[QGIS-Developer] timzonefinder vs point in polygon

C Hamilton adenaculture at gmail.com
Tue Feb 9 08:52:29 PST 2021 

It is highly likely that you have a much faster machine than mine so I can
only look at comparative times between timezonefinder and the PyQGIS code
included below. You will notice that I take a snapshot of the time before
and after I iterate through the 10,000 points using both timezonefinder as
a look up and the code Nyall proposed. That is how I am getting the times.
What I am really trying to determine is the fastest way to look up a point
using either timezonefinder or PyQGIS code. Perhaps what I have with PyQGIS
code is fast enough, but I am not satisfied if it is not close to the speed
of timezonefinder.

Ultimately, I am trying to determine whether I should continue to use
timezonefinder as the method to look up time zones in my
datetimetools plugin or should I have the gpkg time zone polygon layer as a
part of the plugin and do lookups using it. Right now I am just doing
single point lookups and it is probably fast enough, but I don't like that
it is slower than timezonefinder. I am likely to expand datetimetools
plugin to have a processing algorithm to do a lookup on a whole layer and
in that case the PyQGIS method is better.

Those are my thoughts and if there is a faster method using the gpkg time
zone polygon to do the lookup, let me know.  One thing I just tried was
using the shapefile from
https://github.com/evansiroky/timezone-boundary-builder. It appears it is
not spatially indexed and was taking too long. I resaved it as a shapefile
and made sure it had a spatial index, but the gpkg conversion of the
shapefile is even faster to access.

Calvin

On Tue, Feb 9, 2021 at 11:19 AM <mail at groenebij.nl> wrote:

> Hi Calvin,
>
>
>
> Thanks for sending me this from the Developers list. I am not a part of
> the Developer list, so I did not see the Join attribute by location tip
> from Andrea.
>
>
>
> Anyway, what do you mean with not getting fast result with single point
> lookups?
>
> When I select a single point from the 10000 point layer and run the same
> algorithm (check the selected features only option in the dialogue box),
> the result takes 0.05 seconds (12 seconds for all 10000). At least that is
> what is says in the log tab of the algorithm. And it sure looks quick.
>
>
>
> Jeroen
>
>
>
>
>
> *Van:* C Hamilton <adenaculture at gmail.com>
> *Verzonden:* dinsdag 9 februari 2021 16:25
> *Aan:* Nyall Dawson <nyall.dawson at gmail.com>
> *CC:* Andrea Giudiceandrea <andreaerdna at libero.it>; qgis-developer <
> qgis-developer at lists.osgeo.org>; Groene Bij <mail at groenebij.nl>
> *Onderwerp:* Re: [QGIS-Developer] timzonefinder vs point in polygon
>
>
>
> Nyall,
>
>
>
> Using "Join attributes by location" on a large set of points is fast, but
> if you are doing single point lookups it bothers me that I am not getting
> that fast of results in comparison to timezonefinder. The 10,000 points is
> intended to test the speed of many single point lookups. If I were going to
> actually do 10,000 point lookups I would use the Join attributes by
> location, but here are my results of single point lookups using
> timezonefinder vs. your method. Note that the assumption is that they can
> be anywhere in the EPSG:4326 bounding box. I am not trying to constrain
> them to the QGIS canvas.
>
>
>
> timezonefinder: 44 seconds
>
> Your method: 144 seconds
>
>
>
> Here is my code for each:
>
>
>
> *timezonefinder*
>
> from timezonefinder import TimezoneFinder
> import time
> tf = TimezoneFinder()
>
> l = iface.activeLayer()
> features = l.getFeatures()
>
> start_time = time.time()
> for f in features:
>     pt = f.geometry().asPoint()
>     name = tf.certain_timezone_at(lng=pt.x(), lat=pt.y())
> print('Time {}'.format(time.time() - start_time))
>
>
>
> *Your method*:
>
> import time
> tzlayer = QgsVectorLayer("W:\\My
> Documents\\GitHub\\timezone_speed_lookup_test\\timezones.gpkg",
> "timezones", "ogr")
>
> l = iface.activeLayer()
> features = l.getFeatures()
>
> start_time = time.time()
> for f in features:
>     pt = f.geometry().asPoint()
>     rect = QgsRectangle( pt.x()-0.0000000001, pt.y()-0.000000001,
> pt.x(),pt.y())
>     request = QgsFeatureRequest().setFilterRect(rect)
>     zones_intersecting_bounding_box = tzlayer.getFeatures(request)
>     for tz_feature in zones_intersecting_bounding_box:
>         if tz_feature.geometry().intersects(rect):
>             name = tz_feature[1]
>             break
>
> print('Time {}'.format(time.time() - start_time))
>
>
>
>
>
> I'd suggest using similar logic to what join attributes by location
> does internally. In pseudocode:
>
> search_point = ...
> search_bbox = QgsRectangle( search_point.x() - some tolerance,
> search_point.y() - some_tolerance, ... )
> request = QgsFeatureRequest().setFilterRect(search_bbox)
> zones_intersecting_bounding_box = tz_layer.getFeatures(request)
> for time_zone_feature in zones_intersecting_bounding_box:
>     if time_zone_feature.intersects(search_point):
>         # found a hit!
>         break
>
> This will take advantage of whatever spatial index you have on the
> time zone layer (e.g. the internal shapefile/gpkg/... index)
>
> There's quite a number of extra optimisations which could be done here
> if the speed isn't sufficient for mouse movements, e.g. you could
> fetch all the zone features in the canvas extent and "prepare" their
> geometries for ultra fast point in polygon tests, and then invalidate
> this cache and rebuild on each canvas extent change. But I'd only do
> that kind of thing if needed...
>
> Nyall
>
>
>
>
>
> >
> > Calvin
> >
> > On Fri, Feb 5, 2021 at 6:30 PM Nyall Dawson <nyall.dawson at gmail.com>
> wrote:
> >>
> >> On Sat, 6 Feb 2021 at 07:54, Andrea Giudiceandrea <
> andreaerdna at libero.it> wrote:
> >> >
> >> > C Hamilton wrote
> >> > > Using the algorithm Vector->Geoprocessing Tools->Intersection: 4
> minutes,
> >> > > 4
> >> > > seconds
> >> >
> >> > Hi Calvin,
> >> > maybe it could be better to use the "Join attributes by location"
> algorithm.
> >> > It only takes few seconds to preform the join.
> >>
> >> Confirmed - for me it only takes ~2 seconds, and that's on an
> >> non-optimised debug build! There may be a few % more performance boost
> >> on the proper QGIS release builds.
> >>
> >> "Join attributes by location" has a bunch of extra logic to optimise
> >> the method that the join is performed, which really pays off in this
> >> particular situation (matching points to polygons, where number of
> >> points >> number of polygons).
> >>
> >> Nyall
> >>
> >>
> >>
> >> >
> >> > Regards.
> >> >
> >> > Andrea
> >> >
> >> >
> >> >
> >> > --
> >> > Sent from:
> http://osgeo-org.1560.x6.nabble.com/QGIS-Developer-f4099106.html
> >> > _______________________________________________
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