Miodrag Roiæ
e-mail: mroic@public.srce.hr
Institute of Engineering Geodesy, Faculty of Geodesy, Zagreb,
Croatia
Institut fü r Ingenieurgeodä sie, Geodä tische Fakultä t
Zagreb, Kroatien
L' Institut pour la gé odé sie d'engineering, Faculté gé odé
sique, Zagreb, Croatie
ABSTRACT
Cadastral maps in official use in the Republic of Croatia were made in different time periods and according to different legal systems. The constant disregard of the cadastre, especially in the last fifty years, has led to the present condition, in which the cadastral maps are less and less capable of fulfilling the demands that the cadastral system is being faced with. A uniform cadastral map on a state level can be achieved through improvement of the existing data along with the transition to digital recording systems. The improvement process described in this paper is based on the experiences of countries that have legal systems similar to Croatia’s, with adjustments both to particularities in Croatia and to the most recent technological advances.
ZUSAMNNENFASSUNG
Die in der Republik Kroatien offiziell benutzte Katasterplä ne wurden in verschiedenen Zeitperioden und auf Grund verschiedener Vorschriften gemacht. Systematische Vernachlä ssigung vom Kataster, besonders in den vergangenen fü nfzig Jahren, hat den heutigen Zustand verursacht, in dem die Katasterplä ne die dem Katastersystem zugeteilten Aufgaben immer schwieriger erfü llen kö nnen. Ein einzigartiger Katasterplan auf der Staatsebene kann durch die Verbesserung der bestehenden Daten mit der Übertragung in digitale Form erreicht werden. Das dargestellte Verfahren basiert auf den bestehenden Erfahrungen der Lä nder, die ein demjenigen in Kroatien ähnliches Rechtssystem haben, wobei die Anpassungen den spezifischen Bedingungen in Kroatien, wie auch der neusten technologischen Entwicklung nö tig sind.
RESUME
Les plans du cadastre dans l' usage officiel dans la Ré publiqeu Croatie, sont faits dans les pé riodes de temps diffé rentes et en vertu des pré scriptions diffé rentes. Une né gligence systé matique pour le cadastre, particulièrement dans les 50 anneé s pasé e, a influncé et causé cette situation d' aujourd'hui, dans laquelle les plains du cadastre ne peuvent pas ré pondre aux tâ ches, qui doivent être resolues par le système du cadastre. Le plan du cadastre unique au niveau d’ État peut être realisé par l’ amelioration des données existantes, en les faisant passer dans l’ enregistrement digital. Le procédé présenté de l’ amelioration est basé sur les expériences existentes des pays, qui ont un système juridique sembleble au système croate, avec les adaptations tant aux particularités en Croatie, qu’ au développement technologique le plus nouveau.
INTRODUCTION
Land cadastre as a record of position, form, area, culture, manner of exploitation and owner of each individual parcel is used for technical, economic and statistical purposes and for creation of land registers, and also as the basis for the land taxation. In order for the land cadastre to serve its purpose, it must be continually kept in accordance with the actual situation and renewed as necessary. Cadastral maps have their own aspects and specifications. These differences are due to historical development, legal controls, technological sophistication, institutional structures, economic aspects and the needs of the different users (Williamson 1996).
During the historical development of the land cadastre in Croatia great differences were created in the quality of information in different areas. One of the major characteristics of the cadastral system in Croatia is the fact that in most areas (app. 80%) 19th-century maps produced using the plane table in several co-ordinate systems are in official use.
Maintenance and renewal of cadastral maps also changed through history, depending on the changes of the legal systems and the available technology. But, during all that time it was necessary to incorporate the surveying data into the existing cadastral maps and the borders of the existing parcels of lower accuracy. As the data gathering technology improved, situa-tions were created in which the “accurate” data were incorporated into the “inaccurate” cadastral maps. Additionally, the cadastre was not kept in accordance with the actual situation and that the regulations did not follow the development, the shape of the cadastral maps becomes worse and worse even only in respect to the data. In over six decades elapsed since the introduction of the Gauss-Krüger projection in Croatia, the cadastral maps based on numerical land surveys methods have been created for only about 20% of the country. This, as well as other facts, such as the experiences of other countries, demands that other solutions be sought in order to obtain a uniform cadastral map on a state level.
Only a cadastral map in the national co-ordinate system can serve as the basis for a modern cadastral system, and since it is not possible to create new geodetic bases through numerical land surveys methods in a short period of time, it is possible to achieve the improvement of the existing ones by translating them to a digital form. Improvement of cadastral maps is necessary, as in order to satisfy the growing demands of the users, as well in order to take advantage of the possibilities offered by new technologies in gathering, processing and representation of data in space (GIS). The process of improvement of the cadastral maps is under way in other countries with good results (Carosio, 1991; Ernst & Mayer, 1994; Morgestern & others, 1988). Significantly amplified activities start with the occurrence of computers and software (CAD programs) that considerably speed up the work on the improvement of cadastral maps.
The difference between the political and legal systems in Croatia and other countries does not allow the simple take-over of experiences, but demands a more thorough consideration of the problems and the adjustment of the improvement process. The cited problem is not easily solvable, especially not in the period of the creation of basic regulations that influence the cadastre (Ownership and Other Real Rights Law, Land Register Law, Dispossessed Property Compensation Law etc.).
GOALS
Quality improvement (Qualitaetsverbesserung, BEV 1993) generally leads to the transfer of analogous cadastral data to the digital form in the state reference co-ordinate system. Digital data is stored in a 1:1 scale, and the creation of plots can follow in almost any scale desired.
The problems arising from the use of different co-ordinate systems are known to us from our everyday life, and as the Introduction stressed, the cadastral system has to aim at achieving a uniform reference co-ordinate system. The improvement therefore transfers the data from the existing cadastral maps to the state co-ordinate system, which rapidly leads to the creation of a uniform cadastral map of the entire country.
Unlike the incorporation, mentioned in the Introduction, of the “accurate” into the “inaccurate” data, in the process of improvement the approach is the opposite one, i. e. the data from the existing cadastral maps is incorporated into the information obtained through numerical survey methods in the process of improvement or obtained from the mutations through which the cadastre was maintained. Further changes after the completion of the improvement are facilitated, and the surveys are carried out from the network of control points exclusively.
Equipment, hardware, software and organization of work
In carrying out the field surveys the equipment of the cadastral office in Rijeka was used. The hardware and software for the data processing consisted of one graphical work station with a plotter and a digitalizator at the Faculty of Geodesy (Figure 1).
Figure 1 Hardware
The work on the project was organized in such a way that certain tasks were assigned to individuals and certain other jobs, such as e. g. the global incorporation, were performed jointly. In order to enable the exchange of opinions among the project participants, certain tasks that could have been completed independently were carried out jointly.
PROCEDURE
The improvement process itself does not change the position of cadastral data in a legal sense. The representation of cadastral parcels is partially different after the improvement, but since the topological relations are preserved, it is considered that there has been no change in the position, form or area of the parcel. The alteration of area can be effectuated (when significant discrepancies are found), but according to standard procedure for alteration of established errors, an administrative procedure. The improvement process, therefore, does not interfere with the established rights, so long and expensive administrative procedures are not necessary, as they are with the cadastral surveys.
As it is shown in Figure 2, the basic foundations that are part of the improvement are the cadastral maps created by the means of the plane table. In order to achieve the improvement of this data the following information was used in the process of digitalization-transformation:
and additional surveys were performed in order to determine the convenient number of correspondent points necessary for the transformation ad the recording of the actual situation in nature.
Figure 2 Improvement of cadastral maps
The improvement of cadastral maps was carried out fully in the cadastral unit of Bakarac, with total area of 160 ha and 1683 parcels. By execution of this project the practical training of associates on a concrete example was realized, and the knowledge obtained that is necessary for the creation of directions for the improvement of cadastral maps in Croatia and for further education.
Firstly, the homogeneity of the control points field on the cadastral unit level was examined. This homogeneity is the necessary requirement for a justified implementation of the improvement process. The control points network project was carried out in the Geodetic Institute of Rijeka to serve the needs of the cadastral land survey. It was decided that the well-developed network of control points, standardized at the same time, was homogeneous and accurate enough for the needs of the project (Figure 3).
Figure 3 Control points network
The existing data
For a considerable period of time cadastral operates have been kept up on the basis of individual surveys that are in themselves much more accurate than the original survey upon which the cadastral map was built. The incorporation of this data into the existent cadastral map, as a method of maintenance, discredits the effort and the overall technological advance.
If these surveys (geodetic mutations) are on file in their original form with the survey data, they contribute greatly to the success of the improvement process. Geodetic instruments used in surveying today enable the recording of original surveys in digital form.
All the mutations used in the maintenance of the cadastral operate of the cadastral unit of Bakarac up to now, that are on file, were processed and other data were gathered (utility cadastre…). In order for them to be connected to the state co-ordinate system the necessary surveys were carried out. Figure 4 shows the spatial arrangement of the mutations.
Figure 4 Mutations
Co-ordinates and parcels data bases
The basic principle of modern cadastre, the guaranty of the parcel’s boundary point position, unlike the one of the tax cadastre, which guarantees the parcel’s area, calls for the formation of boundary points co-ordinates data bases with all the necessary attributes (Roiæ, Masteliæ-Iviæ 1992).
Boundary points are classified according to:
Each boundary point receives, in the base, the co-ordinates that, according to the assessment of accuracy, have shown the minimal departure from the most probable values. It should be mentioned that this data base is the core of the digital cadastral operate and that all other technical data (displays, areas…) is derived from it. Further topological spatial relations, based on boundary points co-ordinates, are upgraded through CAD and GIS tools.
Correspondent points and field work
One of the most important parts of the procedure is the choice of the basis that will provide the mathematical parameters for digitalization. In the improvement process the basis for the determination of parameters are the correspondent points identifiable by their co-ordinates in both systems (Figure 5).
The choice of correspondent points is the best indicator of the knowledge and experience of the geodetic expert who is well-acquainted with the local field conditions, but also with the existing cadastral maps. No mathematical model can eliminate the errors that can be made in this stage of the works.
The number of correspondent points required by the transformation is set according to the entry data at about 25 to 30 points per cadastral map. In certain cases these values will not be satisfactory, depending on:
In order to determine the correspondent points co-ordinates in the system undergoing the transformation, surveys were carried out from the previously established control points network (Figure 3).
The procedure of a new survey by one of the geodetic methods can be applied to an area for which even higher quality data than provided by the improvement is required (e. g. urban areas). As it is usually built-up areas that are in question, more terrestrial methods than photogrammetry will normally be utilized.
For those areas a digital cadastral map will be made based on the survey data, and for other areas the improvement process will be implemented and the surveyed points will be used to determine the transformation parameters.
The choice of correspondent points and the correspondent lines was made on the photo-material., the copies of the existing cadastral maps and through field work. The correspondent lines were taken over from the existing photogrammetric surveys, while field surveys were carried out to determine individual correspondent points.
Digitalization and transformations
Most data on the cadastral maps is today available to us only in graphic form. That data is easy to interpret. The accuracy of the data is limited by the surveying method, mapping technique, background quality and so on, and is habitually lower than the accuracy of the original survey.
If we reconstruct the data from the graphic background by digitalization, we normally obtain good topological and thematic information. However, metrical information is subject to loss of accuracy. Still, digitalization imposes itself as one of the ways to obtain large quantities of numerical data at a lower price relatively quickly.
In order to put together the spatial geometry data from different sources, in our case the co-ordinates of control points, correspondent points and digitalized co-ordinates, we will use one of the well known transformations. Almost all software packages that enable computer drawing offer transformations such as Helmert’s, affine and so on. Calculation of surplus correspondent points for determination of transformation parameters makes an assessment of accuracy possible, and by linking correspondent and digitalized points we achieve homogenization of the cadastral map data.
As far as the arrangement of the chosen correspondent points goes, the area of the cadastral unit was divided into zones for which the transformation parameters were determined separately. In order to avoid discrepancies on the boundaries of the area, neighbouring areas points were also included in the calculation of parameters (Figure 5).
Figure 5 Transformations areas
Controls, comparisons and analyses
Result controls can be generally divided into:
These controls can be carried out visually after the plotting or with help of computers. Computer supported controls have great significance in speeding up the improvement process and bringing down its cost.
During the result analysis, the homogeneity of the cadastral unit maps was assessed, individual cases of errors were examined, as well as the possibilities of the maps updating. The existing new survey data significantly contributed to the reliability of these analyses.
The entire cadastral unit was treated as a single entity, which assured freedom in creating different cadastral maps. During the calculation of parcel areas the consistency of areas was examined. The treatment of the entire cadastral unit must definitely be recommended not only because of problems with the consistency of parcel areas on the edges of maps.
Automatic controls have been carried out of (DGU 1997):
Topological treatment of parcels has enabled a wide control of the original survey, and especially a control of the later maintenance. In that way it was established that in the operate records a surplus of 9 parcels was recorded of 1821 sq. m. in area, which were not on the map. The basic metrical control was the comparison of the parcel areas after the improvement and of their areas in the cadastre.
Results
Through the comparison of parcel areas in the cadastral operate and the areas after the improvement it was established that in 85 parcels or 5% of total number there are differences greater than the adopted border values (Figure 6).
Figure 6 Differences in areas
All the differences greater than 20% were examined one by one. For the same purpose a list of parcels was created and they were presented differentially in space (Figure 1). It must be emphasized that more than half of the parcels are grouped around the edge of the cadastral map and that they are mostly parcels in urban areas. Their arrangement also corresponds to the arrangement of the mutations.
Figure 7 Spatial plot of differences
CONCLUSION
The improvement process has been under way for some time in other countries as well. The process itself can be implemented only if there is a homogenous network of control points and the necessary technological equipment. One must not forget that the most important factor of a successfully implemented process is an experienced geodetic expert familiar with modern technologies. The improvement of cadastral maps is a multi-tier task and in order to obtain higher quality maps from the existing ones, we must analyze them from different points of view and take into consideration all the factors within the framework of the project.
The improvement process does not in itself resolve the co-ordination of the situation in cadastre with the actual situation in nature nor the co-ordination with the land register, which is a special problem of the Croatian cadastre, and the transfer of data into digital form is just a secondary result of the process. Generally speaking, the improvement raises the level of accuracy and homogeneity of the existing data and creates the assumption that each new survey of even the smallest part of a cadastral unit will not disturb that data when it is entered into the digital cadastral map. As far as the situation of the cadastre in Croatia is concerned, it can be said that additional surveys of the cadastral units will have to be carried out as part of the improvement process, which will greatly contribute to the success of the process. These surveys need not be large scale ones, which will depend on the state of the cadastral operate in each cadastral unit.
In order for the improvement process to be carried out as efficiently as possible, the governing bodies must issue directions and guidelines for work and recommend tools, and where necessary provide software applications as a support for the cadastral maps improvement process.
Acknowledgement. The author would like to thank the State Geodetic Administration that financed the project “Cadastral map improvement in the process of transition to boundary cadastre”, and everyone else who in any way contributed to the creation of the project.
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