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https://github.com/zsiki/realcentroid

QGIS plugin to create points on surface
https://github.com/zsiki/realcentroid

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QGIS plugin to create points on surface

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README 

        
Real Centroids V1.0

===================

![icon](icon.png)



RealCentroids plugin creates a point shape file with internal points of a

polygon shape, similar to PostGIS (GEOS) ST\_PointOnSurface. The point will be

inside the polygon in all cases. Not only the points are created but the

attributes are also copied from the polygons to the internal points.

A single point is generated for multipart geometry. The internal pont will be

in the part with the largest area.



New features in this version

----------------------------



*   upgrade to QGIS 3.x

*	moved to vector menu



Installation

------------



Use the plugin manager or unzip the downloaded realcentroids.zip from the QGIS

plugin repository (http://plugins.qgis.org/plugins/realcentroid) into yours

plugins directory.



You can download the plugin from GitHub (https://github.com/zsiki/realcentroid), too.



Usage

-----



Enable the plugin in the *Plugins/Manage and Install Plugins ...*

dialog. Open the polygon layer you want to create centroids. Start the plugin

clicking on the icon in the plugins toolbar or select it from the plugins menu.

The following dialog appears



![dialog](help/images/dialog.png "Dialog of the plugin")



In the *Polygon layer*

list you can select one from the loaded polygon layers. If the active layer is

a polygon layer then it will be selected in the list. The

*Selected features only* is enabled if there are selected features in the

selected layer. Select a new shapefile for the output clicking on the

*Browse* button, . If you select an existing shapefile, you get a warning

weather to overwrite that shape. If you check *Add to map canvas* checkbox,

the point shape with the internal points is added to the current project. The

attributes of the polygons are also copied to the target point shape file. If

*Polygon layer* or *Output point on surface layer*

is empty, a warning will be displayed.



Why do we need such plugin? There is a *Polygon centroids*

option in the *Vector/Geometry Tools* menu.

This will create centroids at the weight point of the vertexes of the polygon.

So it can be outside the polygon in a concave or multipart case. Realcentroid

will place the point always inside the polygon. See figures below to compare

the results of the two methods. From QGIS 2.4 the GEOS pointOnSurface function

is available from Python/C++ API too, but the standard Polygon centroids tool

doesn't use it. My plugin uses pointOnSurface function for QGIS version 2.4 and

newer.



![weightpoints](help/images/weightpoint.png "Centroid (weight points) created by Vector/Geometry Tools/Polygon centroids")



![pointonsurface](help/images/pointonsurface.png "Points on surface generated by RealCentroids plugin")



Algorithm used before QGIS 2.4

------------------------------



For each polygons the weight point is generated first. If the weight point is

outside, the intersection of the horizontal line through the weight point and

the polygon is generated. The midpoint of the longest line segment from thes

intersection result will be used. See the figure below.



For multipart polygons the algorithm is used for the largest part.



![algorithm](help/images/algorithm.png "Internal point generation")



In QGIS 2.4 and after the pointOnSurface (GEOS) function is used.



Thanks to Seal Phone, Jukka Rahkonen, ozak, pxp44, lokal-profi (github) giving advices,

bug reports, pull requests to improve the plugin.