One of the recurrent problems regarding the management of the geographical layers results from the differences between the layer entities, depending on their origin . The same layer produced by two different sources will have slight digitalisation differences. But the term “slight ” does not apply only to our visual perception . In the case of our GIS software , two entities with some shifted points , are as different as two entities located at the antipodes.
The usual solution is to take a layer as reference and applying spatial analysis operators to recover quite poorly the attributes of the other ” similar ” layers .
Anyway! In QGis 2.12 you have a new tool that allows you to modify the geometries of “similar ” layers to match them exactly with the layer you have selected as reference .
Let’s look this tool with a simple example . Let’s take as example the Corine Land Cover layer.
This will be our reference layer. Suppose now that we receive a layer made from Corine Land Cover containing new data :
The red polygon is our new layer . If we look in detail, we see that the geometry is not exactly the same :
To correct this problem , we will use a new plugin available in QGis version 2.12: snapping geometries.
You have to activate it because the plugin is installed automatically with version 2.12 but it is not activated . Click on the Extensions menu ->Manage/ Install Extensions to open the window of the extension manager:
Check the box in front of Snapping geometries. To open the plugin window, click the Vector menu ->GeometryTools->Snapgeometries
The configuration window opens :
The input Vector layer corresponds to the layer where you want to correct the geometry .
The reference layer is the layer that will assume the entities correct geometry.
In Maximum snap distance enter the distance from which you do not want the geometry to be corrected . In our example, we will look for points in the reference geometry with a distance of 100 meters from the geometry to be corrected .
You can correct directly the input layer ( always not recommended ) or create a new layer ( always advised ). Click on Launch, the corrected layer is loaded in the cartographic window :
The new polygon (purple) corresponds exactly to the polygon of the reference layer, while keeping its attribute table . Let ‘s leave aside the example and see what the plugin does:
For nodes that are in the same locations in both geometries -> no action .
For a node that is not exactly in the same location ( Displaced node in the figure), but in the search radius defined in the setting window (Snap area of the figure) -> movement of the geometry node to locate the correct exactly at the location of the reference geometry.
For a node present in the geometry to correct but which has no correspondence in the reference geometry in the search radius defined in the setting ( Extra node ) -> no action , the nodes will kept in geometry corrected .
For nodes of the reference layer which have no correspondence in the geometry to be corrected (missing node), but which are in the search area (Snap area) -> addition of these points in the geometry corrected .
