This thesis was completed with the contribution of many people to whom I want to express my sincere gratitude.

I thank Prof. Franz Leberl for the fruitful discussions concerning different aspects of the research as well as for the guidance through the administrative labyrinths toward the doctoral degree. My very special thanks go to my supervisor Dr. Klaus Tempfli for his criticism, concern and valuable scientific remarks regarding various issues in this research.

I thank Dr. Michael Gruber for the constructive conversations at the first stages of my PhD project when he was appointed as my second supervisor. I am indebted to Dr. Ben Gorte who helped me with the set up and maintenance of the prototype system, and provided a number of programs used in the research. I owe thanks to Mr. Jan Hendrikse who gave me critical remarks on the formal definition of the model and made available some operators in ILWIS needed for the project. I am grateful to Prof. Peter van Oosteroom, the most critical reader of Chapter 6.

I had stimulating discussions and meetings with many ITC staff members: Alan Brown, Ard Blenke, Barrend Kobben, Bert Raidt, Dick Groot, Ed Sides, Elisabeth Kosters, Janice Collins, John Horn, Karl Grabmaier, Meno- Jan Kraak, Rob Lemmens, Rolf de By, Sockon Phem, Teo Bouloukous, Van Bakx, Wim Bakker, Wolfgang Kainz; PhD candidates: Ale Raza, Maria Bastidas, Morakot Pilouk, Patrick Ogaio, Rudiger Genz, Saadi Mesgari, Tao Cneg, Wanning Peng; MSc students: George Sithole, Jonas Paitsil, Juranuwat Sakseau, Lameck Mweva. Dear ITC people, thank you all for being so kind to me! I express my deep gratitude to ITC for giving me the opportunity to work on this project.

My Bulgarian colleagues were always with me. Bano Banov, Benjamin Cohen, Borislav Marinov, Georgi Milev, Ivan Katzarsky, Mosko Aladjem, Penio Penev, Radka Yaneva, Temenuzhka Bandrova, Yordanka Zaharieva, thank you for your support. I am in dept to the Faculty of Geodesy at the University of Architecture, Civil, Engineering and Geodesy (Sofia, Bulgaria) and the Union of Bulgarian Surveyors and Land Managers for their interest in my work and the frequent contacts. Many thanks go also to my colleagues at the companies participating in the investigation of user requirements.

Although I did not spent much time in Austria, I received valuable, "just on time" help by my Austrian colleagues Marcus Maresh, Michael Kofler, Michaela Tretter, Renate Hoevel. I had enjoyable discussions with many people during conferences and via e-mails. I would like to mention the names of Henri Dekker, Jan van Eck, Lars Bodum, Peter Lindstrom and Weiso Chen.

Many other people not particularly involved in the research gave me encouragement and support: Atanas Kisyov, Boyan. Nedkov, Ganka Peycheva, Elisabeth Muller, Fernando Sifuentes, Jatin Pandya, Konstantin Goranov, Lyubomir Ninov, Milko Petrov, Sharleth, Petia Tzaneva, Tatiana Ouzunova, Vania Orosova, Maartje and Frank, Didi, July and Galia, Mony, Tony and Rony. Thank you all!