I do that type of things with GMT’s mask module (-D option) https://docs.generic-mapping-tools.org/latest/mask.html
From: gdal-dev <[email protected]> On Behalf Of Paul Meems Sent: Friday, March 3, 2023 2:47 PM To: [email protected] Subject: 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: [cid:[email protected]] 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]<mailto:[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]<mailto:[email protected]>> Puolesta Paul Meems Lähetetty: perjantai 3. maaliskuuta 2023 14.52 Vastaanottaja: [email protected]<mailto:[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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