Select by location in Python

Question

I have a polygon layer and a point layer (with coordinates for each point). The polygon layer got created using the MMQGIS plugin (Hexagonal Polygons). I want to select all the points inside each polygon.

I worked with ArcGIS before but I want to change to open source. Furthermore, I want to approach that through a Python script since I want to analyse the extracted data. I have QGIS 2.8 installed. I am a bit overwhelmed by all the GIS libraries there are for Python.

Can I approach that sort of selection with a GIS library that is already installed with QGIS? If so, could anyone pin point me into the right direction?

Related question and answer: https://gis.stackexchange.com/questions/256569/select-features-from-another-layer-based-on-a-selection-in-pyqgis — Comrade Che, Dec 19 '17 at 12:58

score 11 · Accepted Answer · edited Apr 13 '17 at 12:34

11

Assuming you want to run your script within QGIS (from a script file or Python console), you can use the following:

import processing

processing.runalg("qgis:selectbylocation", INPUT, INTERSECT, METHOD, OUTPUT)

Here is the help description provided by the Python console which defines each parameter:

processing.alghelp("qgis:selectbylocation")
ALGORITHM: Select by location
    INPUT <ParameterVector>
    INTERSECT <ParameterVector>
    METHOD <ParameterSelection>
    OUTPUT <OutputVector>

METHOD(Modify current selection by)
    0 - creating new selection
    1 - adding to current selection
    2 - removing from current selection

To create a script entirely from scratch which doesn't call on QGIS functions, the following links might help in developing similar functionality:

EDIT:

There are a couple of posts which might be of some use with writing scripts outside QGIS for Mac:

edited Apr 13 '17 at 12:34

Community

1

answered Apr 02 '15 at 12:50

Joseph

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Hello Joseph. Thanks for your reply. I want to use QGis functions! But I think I do not want to make that in the QGis console but call a script from the console of my computer (OS X 10.10). Which library are you using there? Or which library is QGis providing and how can I access that API? Is there anything to read on that? – four-eyes Apr 02 '15 at 13:23
No problem Stophface! I've edited my post to include links which describe how to use scripts outside QGIS for Mac. I use Windows :) In terms of scripting, I don't think there's much difference, the biggest being setting the PATHS I think (I'm also a beginner!). – Joseph Apr 02 '15 at 13:38
Yes, that path setting is something which wont work over here at all. But once thats fixed, it will save a lot of work later on :) Is there a documentation on the library QGis is using? That way I do not have to come back here but just look things there up. Is it PyQGis? – four-eyes Apr 02 '15 at 13:40
Yes, it uses PyQGIS :) – Joseph Apr 02 '15 at 13:49
Most welcome buddy! – Joseph Apr 02 '15 at 14:13

score 4 · Answer 2 · answered Jan 19 '22 at 18:13

Joseph's answer is correct, although there needs to be another parameter included: "PREDICATE".

ie:

processing.run("qgis:selectbylocation", {
    "INPUT":lyr_input,\
    "PREDICATE":0,\
    "INTERSECT":lyr_intersect,\
    "METHOD":0,\
    "OUTPUT":path}
)

From processing.algorithmHelp("qgis:selecybylocation"):

PREDICATE: Where the features (geometric predicate)
Parameter type: QgsProcessingParameterEnum

Available values:
    - 0: intersect
    - 1: contain
    - 2: disjoint
    - 3: equal
    - 4: touch
    - 5: overlap
    - 6: are within
    - 7: cross

Accepted data types:
    - int
    - str: as string representation of int, e.g. '1'
    - QgsProperty

Select by location in Python

2 Answers2