The DWG format is an Autocad proprietary binary format. When we want to load layers in QGis from this format, multiple problems arise.
QGis uses the GDAL library to read / write external formats , such as DWG.
Firstly a clarification: do not confuse the DWG format and DXF format. Often references for DWG / DXF format are found which is a misuse of language. Both formats are different . The DWG format is the native format for Autocad software. It is a proprietary format and, in theory, subject to license. The DXF format is a data export format for Autocad software. It is public and not subject to license.
If you have at your disposal DXF files none of the problems raised in this article applies.
DWG version
The first concern arises from the fact that several versions of the DWG format exist.
The GDAL library used by QGis, is compiled with the libopencad library that manages some of these versions, but not all . You will get a message such as the following
In the case where the version is supported by libopencad you will get a window similar to:
Drawing Software / GIS Software
In the previous window we can see the conceptual difference between CAD and GIS software. In the drawing software , the objects are grouped in “layers ” in this window . We must not confuse the layer concept with the GIS layer.
GDAL tries to move from one notion to another and that is why you have in this window a proposal for GIS layers built from layers of the DWG drawing file.
The first line suggest a 0-FB1-archive layer with 7 entities of linear type . It corresponds to a layer of that name where the designer has placed just the linear objects . For this layer / layer there is, a priori, no problem.
Nevertheless, a a little further down, we have a “BORNES” layer with 39 entities but with a “Geometry Collection” type geometry . This, clearly, means that the objects drawn on this layer are of different types : lines , points, text , etc …
GDAL cannot create a single GIS-type layer from the layer without losing, eventually, important information. For the time being, QGis does not support layers with different geometries (I believe, only Geomedia Intergraph manages multi-type geometries). If you select TERMS and click OK, you will see a new layer in the layers panel , but of the DATASET type
You will not see any entity on your cartographic window. Nevertheless, if you open the attribute table you can see the contents of this DATASET
You can see that the original layer contained circles and text. Nevertheless, this layer is unusable in QGis because no geometry is associated with the layer and, therefore, we cannot display it on the map.
Entities Content
The other problem that arises but, usually, when importing data from the CAD software, is how to manage symbology entities. In GIS, the symbology is managed outside the entities data. It is not stored as data, at the most it can be stored in separated files, such as QML or SLD.
In the list of layers shown above you can see a FAS-hatch- built layer that contains hatches of buildings .
These layers are useless during the integration in a GIS.
A workaround
Until GDAL can solve all these drawbacks here is a workaround.
You can install and use a free program to turn DWG files into DXF, before loading them into QGIS.
ODA File Converter (Open Design Alliance) allows converting the different versions of DWG to the DXF format.
You can download the following link .
Once installed , when running it, you will get the following window:
You can select the most recent version of the DXF format as it does not pose any integration problem with GDAL and QGIS. The program converts all the DWG files of a directory , and possibly subdirectories (if you check the Recursive folders box ).
Besides, the fact of having a file in a readable format by QGis, the program groups all entities in three types of entities : points, lines and polygons , plus one Geometry Collection class .
Unlike GDAL and Libopencad , you will see a reduced number of Geometry Collections. Only the collections that ODA cannot interpret (eg CADAttdef) will remain as collections.
Others, such as the layer TERMINALS we have discussed above, will be converted as points for the texts (with TEXT attribute ) and lines for the circles . The result , if you load the three layers (Point, LineString and Polygon) will be of the following type:
A cleaning job to remove drawing artefacts will be essential to retain only the data itself:
This can be easily done since the attribute table generated contains the name of the original layer for each entity .
