One of the recurrent problems regarding the management of the geographical layers
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
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
and applying spatial analysis operators to recover
poorly the attributes of the other ” similar ” layers .
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
Let’s look this tool with a
Let’s take as example
the Corine Land Cover layer.
This will be our reference
Suppose now that we receive
made from Corine Land Cover containing new data :
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:
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
the window of the extension manager:
Check the box in front of Snapping
To open the plugin window,
click the Vector menu ->GeometryTools->Snapgeometries
configuration window opens :
The input Vector layer corresponds to the layer where
you want to correct the geometry .
The reference layer
the layer that will assume the entities correct geometry.
In Maximum snap
the distance from which you do not want the geometry
be corrected . In our
we will look for points in the reference geometry with a
distance of 100 meters from the geometry to be corrected .
You can correct
the input layer ( always not
recommended ) or create a new
layer ( always advised ).
Click on Launch, the corrected layer is loaded
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
the plugin does:
For nodes that are in the same locations in both
-> 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
the geometry to correct but which has no
correspondence in the reference geometry
the search radius defined in
the setting ( Extra node ) -> no action , the nodes will
geometry corrected .
For nodes of the reference layer which have no correspondence
the geometry to be corrected (missing node), but which are in
the search area (Snap area) -> addition of these points in the geometry