Inter-change: a software support for interpreting land cover changes. (2003)

The IMAGEC (2) interpretation and mapping of land cover changes for Europe. As part of Hungary's preparation for the project, a computer-assisted photointerpretation tool has been developed under ESRI's ArcView software to interpret land cover changes. The system provides interactive facilities for the photointerpreter to simultaneously revise the CLC90 database and interpret land cover changes between 1990 and 2000. Satellite images taken in the 90's and 2000 are used to control both processes. Powerful tools are built in to fulfil specifications of the project and avoid database errors. No sliver polygons are produced during updating. 1 THE CORINE LAND COVER DATABASE AND ITS UPDATING IN EUROPE The idea of producing a uniform pan-European land cover database dates back to the early 80’s. It has been recognised that land cover is a basic information for the management of the environment and natural resources. Land cover mapping has become an integral part of the CORINE (Co-ordination of Information on the Environment) Programme, started in 1985 by the European Commission DG Environment with the main aim of compiling consistent and compatible information on the environment for Member States of the European Union. Information obtained by earth observation satellites gives the base data for the production of a land cover inventory. In 1994, the European Environment Agency (EEA) in Copenhagen undertook the maintenance and use of the CORINE Land Cover (CLC) database. The CORINE Land Cover project has been implemented in most EU countries, as well as in the 13 Phare partner countries in Central and Eastern Europe and in two North-African countries (Morocco and Tunisia). The last countries to finish their CLC inventory were Bosnia-Herzegovina and Macedonia using satellite images taken in 1996-98. In each country local team(s) implemented the project under a central supervision. 28 European countries have been involved in the CORINE Land Cover mapping project resulting in a total area of 4.4 Mkm being mapped (Buttner et al. 1998a). The basic aim of the CORINE Land Cover project is to provide an inventory of the Earth surface features for managing the environment. CLC is consistent and comparable across the continent. Computer-assisted visual interpretation of satellite images has been chosen as preferred mapping technology. The choice of scale (1:100,000), minimum mapping unit (25 ha) and minimum width of linear elements (100 metres) represents a trade-off between the costs of production and the details of land cover information derived (European Commission 1993). Today, CLC is recognised by decision-makers as an essential reference data set for spatial and territorial analyses on different territorial levels. Within the European Commission Services such as DG-Regional policy, DG-Environment and DGAgriculture, as well as in EEA and its European Topic Centres (ETCs) there is a growing need of integrating spatial and territorial analysis into the assessment (EEA-JRC-ETC/TE 2002). A new project called IMAGEC area change should be minimum 5 ha Faster turnaround time 10 years 3 years Lower production costs 6 /km 3 /km Improved documentation incomplete metadata standard metadata Easier access to the data unclear dissemination policy agreed dissemination policy from the start 2 BASIC CONSIDERATIONS 2.1 Characterisation of CLC1990 data The CLC1990 database is produced with a traditional photointerpretation technique. A transparent overlay was fixed on top of the satellite image hardcopy and the photointerpreter drew polygons on it. The polygons were coded according to the CLC nomenclature (European Commission 1993). Although the display and interpretation of satellite imagery using an image processing system was recommended to compensate the drawbacks of using hardcopy images, these facilities were not always available. After completing the interpretation and supervision, the polygons were digitised, a topology was created and the CLC codes were entered. This procedure often resulted in several different types of error: − geometrical errors caused by an imprecise hardcopy image size, distortions of the hardcopy image, improper alignment of overlay and image, errors of digitisation; − thematic errors made by the photointerpreter because of the limited interpretability of hard copy image and / or because of misuse of the nomenclature. Thematic mistakes could have been introduced also during the database-coding phase. 2.2 Approaches of updating For the reasons above, national CLC1990 databases frequently include several mistakes and in several cases, the geometric and thematic accuracy is below the specification (Table 1). In many countries the 25 ha minimum mapping unit was not respected, which makes data hardly comparable among countries. Therefore the updating strategy should also include revision and correction of CLC1990, otherwise some of the differences between CLC1990 and the updated CLC2000 would be not real changes, but due to the mistakes of CLC1990. Figure 1. Approaches of updating and derivation of CLC changes There are different approaches of updating CLC data. According to approach a) on Figure 1, first CLC1990 is revised and corrected using IMAGE1990 data, and then the photointerpreter modifies the database according to the status seen on IMAGE2000 imagery, which yields the CLC2000 database. CLC-Changes will be computed accordingly: CLC-Changes = CLC2000 ( ) CLC1990 Where ( ) means the following operation: CLC1990 and CLC2000 is intersected; all polygons will have two CLC codes: codeold and codenew; then polygons with codeold = codenew are deleted. Experience has shown that the revision and the updating can not be fully sequentially organised. If during the updating process a new mistake is discovered in CLC1990 (e.g. missing LC polygon), it has to be drawn into the CLC2000 database as well. Because the lines in the two databases will differ in some extent, slivers a) Updating Changes b) Changes Updating revision of