and have succesfully
>> imported our las files using the awesome documentation provided. So we have
>> a working pipeline LAS->PDAL->pgPointCloud and can perform all kinds of
>> queries on the data so that part works.
>> >
>> > We can do the spatial join between the shapefiles containing all our
>> individual crowns (there can be thousands) and then extracting all the
>> points under those. What we would like to do is update all the points
>> selected under a crown with that crowns ID so we do not have to do these
>> spatial joins all the time eg. we can just query the crown number and get
>> the points that way. We can select all the points, we just cannot update
>> the points with the proper info. The spatial join is quite long with
>> thousands of crowns, we would like to only run it once, update the points
>> with the proper crown ID and then work with the data that way.
>> >
>> > I believe part of the solution is the shema used in the pipeline
>> process, I think we would have to add a field in a new schema for the
>> crownid which could then be updated with the crown id. Just not quite sure
>> how to get there.
>> >
>> > Not afraid to learn on my own and digging in, just in need of a push in
>> the right direction!
>>
>> Ok, let me attempt to rephrase. You want to update some kind of
>> classification or other attribute for each point in a pgpatch that is
>> inside a crown polygon? I think what I would do is ingest the crown
>> shapefiles into PostGIS, enable the pointcloud_postgis extension, and then
>> write SQL to update the points inside the crown polygons. There is very
>> likely a Classification attribute as part of your data, and I would just
>> repurpose that to flag points as your tree crowns in some way. Is this on
>> the right track?
>>
>> Also, make sure your read Stephen Mather's blog posts on pgpointcloud and
>> PDAL. He's doing something very similar to your usage and there might be
>> some pointers in there https://smathermather.wordpress.com/tag/pdal/
>>
>> Howard
>
>
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