Piekka generalizes topographic data for small scale maps
By Veijo Pätynen & Riitta Sairinen
Pdf-file of the article, from Positio ICC Special Issue 2011
An application for vector-based generalization of geographic data was developed at the National Land Survey of Finland and has been used for the production of small scale maps for a couple of years now. The quality of map databases as well as the efficiency of the production process is well improved.
Until 2006, the production of digital small scale maps at the National Land Survey of Finland was organized as separate production lines of each map scale. The 1:100 000 scale map was generalized from the Topographic Database in 1:10 000, but the rest were scanned and vectorized from printed maps. The needs of the customers of that time were fulfilled, but the suitability of the produced digital maps for present-day needs would have been inadequate.
A far automated process
The development project of the new production system was carried out in 2003-2006. Three system designers and three application developers worked in the project that created the generalization application called Piekka. The basic idea of the production system was to avoid interactive work on the generalization and to use automation whenever it is possible. The production process is general in a sense that it is not intended for any specific map portrayal.
The generalization application is based on ArcGis by ESRI. The ArcObjects technology provides a wide component library for developing your own generalization tools. The tools in Piekka are developed using Visual Basic and C# scripts and integrated into an ArcMap application. The database format is ArcGis FileGeodatabase. In the database, the topological relationships between features are maintained with ArcGis topology tools.
The whole generalization process is done in vector format. The tools can be used for the generalization of different scales after the parameters of a desired generalization step have been defined.
The Piekka application contains all necessary generalization tools for the selection, simplification, combination, collapse, typification and enhancement of points, lines and polygons. The tools are listed in Table 1 below. They include tools for both automated and interactive editing.
Parameters control the generalization
The parameters of the generalization steps are based on the minimum sizes or widths of objects or the minimum distances between objects, depending on the desired scale of generalized data. All parameters for controlling the generalization process are pre-defined and stored into the tables of a separate control database. This ensures that all data are generalized in the same way and the users of the system cannot use wrong parameter values by mistake.
Many functions of the Piekka application are based on finding and exploiting the center lines of polygons; Delaunay triangulation is applied to find these center lines. For instance, there is a tool for collapsing narrow rivers represented by polygons to lines. This tool finds narrow sections of river polygons, automatically creates the center lines and connects lines to tributaries, and then deletes the redundant river polygons. This is shown step-wise in Figure 1.
The tools for polygon aggregation, especially, are very versatile. All the different classes of land cover (agricultural areas, rocky areas, wetlands, etc.) can be generalized in the same process as the application controls the priority and processing rules of different area classes. The rules for controlling the process are pre-defined and stored into a separate table of the control database.
The different types of rules are:
- Priority rules: Specifying the importance of features
- Geometry rules: Concerning distance, size, or shape
- Blocking rules: For prevent areas to spread over certain areas
- Similarity rules: To allow fusion of similar features
Two map databases are produced already
The source data for the generalization is derived from the Topographic Database that incorporates the most accurate geographic data about the Finnish topography corresponding the scale of 1:10 000. In the first phase, the 1:100 000 scale map database was produced. The full coverage of Finland, 390 000 km2, was achieved at the end of 2009. The production continued with the 1:250 000 scale map database that was generalized from the 1:100 000 scale data. This database was completed at the end of 2010. Production continues with the map databases on scales 1:1 million and 1:4.5 million. Around 10 operators are involved in the production.
Efficiency of the new process is remarkably better than that of the old production system. The Piekka application enables the establishing of homogeneous, high-quality small scale databases that cover whole Finland. The application can be used for the generalization of different scales just by defining the parameters. The Piekka application in its current form is intended for the creation of databases. Development work for an appropriate maintenance system of these databases is still to be done.
Veijo Pätynen works as a Senior Application Specialist and Riitta Sairinen as an Expert in the Development Centre of the National Land Survey of Finland, contact: veijo.patynen[at]nls.fi and riitta.sairinen[at]nls.fi
6.8.2011 10:40