Athens - View of Plaka area and Likavitos Hill from the Acropolis
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Millions of land transactions occur around the world every day - subdivisions are platted, lot lines are adjusted, parcels are combined, and utility easements are created. This daunting number of cadastral changes necessitates a large workload of time-consuming ground surveys to create legal records of land-boundary changes. It also creates a data management nightmare for many municipalities because, even though they may have accurate paper records of cadastral changes, their maps may be out of date or infrequently updated. Plus, cities, counties, provinces, states, regions, territories, and national agencies may maintain data in different scales or datums - making it even more difficult to reconcile maps if, for example, a city annexes a parcel under county jurisdiction.
To tackle land-boundary record and mapping challenges, efforts worldwide have been afoot for years to establish cadastral record and mapping standards between local and regional authorities. Greece, however, has undertaken an even more ambitious project - to establish an integrated, cadastral system for the entire country based on geographic information system (GIS) technology.
Cadastral Mapping with GIS
A GIS links spatial data to alphanumeric information to produce a geographically referenced database. Such a database is organized into layers or themes (or objects), each one belonging to a specific topological type and relating to a specific type of data, such as cadastral data.
Data kept in a GIS may include, among others, cadastral parcels, land use, topographic mapping and town plans. The nature of the data to be entered into the system, its sources, and its forms must all be defined: alphanumeric, graphic inputs such as maps or technical drawings or external files from existing systems. Logical layers may be defined, including the inter-relationships among the various layers.
GIS data is captured from such sources as:
- digitized maps
- manually entered alphanumeric data on various types of facilities
- scanned photographs and maps.
The heroic efforts involved in creating a national cadastre are not new to Greece. Hopefully, the Hellenic Cadastre will be around for as long as the Parthenon in Athens.
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It may be necessary to check the quality of the data and verify it in the field, especially for infrastructure layers.
In the process of creating the system, known as the Hellenic Cadastre Project (HCP), partners have developed an innovative method of using GIS and photogrammetry to quickly produce and update local/municipal digital infrastructure and land-boundary records and maps.
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An Epic Undertaking
Greece began the process of establishing a nationwide cadastral system in 1993. In 1995, the Ministry of the Environment, Public Works, and Planning founded the Ktimatologio S.A. firm to create and promote the HCP. Work on the project was to be supervised by HEMCO in cooperation with the Hellenic Cadastre Consult, a private consortium. The initial goal was to implement a system of 54 land information centres across the country, one in each county, linked to municipal cadastral offices, through a wide-area network for local-scale services. Each land information centre would store and maintain data using GIS and an RDBMS according to technical specifications created by HEMCO in 1995. For maintaining land records, each municipality's cadastral office would also have its own local RDBMS and GIS associated with its respective regional land information centre data. Thus, as changes were made to the local cadastre, revised data could be uploaded to the regional RDBMS at periodic intervals and checked for conformance with HEMCO specifications.
Figure 1 By digitising and over-laying older land-boundary maps on newly acquired aerial imagery, the photogrammetry lab at the National Technical University of Athens was able to update the cadastral base layer for the Ag Paraskevi municipality within the Athens metro area.
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Data tragedy. By 1997, HEMCO had contracted and supervised pilot projects for implementing the HCP in two stages - one encompassing 66 municipalities (many within the Athens metro area) and the other comprising 54. As historical data, general plans, zoning maps and diagrams, nonspatial alphanumeric data, and other information were loaded into the database and GIS for the pilot municipalities, the difficulty of integrating existing cadastral data (established by various state services under different regulations) became clear. In addition, HEMCO and Ktimatologio S.A. realised that much of the cadastral record was incomplete or outdated.
Greece has about 8,000 square kilometres of urban areas, but modern cadastral records and maps (required by a 1983 planning law) existed for only about 450 square kilometres of that area. For another 850 square kilometres of urban area, town-planning diagrams were available, but at different scales and compiled in several datums in accordance with a variety of regulations. Additionally, data for the greater Athens area consisted of photogrammetrically adjusted diagrams at a scale of 1:1,000, but compiled in 1974-1975.
Because cadastral data were disparate, incomplete, and very possibly outdated, HEMCO decided that for the pilot project it would be more effective to build a new cadastral base, rather than try to create the base layer from existing maps and data. They knew, however, that it was infeasible to create the new cadastral basemaps by totally relying on traditional ground-based surveys. The process would simply take too much money, too many people, and decades to complete.
FigureS 2a and 2b In creating the new cadastral layers, lab staff edited the topology of the 1974-1975 data (2a) by adding and erasing boundaries and buildings. The end result (2b) was a new topology having three coverages for blocks, buildings, and properties.
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Adding Photogrammetry
As a creative solution, in 1999, the Laboratory of Photogrammetry in the Department of Topography at the National Technical University of Athens suggested using aerial imagery and photogrammetric techniques to more efficiently create base layers for cadastral input. Not only would this method make it easier to create a seamless cadastre with single datum and scale, but it would provide more up-to-date land information. Their idea was that a photogrammetric approach would accelerate the process of filling in information islands in the cadastral data and provide more timely information about land-use and land-boundary changes.
Photogrammetric Odyssey. Though aerial photography is commonly used for gathering detailed land-use information and valuable for detecting land-cover changes and trends, rarely has it been used for survey-quality land-boundary delineation, especially in urban areas. To test whether photogrammetric land-boundary demarcation was feasible in urban areas, the photogrammetry laboratory developed and tested a technique for creating cadastral records based on aerial imagery for the municipalities within the greater Athens metro area that were part of the first pilot project. (HEMCO contracted other firms and agencies to follow a similar method for creating the cadastral maps for the other municipalities taking part in the pilot project.)
