Geometry validation with QGis 2.12.0

We have already discussed the geometry validation in a series of
articles. Particularly, you can read the article The  geometry validation  tools (5) with QGis for the validation tools with
QGis 2.8

Version 2.12 brings a new plugin that changes the game. Here is a list
of  those main characteristics .

Firstly, let’s see what does not change: there is no documentation regarding
the tools used by the plugin. It is true that in order to develop those tools implies
a very hard job, but using a tool without knowing what is actually done is not
very reassuring .



We are going to proceed as for the aforementioned series of articles, by
taking a vector layer with errors and following the behaviour of the plugin. We
will be using the same working layer .

”   We will use a layer of Italian communes provided by ISTAT,
the Italian Statistical Institute. This is the layer used in the geometry
validation page of Spatialite: SQL functions based on
liblwgeom support  version 4.0.0
.

The layer is downloadable in this link: http://www.istat.it/uploads/com2011.zip .   ”

The QGis version used is 2.12.0

Activation of the plugin

The plugin is installed by default with version 2.12 but not activated .
You must activate it :

Extension -> Install / Manage Extensions
then check the box in front of Check geometry

Launching the plugin

Load the data layer you want to check and then
click the Vector menu ->GeometryTools->Checkgeometries

The plugin settings window displays.

As you can see, the window includes several blocks of parameters .

These blocks correspond to the different types
of geometry error supported by the plugin. Here is a graphic synthesis :

The first one includes the basic elements of geometries validation: the
auto-intersections, the duplicated nodes and the polygons with less than three
nodes (not shown in the previous image).

In the second block you must choose the type of geometry to validate
(point, line , polygon , … ).

The third block corresponds to the management of the holes and
archipelagos. You authorize the polygons with holes or not, according to your
knowledge of the data , and you authorize archipelagos polygons or not.

The fourth block used to validate data according to minimal parameters of
segment length, angle, area and width. It becomes obvious that a deeper
knowledge of the data is essential to configure these parameters .

The last block allows topological validation , by looking for
duplicates, entities within other entities , partial entities overlays and
interstices between entities .

Plugin test

Firstly, let’s test a simple configuration, only
taking in account the geometry validity (first block of parameters ), to
compare the result with the tools of previous QGis versions.  

The number of detected errors is 476. This figure must be compared to
those obtained in previous tests :

Display

items

Search :

Edit
Software   Nb errors found
ArcGis
10.3.1
  0
SpatiaLite

  19
postgis

  19
FME
2015
  19
Geomedia

  28
Qgis
2.8
Verify
topol .
19
  Valid.
Geom .
129

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items 1 to 7 of 7 items

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Being 4 times larger than those observed with the geometry validation
tool available in QGis 2.8 it would be interesting to see what are these new
errors detected .

Nevertheless, the only  available indication
is the name of the error : Type of geometry

( multipolygon ) and the coordinates of the centroid of the polygon
concerned .

Despite the efforts deployed, it was impossible to see the errors in
most communes indicated as error .

Therefore, we have tried the automatic correction options of the plugin
but despite the installation on several positions with different systems and
configurations, we have got a QGis crash with every attempt of correction.

Conclusions

The plugin appears be more effective in the detection of geometry errors
, but it presents, in its current stage of development two major problems:

  • a lack of accurate location of the error for the
    concerned geometry concerned
  • a regular crash of the automatic correction module .

To follow with interest, as it looks  very promising .

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