[postgis-users] Understanding of Geohash of a line feature

[Shaozhong SHI]

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 <lr at pcorp.us> 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:postgis-users-bounces at lists.osgeo.org] *On
> Behalf Of *Shaozhong SHI
> *Sent:* Wednesday, December 14, 2022 1:03 PM
> *To:* PostGIS Users Discussion <postgis-users at lists.osgeo.org>
> *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 <lr at pcorp.us> 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:postgis-users-bounces at lists.osgeo.org] *On
> Behalf Of *Shaozhong SHI
> *Sent:* Monday, November 14, 2022 3:46 AM
> *To:* PostGIS Users Discussion <postgis-users at lists.osgeo.org>
> *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
>
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>
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