Wednesday, December 17, 2014


The BGT.

The Registration Large Scale Topography (BGT) is a detailed topological digital map of the Netherlands in which all physical objects such as buildings, roads, water and land cover are unambiguously registered.

The last years the entire geo sector of the Netherlands is busy constructing this polygon based map that will ultimately replace the old line based map (GBKN).

This huge national endeavor encompasses ministries, national agencies, municipalities, provinces, companies and the national cadastral agency.

The transition from a line based map to a polygon map is not an easy one and requires continuous tunning and collaboration between the parties involved.


FME and the BGT. 

Pricipally this is a task that can be described as CAD to GIS conversion in FME terms.
Most of the dutch GIS companies are in someway involved by assisting the parties involved to assemble their part of the map.

Unfortunately a large sector of the civil engineering CAD users need to adjust their work methods to a polygon based map.

This is where FME can be used to reverse engineer the GIS map into a line based map, something that is more commonly used in the civil engineering CAD based sector.

BGT - CityGML format.

GIS 2 CAD with FME.

To demonstrate how easily this task can be done with FME, I am making use of a small part of the publicly available BGT obtained from the dutch SDI (PDOK). For more information on CAD 2 GIS translation and resource with FME see FMEpedia.


Step 1: Polygons 2 lines.

Converting polygons to lines is a no-brainer for a seasoned FME user, but you do need some tricks up your sleeve to successfully accomplish that for all of the polygon objects.

Step 2: Lines priority.

To prioritize the resulting lines I am using the AttributeValueMapper transformer, this is just one of the many way to do this, but for this example it is sufficient.

Step 3: Generating prioritized lines.

Once the priority is assigned it is a matter of making use of the priority by testing and reordering to achieve the desired results.

Final step: Writing to CAD.

Result DWG.
For the purpose of this demonstration I have created a DWG file for viewing the results. The visualization of the lines is not according to any template, but that is something that can easily be done with FME.

Real world example.

This demo is based on a solution already used by the municipality of Gorinchem.
The solution created allows the user to transform the polygon map into a DWG defined by a template file.

The solution also provides the user the ability to reorder the generated lines, set new visualization rules and decide with layers should be included in the output.

If you don't believe me, just ask Hans......

Small tip for reading the BGT in FME, use the CityGML reader with the imgeo xsd provided by Geonovum or simply download this workspace and follow the instructions.