In many planning, construction and network management projects, technical drawings are supplied in DWG format (or sometimes DGN), produced by Computer-Aided Design (CAD) tools such as AutoCAD, BricsCAD or MicroStation. However, these files are poorly suited to direct use in a Geographic Information System (GIS).
GIS teams therefore need to transform these files so they can load them into their spatial database, overlay them with other layers (land parcels, utility networks, regulatory constraints, etc.), or simply view them in their spatial context.
In this article, you’ll discover how FME software handles this conversion, along with 3 examples of CAD-to-GIS conversion.
Understanding the structure of a CAD file
A DWG file contains graphic entities (lines, polylines, blocks, circles, text, etc.) organised into layers. Each layer often corresponds to a theme (e.g. networks, street furniture, roads), but this organisation depends heavily on the drawing conventions used.
Some key characteristics to bear in mind:
- No spatial projection: coordinates are local (often in metres), with no explicit geographic reference.
- Few or no attributes: entities are rarely enriched with usable attribute data.
- Graphical structure: the information is visual, not semantic. Two identical lines can represent very different objects.
These elements therefore need to be interpreted to extract coherent geographic features that can be used in a GIS.
The challenges of GIS integration
Integrating a CAD file into a GIS is about more than a simple format conversion. It serves specific business objectives:
- Visualising the drawing in a geographic context: overlaying it with existing data (land parcels, zoning, roads).
- Analysing the content: measuring, filtering and cross-referencing with regulatory or technical layers.
- Reusing the data: loading features into a GIS database to update a reference dataset or feed a web application.
DWG to GIS: 5 conversion steps with FME Form
Here is a typical process for transforming a DWG file into usable GIS layers with FME Form:
1. Reading the DWG / DGN
FME reads DWG and DGN files natively through its readers. Each entity (line, block, text, etc.) is imported as a “feature” in FME Workbench. Layer names, object types and geometric properties are all available for filtering.
You can use the DWGStyler transformer to interpret layer styles and prepare an initial structure.
2. Selecting the relevant layers
Not all layers are relevant. It is essential to isolate those containing the features of interest (e.g. networks, kerbs, land parcels) and exclude superfluous graphical elements (dimensions, hatching, annotations).
For example, in a road layout drawing, we might keep the "carriageway", "kerbs" and "manholes" layers, and exclude the "text", "dimensions" and "hatching" layers.
3. Georeferencing
Because CAD drawings are often drawn in a local, “flat” coordinate space, they need to be repositioned in the correct coordinate system. This can be done:
- Using known control points
- Through an affine transformation (with the AffineWarper transformer)
- By applying an XY offset if the offset is known
For example, a drawing in local coordinates can be repositioned into British National Grid (EPSG:27700) for use in a UK GIS, or into Lambert 93 (EPSG:2154) in France.
4. Transformation and feature typing
CAD entities need to be translated into GIS features: lines into roads, blocks into assets, or polygons into land parcels.
This requires:
- Converting blocks into points with attributes (DWGBlockAttributeExtractor)
- Filtering by type (Tester, AttributeFilter)
- Enriching the data (attributes, types, identifiers)
A block representing a lighting column can be converted into a point feature with the attributes "Type = Street lighting" and "Height = 5m".
5. Converting to a GIS format
Once the data has been cleaned and typed, it can be exported to a standard GIS format:
- Shapefile or GeoPackage for standard integration
- GeoJSON for web publishing
- PostGIS for database storage
You can add a GeometryValidator before export to check that the geometries are valid.
3 CAD-to-GIS use cases with FME by Avineon Tensing
After processing in FME, the DWG file becomes structured GIS layers that are ready for analysis. Here are some real-world examples:
1. Integrating CAD data into a GIS with FME
A public lighting operator (article in French) automated the conversion of CAD drawings into reliable, consistent GIS data with FME, using an intelligent workflow to improve consistency and efficiency.
2. Checking data quality for a PCRS with FME
Before producing a PCRS (France’s large-scale reference base map; article in French), data validation is crucial. FME automated the detection and correction of inconsistencies between cadastral data and French tax authority (DGFiP) records, ensuring a reliable and complete map.
3. Publishing DWG street furniture as Open Data with FME
Publishing a street furniture drawing as Open Data requires reliable, usable data. FME automated the conversion of DWG files to GeoJSON, identifying and enriching street furniture features to deliver clear, standardised data ready to share on a public portal.
In summary, converting a DWG file to a GIS format is a strategic operation. It makes technical drawings interoperable, usable and future-proof. FME software provides a powerful, visual and automatable solution for successful integration, whatever your business context. We offer FME training for all levels, from beginner to expert. For any bespoke requirements, please get in touch with our team.
