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 < [email protected]>: > 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 <[email protected]> > *Lähetetty:* perjantai 3. maaliskuuta 2023 16.47 > *Vastaanottaja:* [email protected] > *Kopio:* Rahkonen Jukka <[email protected]> > *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 < > [email protected]>: > > 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 <[email protected]> *Puolesta *Paul > Meems > *Lähetetty:* perjantai 3. maaliskuuta 2023 14.52 > *Vastaanottaja:* [email protected] > *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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