Martin Salzmann, Auke Hoekstra and Ted Schut
Cadastre and Public Registers Agency (Cadastre) of the
Netherlands
P.O. Box 466, 7300 AL Apeldoorn, The Netherlands
e-mail: {M.A.SALZMANN|A.V.HOEKSTRA|T.G.SCHUT}@AP.KADASTER.NL.NET
ABSTRACT
Currently the Netherlands’ Cadastre undertakes a major effort to improve the digital cadastral map of the Netherlands. It is renovated to meet strict quality specifications. At the same time it is harmonized (reconciled) with the large-scale base map of the Netherlands. A pivotal role in the map renovation process is played by the so-called reconciliation. Attention is also paid to the renovation project and its control. The concept of reconciliation is discussed as well as the map renovation process implemented by the Dutch Cadastre.
INTRODUCTION
Since January 1st, 1998 the cadastral map of the Netherlands is completely digital. After the digitization process the Netherlands’ Cadastre has started a major map renovation project to let the digital cadastral map meet strict geometric and semantic specifications. At the same time the large-scale base map of the Netherlands is nearing completion.
It is foreseen that the cadastral map will be used increasingly in combination with the large-scale base map. It is evident that inconsistentcies between both maps become apparent immediately. Furthermore the quality of the cadastral map is not homogeneous, which is caused by the permanent revision process since its inception in 1832.
Therefore the Cadastre has set up a map renovation programme to have the cadastral map meet its specifications and to allow its combined use with the base-map.
This contribution describes the map renovation project, its control, and the technical activities which are part of the map renovation process. We sketch the objectives of map renovation and outline the source material underlying the map renovation process, namely the cadastral map and the large-scale base map. Then we discuss the map renovation itself and pay particular attention to the reconciliation operation.
OBJECTIVES
For many of our customers the cadastral map is the framework of their large-scale mapping activities or the geometric reference for their administrative data. Therefore it is important that they can rely on the quality of the map. This necessitates that the Cadastre warrants that the cadastral map meets certain clearly defined quality specifications. Furthermore it is the policy of the Cadastre that the cadastral map can be used in combination with the large-scale base map. In other words a user comparing the cadastral map with the base map may not misinterpret the result of, or take false decisions based on, the overlay operation (at the proper presentation scale). If for example a boundary coincides with a topographic element (e.g. a hedge), the user may conclude that the boundary and the hedge coincide. This requires that the boundaries in the cadastral map and the topography in the base map have to brought into accordance with each other. We call this harmonization activity reconciliation.
Summarizing the objectives of map renovation are:
CADASTRAL MAP OF THE NETHERLANDS
The cadastral map of the Netherlands is an object-based seamless digital map with nationwide coverage. Presentation scales are 1:1000 (built-up areas) and 1:2000 (rural areas). The cadastral map has the following contents (see Figure 1):
Figure 1: Fragment of the cadastral map of the Netherlands (not drawn to scale).
Although stored in one database, the map is a patchwork of different types of source-material: remeasured maps of good quality, photogrammetrically created maps of relatively good quality produced during land consolidation projects, and digitized maps of unknown quality. Map renovation will bring the map up to standard quality, which is defined as follows:
In the map renovation process particular attention is paid to the aspects of geometric quality, completeness and attribute accuracy. At the moment the consistency and up-to-dateness have already been achieved to a large extent.
LARGE-SCALE BASE MAP OF THE NETHERLANDS
The large-scale base map op the Netherlands (known in Dutch as the "GBKN") currently covers about 85% of the Netherlands and will be completed by 1999 or 2000. It is a digital map with national coverage. Presentation scales are 1:1000 (built-up areas) and 1:2000 (rural areas). The map is usually produced by photogrammetric stereo plotting with field completion. Height is not an element of the Dutch large-scale base map. The base map is produced under the responsibility of provincial public-private partnerships. Participants are the Cadastre, the utilities (water, telecom, gas, electricity, cable-TV) and municipalities.
Figure 2: Fragment of the large-scale base map of the Netherlands (not drawn to scale).
Most provinces use the so-called 'standard' base map (see Figure 2). It has a full coverage of buildings, roads, waterways, and landscape elements. In particular the map contains: viaducts and other structures, buildings, pylons of high tension power lines and railroad overhead lines, roadsides and road surfacing materials, waterways and ditches, embankments and slopes, names of streets, and house numbers.
The required positional accuracy of the base map is also stated in terms of relative precision. The relative precision between two well defined points has to be better than 40Ö 2 cm in rural areas, or better than 20Ö 2 cm in built-up areas (Osch, 1991).
MAP RENOVATION PROJECT
An overview of the map renovation project is given in Figure 3. Start of the project was the matching of (current and future) customer demands and the existing digital map. This resulted in the explicit specification of the required quality of the cadastral map. Considering the map specifications and the quality of the existing map, budget has been allocated and it has been decided that by 2003 the cadastral map should meet the specifications. For this two systems were needed. A map renovation system, which is implemented as part of our in-house software system LKI (Osch, 1991) and a system for process control. The latter system serves to monitor the progress and the quality of the renovated cadastral map. The digitization of the cadastral map was finished by the beginning of 1998, so that currently all efforts are put into the geometric and semantic improvement of the map.
Figure 3: Overview of the map renovation project.
MAP RENOVATION
The map renovation, which is currently underway, encompasses two aspects, namely the improvement of the geometric and the semantic quality of the cadastral map. The most time-consuming activity is the geometric improvement. It is executed by means of computational methods and interactive-graphical manipulation and here reconciliation is the guiding concept. After improvements by means of computations and reconciling the cadastral map with the base map, the cadastral map meets geometric quality requirements. The second activity involves the checking and improvement of the semantics of the cadastral map. In particular attention is paid to the correctness and completeness of street names and house numbers. In practice geometrical and semantical improvement are often combined.
Geometric map improvement
Geometric map improvement is not executed according to a fixed procedure. The diversity of the source material (cadastral map and base map) is too large. Instead we have chosen to facilitate the map renovation by providing the user with a number of tools. Depending on the situation the user has to choose the best combination of tools. The following limiting conditions apply to our map renovation activities:
Geometric map renovation is a challenging task, because often the geometric quality of the maps is poorly known. The cadastral map-database has allowed from its inception the description of geometric quality as described in (Salzmann, 1996). In areas where these quality descriptions have been maintained over the years (fortunately at a large number of branch offices), these attributes are now very useful in executing map renovation.
In our map renovation process we distinguish the following activities (for a detailed description of the technical issues we refer to (Salzmann et al., 1997)):
Not all steps of the procedure given above are always executed in the map renovation process. The order of the operations, however, is fixed. Considering the situation at hand the operator will decide which steps are necessary. The cadastral draughtsman thus plays a key role in the map renovation process. If, for example, the cadastral map is of homogeneous quality, he generally limits himself to preprocessing, reconciliation, and postprocessing. In complex cases (e.g. inner cities) all steps are executed and also measures from field sheets are used and needed to arrive at acceptable results.
Much effort is also put into bringing the (geometric) quality description up to date. We think that a good quality description will make the revision and the maintenance of the map easier. Besides user demands can be better accomodated.
Map reconciliation
Map reconciliation is a key element in the map renovation process. By reconciling cadastral boundaries with topographic elements two objectives are achieved simultaneously:
The realization of the latter objective is based on the premise that the base-map meets its specifications of geometric quality (which it generally does) and that through reconciliation the cadastral maps "inherits" its quality. This assumption is of course only valid if one actually validates if boundaries can be reconciled with topography.
Part of the map renovation process is also that the buildings of the base map provide the positional reference for both the boundaries and the topography. After map renovation only the buildings of the base map are part of both maps.
Once map renovation has been completed the result of the overlay of the cadastral and topographic map should be comparable with the situation in Figure 4.
Figure 4: Cadastral map and large-scale base map after map renovation (not drawn to scale).
What is reconciliation?
The concept of reconciliation is briefly illustrated by means of a simple example. Consider the following (exaggerated) case where the cadastral boundary should coincide with the middle of a ditch. The map overlay before renovation is shown in Figure 5a.
Once the operator has verified that the boundary coincides with topography (e.g. by using information from field sheets), he will redraw the boundary at its proper position (see Fig. 5b). At that point the boundary will be assigned the geometric quality of the ditch as it is stored for the base map.
Figure 5: Illustration of reconciliation.
PROCESS CONTROL
A project of the size of the Dutch map renovation process requires careful planning and control, not only at the level of project management, but also at the quality level. If lineage information and quantitative information on the geometric quality is available for the source material the planning of the map renovation activities is greatly facilitated. Fortunately many provincial branches have recorded meta-information on the quality and source of the original (analogue) cadastral map sheets and base maps.
Parallel to the extension of our software-system with map renovation functionality, we are at the time of writing (January 1998) developing a system for process control. It is a very simple system, based on proven components as the spreadsheet Excel and the GIS-viewer Mapinfo. The data from this system can be propagated to our project-control and financial systems. The basic unit in this system is the original cadastral map sheet, which is sufficient for planning and control purposes. Most effort in building this system goes into filling the underlying database with meta-information on map quality and origin. Once filled the system can serve as a permanent source of information on the quality of the cadastral database, because it is updated after map renovation. We feel that visualization greatly enhances the planning and progress monitoring.
CONCLUDING REMARKS
Improving the cadastral map under the additional constraint of warranting its combined use with the base map is a challenging task. The Netherlands’ Cadastre is putting much effort in developing tools for map renovation. The execution of the map renovation process remains however to a large extent an activity of human beings. The decisions on which elements of the complete map renovation procedure should be executed as well as on which boundaries are to be reconciled are laid in the hands of our professionals. With this approach we can take advantage of the best of both worlds: the powerful tools of computer graphics and computing and the craftmanship of our experienced employees.
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