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<\/a><\/figure>\n\n\n\nIn fact, the boundary of each locality is the edge of the adjoining road, leaving empty spaces where the roads are. While this may not be a problem in some cases, it becomes one when you add the \u00ab\u00a0Communes\u00a0\u00bb layer (from the same cadastre) to your map:<\/p>\n\n\n\n At this scale it doesn’t look very clean, but if you zoom in it’s even worse.<\/p>\n\n\n\n Et si on And if we look at the locality layer itself, we find numerous topological anomalies:<\/p>\n\n\n\n Yes, it’s the official land registry\u2026<\/p>\n\n\n\n Here’s how to solve both problems:<\/p>\n\n\n\n The following example was created using data from the official cadastre of the 71 department, downloaded from the following link https:\/\/cadastre.data.gouv.fr\/data\/etalab-cadastre\/2023-01-01\/shp\/departements\/71\/<\/a><\/p>\n\n\n\n The solution proposed here requires a PostgreSQL with Postgis<\/a> database. It is possible to adapt it to SQLite to use a geopackage<\/a> file and the QGis DB Manager. But you have to expect very long processing times and work around a few shortcomings and bugs. We’ll look at the best solution here, but we’ll present the solution with Geopackage in another article.<\/p>\n\n\n\n When it comes to managing the spaces between polygons, even if some snapping tools can be used to deal with certain problems, the simplest and most radical solution is to build a buffer around the polygons, so as to transform empty spaces into superimpositions. In the case of localities, empty spaces are of the order of 10m. We build a 6m buffer around the localities, which will give an overlay of the order of 2-3 meters minimum.<\/p>\n\n\n\n Choose a permanent output file in Geopackage format, which will allow you to follow the procedure in the next step without modification.<\/p>\n\n\n\n The result of the buffer layer is as follows:<\/p>\n\n\n\n We no longer have to manage spaces between polygons, we only have overlays.<\/p>\n\n\n\n You can read more about how to load the layer in the article How to load a geopackage in Postgis with QGis<\/a> . Use the same procedure to load shapefiles.<\/p>\n\n\n\n And you can find your table in PostreSQL with pgAdmin<\/p>\n\n\n\n We’re going to use a single script to do all the work:<\/p>\n\n\n\n Here’s the script:<\/p>\n\n\n\n To adapt the script to your case, you need to replace the following elements with a word processor:<\/p>\n\n\n\n Load the new layer into QGis:<\/p>\n\n\n\n We’ve solved the problem of the topological consistency of localities.<\/p>\n\n\n\n If we superimpose the cadastral commune boundaries on our new layer, we can see that they are not consistent with our new lieux-dits, just as they are not consistent with the original lieux-dits.<\/p>\n\n\n\n To obtain coherent communal boundaries, it would be sufficient to group the lieux-dits by commune. The problem is that not all communal areas are divided into lieux-dits. We therefore have \u00ab\u00a0holes\u00a0\u00bb in the lieux-dits layer, which are perfectly normal.<\/p>\n\n\n\n To build complete communes, first create the missing locality polygons, assigning the corresponding commune code, then group the localities together using the commune attribute.<\/p>\n\n\n\n First of all, we’ll create a new polygon layer, emprise<\/em><\/strong>, with a single polygon that completely surrounds our lieux-dits layer.<\/p>\n\n\n\n We then use the \u00ab\u00a0Difference\u00a0\u00bb treatment to keep only the emprise <\/em><\/strong>zones that don’t correspond to the existing lieux-dits in our corrected_table:<\/p>\n\n\n\n You can leave the output of the treatments as temporary layers, as you won’t have any further use for them after the final result.<\/p>\n\n\n\n The result of the difference is:<\/p>\n\n\n\n The result is a single multipolygon\u2026 Before continuing, we need to change this multipolygon layer to single polygons, so that we can remove the area outline.<\/p>\n\n\n\n We use single-piece-to-multi-piece processing<\/p>\n\n\n\n Once the processing is complete, switch the result to edit mode and select the outer polygon.<\/p>\n\n\n\n And it’s being removed:<\/p>\n\n\n\n So now we have a Simple Geometries layer<\/em><\/strong> containing all the places where there are no localities, and a layer of corrected localities. We merge the two layers into one, which will no longer have any holes:<\/p>\n\n\n\n We still need to resolve the problem of attributes for polygons that are not localities. They must have an attribute containing the commune code, so that grouping by commune can take place.<\/p>\n\n\n\n We will use the communes layer of the cadastre and perform a join by location with the new merged layer.<\/p>\n\n\n\n We now have the joined table:<\/p>\n\n\n\n In this image, the first five records correspond to areas that are not lieux-dits. The Commune attribute, which is of interest to us in the next step, is not filled in. However, the id2 attribute, which comes from the location-based join with the communes layer, is filled in.<\/p>\n\n\n\n Open the table in edit mode. Use an expression to select the records that do not have the commune attribute filled in, then open the field calculator and update the commune field with the contents of id2.<\/p>\n\n\n\n Now we can group the lieux-dits by commune to obtain our communes layer consistent with the lieux-dits.<\/p>\n\n\n\n We use the Group treatment, with the layer resulting from the join we’ve just modified.<\/p>\n\n\n\n So we have our new communes. We now have one last operation to perform. As with all mergers, there are a few small drosses that need to be eliminated:<\/p>\n\n\n\n To do this, we’ll use the Remove Holes treatment.<\/p>\n\n\n\n Don’t forget to save the result in a permanent file – it would be hard to have to redo everything!<\/span><\/p>\n\n\n\n We can now compare the result of our new common layer with the original layer<\/p>\n\n\n<\/a><\/figure>\n\n\n\n<\/a><\/figure>\n\n\n\n<\/a><\/figure>\n\n\n\n\n
How to correct the topology of the locality layer<\/h2>\n\n\n\n
1-Create a layer with a buffer around localities<\/h3>\n\n\n\n
<\/a><\/figure>\n\n\n\n<\/a><\/figure>\n\n\n\n<\/a><\/figure>\n\n\n\n<\/a><\/figure>\n\n\n\n2- Load the lieux-dits layer into PostgreSQL<\/h3>\n\n\n\n
<\/a><\/figure>\n\n\n\n<\/a><\/figure>\n\n\n\n3- Running the SQL script<\/h3>\n\n\n\n
\n
DROP TABLE IF EXISTS table_corrigee ; \n\nCREATE OR REPLACE VIEW auto_jointure AS SELECT\n\n tampon.id as id,\n tampon.nom as \"nom\",\n tampon.commune as \"commune\",\n-- we retrieve all the polygons from the buffer table in two collections: one containing the complete geometries and another containing the superimposed parts. For the moment, both collections contain the complete geometries.\n ST_Union(tampon.geom) AS full_geom,\n ST_Union(tampon_bis.geom) AS shared_geom\n FROM tampon ,tampon AS tampon_bis\n WHERE\n --check polygon validity as follows\n ST_IsValid(tampon.geom) AND ST_IsValid(tampon_bis.geom)\n --filter to retain intersecting polygons\n AND ST_intersects(tampon.geom,tampon_bis.geom) \n --eliminate intersecting polygons\n AND tampon.id <> tampon_bis.id \n --for an intersection of 2 polygons, keep only the smaller one\n AND ST_Area(tampon_bis.geom) < ST_Area(tampon.geom)\n --since we're making \"unions\", we need to perform a grouping on the other attributes\n GROUP BY tampon.id,tampon.\"nom\" , tampon.commune ;\n\/*\nWe'll create a table containing the final result: the first step is to remove all overlapping areas. The second step consists in adding the overlapping areas to the smaller of the two polygons. The holes created by the first step are filled by the second step. The final polygons are seamless, with no overlaps or gaps.\n*\/\n\n CREATE TABLE table_corrigee AS SELECT\n id,\n \"nom\",\n commune,\n\/* Intersections are subtracted from the layer containing all polygons, thus removing conflicting parts*\/\n ST_Multi(ST_Difference(full_geom,shared_geom)) as geom,\n ST_Area(full_geom) as area\n FROM auto_jointure\n WHERE ST_IsValid(full_geom) AND ST_IsValid(shared_geom)\n\/*The intersections just subtracted must then be added to fill the spaces created*\/\n UNION \n SELECT\n id,\n \"nom\",\n commune,\n geom,\n ST_Area(geom)\n FROM tampon\n WHERE id NOT IN (SELECT id FROM auto_jointure);\n\/*A primary key is then redefined so that QGIS can load the layer.*\/\n\nALTER TABLE table_corrigee ADD CONSTRAINT pk_table_corrigee PRIMARY KEY (id);\n<\/code><\/pre>\n\n\n\n\n
<\/a><\/figure>\n\n\n
Left: corrected layer, right: original locality layer<\/figcaption><\/figure>\n<\/div>\n\n\nHow to recreate a communes layer consistent with the new lieux-dits layer<\/h2>\n\n\n\n
<\/a><\/figure>\n\n\n\n<\/a><\/figure>\n\n\n\n1-Creating the missing polygons<\/h3>\n\n\n\n
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