National Maps of Switzerland - Automated Production Using FME and axpand

Swiss national maps are high-quality topographic maps at 1:25000 to 1:1 000 000 scales. In order to satisfy the ever-increasing demand for vector GIS data, the Federal Office of Topography swisstopo is working on changes to their production process ranging from the compilation of topographic base data to the production of maps. Over the course of the past 10 years, swisstopo has launched a number of projects to enable this transition. ‘SysDab’ is one of these projects and is designed to automate cartographic generalization of the source model and automate incremental updating of the derived target models. The technology for SysDab is being developed by Axes Systems and is based on our core product "axpand", a data-based cartographic GIS system.

Generalization, along with text placement, is at the core of cartography and for many years was considered a step that could not be automated. Generalization is concerned with making maps readable in spite of limited space in which to graphically display data. Where necessary, important map elements are emphasized and less important ones simplified, changed or eliminated. Generalization is an extremely complex process in which numerous content, geometric and topological rules come into play.

The challenge for automated generalization is to define contextual and geometric rules in a format that a computer system can utilize and then implement them in an effective, logical process. Combinations of complex algorithms that use these rules need to work precisely and take neighboring map elements into account in almost all cases. The required result of the automated generalization in SysDab (system goal) is that fewer than 25% of all objects require manual editing after generalization. axpand already fulfils this SysDab system goal. Recent tests using mid-land map sheets showed that an average of 80% of the objects was correctly automatically generalized according to generalization rules. An important element in having already reached this goal is excellent building generalization as well as a number of other well-functioning generalization operators. Work is ongoing to further improve automatic generalization technology within the SysDab project.

 
The cartographic source model (left) and its automatically generalized DCM25 output from axpand (SysDab- right). Source: swisstopo

The interface between the source data, the destination data and SysDab is built on the technology framework of FME and is delivered using the Swiss standard geo-data interface ‘Interlis’. In swisstopo’s case, the source data comes from a topographic data base (stored in ArcGIS).

FME prepares data for the generalization process and can include transformation from 3D to 2D, joining of attribute tables and thinning of extraneous data in cases where their is more data than necessary for representation in maps.

Once data has been transferred via FME to an un-generalized digital cartographic model in axpand, the data is generalized according to user-defined detailed constraints that produce a minimum 75% degree of correct generalization. Cartographic generalization can include, among many other operations, displacement, parallel adjustment, geometry type changes, aggregation, simplification, merging and scaling.

Zone Specific Generalization Rules
axpand supports the definition of ‘generalization zones’ where data can be generalized using distinct zone-specific constraints and workflows. This enables the execution of generalization workflows specific to map regions including built-up areas, city centers or rural areas within the same generalization process.

Incremental Updates
In the case of SysDab, the delivery of source data in increments for automatic incremental updating is managed through an interface between FME and axpand. Three separate Interlis files that include deletes, modifications and additions are generated using FME. In cases where source features have been modified, an optional relevance check in axpand determines whether the changes are significant enough be imported and merit a re-generalization. Those changes that are deemed relevant undergo automatic re-generalization to update the derived model.

axpand generates and maintains stable feature identifiers of the derived data throughout the generalization process over time and update cycles. In addition to the data in the derived model, axpand also creates and stores relationship links between source features and derived features so that the changes to objects can be traced over time and can be re-used for updating. Once the updating through (re)-generalization has been carried out, the data is then sent back, alternatively as increments of derived data, to the front-end editor (in swisstopo’s case ArcGIS) through the FME-based interface.
 
Changing the Face of Generalization
Thanks to technical developments and innovative solutions, including the combination of FME technology and the automated generalization in axpand, it is now possible to automate much of the work of cartographers. This way, cartographers can concentrate on more complex challenges that require their dedicated know-how, intuition and experience. The swisstopo project SysDab illustrates how axpand and FME technologies complement each other in preparing, generalizing, updating and delivering exceptionally high quality derived data products to a geo-spatial infrastructure.