[gdal-dev] Convex hull with holes

[Paul Meems]

Thanks Jukka for finding this.

It seems exactly what I'm looking for.
Later this week I'll give it a try to use with my CSV-file.

Regards,

Paul

Op zo 5 mrt 2023 om 22:20 schreef Rahkonen Jukka <
jukka.rahkonen at maanmittauslaitos.fi>:

> Hi,
>
>
> I did not know until today that SpatiaLite has also ST_ConcaveHull
> function https://www.gaia-gis.it/gaia-sins/spatialite-sql-latest.html but
> I am glad that I know it now.
>
>
>
> I digitized some points (concave.jml) for a test
>
>
>
> and then I used ogr2ogr and allowed ST_ConcaveHull to create holes
>
> ogr2ogr -f jml -dialect SQLite -sql "select
> ST_ConcaveHull(ST_Collect(geometry),3,1) from concave" out.jml concave.jml
>
> and the result was
>
>
>
> For fine tuning you may want to add ST_Buffer into the SQL.
>
> -Jukka Rahkonen-
>
>
>
> *Lähettäjä:* Paul Meems <bontepaarden at gmail.com>
> *Lähetetty:* perjantai 3. maaliskuuta 2023 16.47
> *Vastaanottaja:* gdal-dev at lists.osgeo.org
> *Kopio:* Rahkonen Jukka <jukka.rahkonen at maanmittauslaitos.fi>
> *Aihe:* Re: [gdal-dev] Convex hull with holes
>
>
>
> Thanks Jukka for your reply.
>
>
>
> I understand it is hard.
>
> In my case, I have a lot of data points making your edge case not a
> problem for me.
>
> The data points are collected driving over a field with 1 or more sensors.
>
> The result can be something like this:
>
> The red dots are the data points and I manually created the black border
> and added a small buffer.
>
> The red dots are transformed into polygons using inverse distance and
> afterward, I need to 'blank' the data using the black border.
>
> I can automate all steps except for creating this field border.
>
>
>
> I'm also struggling to get a proper name for my needs, making it harder to
> Google for a solution.
>
> 'Convex hull' is not correct. What would be a more appropriate name?
>
>
>
> Regards,
>
>
> Paul
>
>
>
> Op vr 3 mrt. 2023 om 14:13 schreef Rahkonen Jukka <
> jukka.rahkonen at maanmittauslaitos.fi>:
>
> Hi,
>
>
>
> So you have an uncategorized bunch of points and you wish to have an
> algorithm that recognizes outer and inner rings automatically? I fear that
> is not as easy as it may appear. Have a look at the annexed image with two
> polygons having the same vertices. How could an algorithm know which
> interpretation is correct? In this simple case you could make a convex
> hull, delete the vertices which were consumed for the outer ring, and
> create a new ring from the remaining points, but generally it will not work
> if there are two holes, for example.
>
>
>
> If the points come from a GPX device, make the application to record
> indexes for both the points and the rings “ring1-point1,
> ring1-point2---ring2-point1,ring2-point2” and construct the polygon ring by
> ring.
>
>
>
> -Jukka Rahkonen-
>
>
>
> *Lähettäjä:* gdal-dev <gdal-dev-bounces at lists.osgeo.org> *Puolesta *Paul
> Meems
> *Lähetetty:* perjantai 3. maaliskuuta 2023 14.52
> *Vastaanottaja:* gdal-dev at lists.osgeo.org
> *Aihe:* [gdal-dev] Convex hull with holes
>
>
>
> How can I algorithmically create a parcel border that accounts for islands
> or holes within the parcel based on a list of coordinates?
>
>
>
> While GDAL+GEOS has a Convex hull method, it only identifies the outer
> border.
>
>
>
> Has GDAL+GEOS a method that I can use to detect inner holes as well, such
> as in a donut-shaped field?
>
>
>
> The output can be json or shapefile, that doesn't matter.
>
>
>
> Thanks,
>
> Paul Meems
>
>
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