Hi, Regina, I need to be confident with a method to use a polygon to filter out all line or point features in and around the polygon so that I have a smaller data set to work on. Is this approach robust? Regards, David
On Wednesday, 14 December 2022, Regina Obe <[email protected]> wrote: > The larger the polygon generally the larger the bounding box. This of > course is not always true. > > You could have a skinny large polygon that doesn’t have much area but is a > huge length angled. > > So the bounding box of that would be big even though the area might not be > so big. > > > > At a certain point, you’ll find that the geohash returns an empty string > because you can’t use it to filter out anything. > > > > > > > > *From:* postgis-users [mailto:[email protected]] *On > Behalf Of *Shaozhong SHI > *Sent:* Wednesday, December 14, 2022 1:03 PM > *To:* PostGIS Users Discussion <[email protected]> > *Subject:* Re: [postgis-users] Understanding of Geohash of a line feature > > > > Hi, Regina, > > > > The matter seems to be more complicated than that. > > > > I just tried the same method on a large polygon and found a table of > geohash values at different granularity, > > > > [image: image.png] > > > > Can one understand that the coverage of this polygon involves all these > geohash grid? > > > > A clear understanding of what these mean is important. > > > > Regards, > > > > David > > > > On Tue, 22 Nov 2022 at 21:40, Regina Obe <[email protected]> wrote: > > David, > > > > Sorry for the delay. For points ST_GeoHash is an exact location. > > For everything else it’s really the hash of the bounding box. Since bound > box covers more area, the length of the hash is shorter. > > > > Here is an example to demonstrate: > > > > SELECT ST_GeoHash(ST_GeomFromText('POINT(-20.234 50.897)', 4326)); > > > > Returns g919e9d098kp4n6m82ys > > > > SELECT ST_GeoHash(ST_GeomFromText('LINESTRING(-20.234 50.897, -20.235 > 50.908)', 4326)); > > Returns: g919e > > > > Note how the hash of the second is a subset of the first? So that means > any geometry that starts with g919e essentially falls in the bounding box > that the linestring forms. > > > > Also note that the above answer is equivalent to: > > > > SELECT ST_GeoHash(ST_GeomFromText('LINESTRING(-20.234 50.897, -20.235 > 50.908)', 4326)::box2d); > > > > Hope that clarifies things, > > Regina > > > > > > > > *From:* postgis-users [mailto:[email protected]] *On > Behalf Of *Shaozhong SHI > *Sent:* Monday, November 14, 2022 3:46 AM > *To:* PostGIS Users Discussion <[email protected]> > *Subject:* [postgis-users] Understanding of Geohash of a line feature > > > > Any detailed explanation on Gehash of a line feature? > > > > The following works and returns output. > > > > ST_geoHash(ST_transform(a_line_geometry,4326)) > > > > However, what exactly the output means? > > > > Anyone knows what the output means? > > > > Regards, > > > > David > > _______________________________________________ > postgis-users mailing list > [email protected] > https://lists.osgeo.org/mailman/listinfo/postgis-users > >
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