[Qgis-user] adding points within polygons

[Brent Wood]

Which version of QGIS are you using? This is in the dev version which I'm using (on Linux), but not in QGIS 2.8 (just looked at that on another laptop - Geometry tools is there - but not the centroid one). I don't know if it is in 2.12 or not.

You can also install the "realcentroids" plugin. This seems to create a replica of the polygon table, but creates a point feature to replace the polygon, which should also do what you want.
I just tried it & it worked in a simple test case. Perhaps your easiest option.

There is also a processing -> qgis geoalgorithms -> polygon centroid function, but this can return a point which does not lie within the polygon if the polygon is concave.
A bit more cumbersome - but you can also:
 edit the polygon layer, 
open the attribute table
start the field calculatorcreate a new (real no with decimals) column as the X value ie: X($geometry)
create a new column as the Y value ie: Y($geometry)save as CSV file
open the CSV file & define it as a point layer using the new X & Y columns for coords

You now have a point layer of centroids (but note that centroids can lie outside a concave polygon):-)

Cheers
Brent
 

      From: rsweeny <rsweeny at mun.ca>
 To: Brent Wood <pcreso at yahoo.com>; "qgis-user at lists.osgeo.org" <qgis-user at lists.osgeo.org> 
 Sent: Tuesday, February 9, 2016 7:54 AM
 Subject: Re: [Qgis-user] adding points within polygons
   
 Brent
 So kind of you to respond. I am afraid "geometry tools" is not an option on my Vector pull down menu or toolbar. Do I need a plug-in?
 I quite agree with you about the centrality of a proper relational database underpinning the gis. It is getting from one to the other that is giving me the headaches.
 Robert
 
 On 08/02/2016 7:01 PM, Brent Wood wrote:
  
  Hi Robert, 
  You can create a layer of points for each polygon using:Vector - Geometry Tools - Polygon Centroids 
  if you have a common field in this and the census data, you can then join the two: Layer - Properties - Join 
  But I recommend the use of an underlying database tool to manage your data, such as Postgis or Spatiallite, and join the data there to be used in QGIS. A collection of files does not provide the power of a database for managing data, even when accessed by QGIS. 
   
  Well managed data is the foundation your analysis, model or GIS sits on...
  
  Cheers
  
  Brent Wood
        From: rsweeny <rsweeny at mun.ca>
 To: qgis-user at lists.osgeo.org 
 Sent: Tuesday, February 9, 2016 2:10 AM
 Subject: [Qgis-user] adding points within polygons
  
 Hi
 People's answers to Grant's question were so clear I'm hoping that 
 something similar might exist to facilitate my task. I have 32k of 
 polygons representing every lot in the city of Montreal in 1903. I also 
 have the 1901 census returns coded by household (95k of them) to those 
 lots. Is there a simple way to have Qgis place points within each lot 
 polygon for the corresponding households?
 Thanks in advance.
 Robert
 
 -- 
 Robert C.H. Sweeny
 Professor of History
 Memorial University of Newfoundland
 St John's NL, A1C 5S7
 Canada
 709-864-8440, 709-864-2164 (fax)
 
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 -- 
Robert C.H. Sweeny
Professor of History
Memorial University of Newfoundland
St John's NL, A1C 5S7
Canada
709-864-8440, 709-864-2164 (fax) 

   
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