In this article we will discuss how to get the same results with QGis.<\/p><\/p>\n\n\n\n
The main difference between the two software (ArcGis and QGis) lies in the fact that in Geostatistical Analyst the tool calculates the Voronoi polygons and offers their display with local statistics. The result is a thematic map where the polygons are displayed with a colour gradient in function of the local value of the chosen statistic.<\/p>\n\n\n\n
For example, if we choose the option \u00ab\u00a0average\u201d, the colour assigned to each polygon will be a function of the average of a column (\u201cAttribute\u00a0\u00bb)<\/p>\n\n\n\n
\u00a0<\/p>\n\n\n\n \u00a0<\/p>\n\n\n\n In QGis, the Vorono\u00ef tools are limited to the calculation of polygons. There is no option to calculate the local statistics simultaneously. Each polygon will have as attributes; the concerning attributes of the point therefore in order to calculate the local statistics extra work is required.<\/p>\n\n\n\n It is possible to do it using different methods. Here, we will follow one of the available procedures; we will use a SQL query.<\/p>\n\n\n\n This procedure consists in:<\/p>\n\n\n\n NOTE: the approach is the same as this with either 2.18 or 3.2 QAGis<\/p>\n\n\n\n You have two tools that allows you to calculate the Vorono\u00ef polygons in the QGis treatment:<\/p>\n\n\n\n \u00a0<\/p>\n\n\n\n One of the tools belongs to the group \u201cVectorial Geometry\u201d, the other is an integral part of GRASS geoprocessing.<\/p>\n\n\n\n You can use them indifferently, but I do prefer GRASS.<\/p>\n\n\n\n \u00a0<\/p>\n\n\n\n Save the result as a shape file. This will allow you to load more easily the file in your database.<\/p>\n\n\n\n \u00a0<\/p>\n\n\n\n In order to use the spatial commands in your layer we will load it into a database allowing this type of operations. You can always create and load the shape file into a Sqlite database, provided by QGis. Or you could, if you have installed Postgresql on your computer, use a Postgresql \/ Postgis<\/a> database.<\/p>\n\n\n\n For importing , you have to follow the same process independently of the chosen database.<\/p>\n\n\n\n Note : To create a Sqlite database , go to QGis Explorer<\/em><\/strong> panel, right click on Spatialite<\/strong> , then on Create Database<\/strong><\/p>\n\n\n\n \u00a0<\/p>\n\n\n\n To import your shape file with Vorono\u00ef polygons you will use the database Manager:<\/p>\n\n\n\n Tick your shape file and name to the table you want to create. Click OK.<\/p>\n\n\n\n A message will appear indicating that the import has been performed.<\/p>\n\n\n\n While remaining in the database manager, select the window \u201cproviders<\/strong>\u201d, the table you just created, and then go to Table-> Edit Table <\/strong>in the manager menu.<\/strong><\/p>\n\n\n\n \u00a0<\/p>\n\n\n\n Click on the Add<\/strong> a<\/strong> column<\/strong> button and fill in the characteristics of the field you want to calculate.<\/p>\n\n\n\n \u00a0<\/p>\n\n\n\n In our example we will calculate the average of each polygon and its immediate neighbours.<\/p>\n\n\n\n Before discussing the syntax of the query, let’s see what we want to calculate ;<\/p>\n\n\n\n For each polygon we want to find the polygons that have a shared side with the polygon into consideration, then to calculate the average of an attribute for this set (the value of the central polygon plus the values of the adjacent polygons).<\/p>\n\n\n\n For a Postgesql \/ postgis base, the query is as follows<\/p>\n\n\n\n![\"\"](\"https:\/\/i0.wp.com\/www.sigterritoires.fr\/wp-content\/uploads\/2018\/08\/poly1.png?resize=561%2C759&ssl=1\")
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Calculate<\/strong> the<\/strong> Voronoi<\/strong> polygons<\/strong> with<\/strong> QGis processing<\/strong> tools<\/strong><\/h2><\/p>\n\n\n\n
![\"\"](\"https:\/\/i0.wp.com\/www.sigterritoires.fr\/wp-content\/uploads\/2018\/08\/poly2.png?resize=307%2C249&ssl=1\")
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<\/figure>\n\n\n\n![\"\"](\"https:\/\/i0.wp.com\/www.sigterritoires.fr\/wp-content\/uploads\/2018\/08\/poly4.png?resize=640%2C402&ssl=1\")
<\/figure>\n\n\n\nHow to import the<\/strong> result<\/strong> in<\/strong> a<\/strong> database<\/strong> (Sqlite<\/strong> or<\/strong> Postgres)<\/strong><\/h2><\/p>\n\n\n\n
![\"\"](\"https:\/\/i0.wp.com\/www.sigterritoires.fr\/wp-content\/uploads\/2018\/08\/poly5.png?resize=363%2C419&ssl=1\")
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![\"\"](\"https:\/\/i0.wp.com\/www.sigterritoires.fr\/wp-content\/uploads\/2018\/08\/poly6.png?resize=640%2C497&ssl=1\")
<\/figure>\n\n\n\nAdd<\/strong> a<\/strong> field to<\/strong> include<\/strong> the<\/strong> desired<\/strong> local<\/strong> statistics<\/strong><\/h2><\/p>\n\n\n\n
![\"\"](\"https:\/\/i0.wp.com\/www.sigterritoires.fr\/wp-content\/uploads\/2018\/08\/poly7.png?resize=640%2C492&ssl=1\")
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<\/figure>\n\n\n\nHow to perform<\/strong> a<\/strong> SQL<\/strong> query<\/strong> to<\/strong> fill up<\/strong> this<\/strong> field<\/strong><\/h2><\/p>\n\n\n\n
![\"\"](\"https:\/\/i0.wp.com\/www.sigterritoires.fr\/wp-content\/uploads\/2018\/08\/poly9.png?resize=640%2C483&ssl=1\")
<\/figure>\n\n\n\n![\"\"](\"https:\/\/i0.wp.com\/www.sigterritoires.fr\/wp-content\/uploads\/2018\/08\/poly10.png?resize=640%2C483&ssl=1\")
<\/figure>\n\n\n\n![\"\"](\"https:\/\/i0.wp.com\/www.sigterritoires.fr\/wp-content\/uploads\/2018\/08\/poly11.png?resize=640%2C368&ssl=1\")
<\/figure>\n\n\n\n![\"\"](\"https:\/\/i0.wp.com\/www.sigterritoires.fr\/wp-content\/uploads\/2018\/08\/poly12.png?resize=640%2C393&ssl=1\")
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