[gdal-dev] Convex hull with holes

[Paul Meems]

Just to let you know.
I tried the ST_CONCAVE as Jukka suggested and it works great.

Here's my code:

ogr2ogr -f "ESRI Shapefile" -oo X_POSSIBLE_NAMES=X -oo Y_POSSIBLE_NAMES=Y
-a_srs "EPSG:28992"  \
             -dialect SQLite -sql "select
ST_Buffer(ST_ConcaveHull(ST_Collect(geometry),2.5,1), 2.5) from 'MyData'" \
             "MyData-border.shp" "MyData.csv"

Thanks,
Paul

Op ma 6 mrt 2023 om 15:12 schreef Paul Meems <bontepaarden at gmail.com>:

> 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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