From 1969 to 1975 he worked as Research and Teaching Associate, and until 1981 as Chief Engineer at the Institute of Photogrammetry and Cartography, Technical University Munich.
Gruen served as the head of the Department of Geodetic Sciences 1996–97 and as the dean of faculty "Rural Engineering and Surveying" of ETH Zurich (1996–98).
He has organized and co-organized/co-chaired over 35 international conferences and has served as a consultant to various government agencies, system manufacturers, and engineering firms in Germany, Japan, Korea, Switzerland, US, and other countries.
He is corresponding member of the German Geodetic Commission (Bavarian Academy of Sciences and Humanities, Munich), Ordinary Member of the German Archaeological Institute (DAI), holds honorary professorships of the Wuhan University, Wuhan, China and Yunnan Normal University, Kunming, China, is honorary member of the Japan Society of Photogrammetry and Remote Sensing (JSPRS) and International Society of Photogrammetry and Remote Sensing (ISPRS).
Gruen's group's major scientific achievements include (in chronological order): His group's most recent major projects in 3D modeling of Cultural Heritage, based on photogrammetry, laser scanning and structured light, include: Nasca Lines/Geoglyphs, Adobe pyramids of Tucume, Machu Picchu, Pinchango Alto, petroglyphs of Chichictara (all Peru), Mount Everest, Ayers Rock (Australia), reconstruction of the two Great Buddhas of Bamiyan, pre-Columbian site of Xochicalco (Mexico), Maya site of Copan (Honduras), Bayon/Angkor Wat (Cambodia), Drapham Dzong (Bhutan), Weary Herakles (Antalya, Turkey), Khmer Head (Rietberg Museum), Zurich 1800 city model relief, Alfred Escher memorial, St. Gallen Globe (all Zurich), Pfyffer Relief (Lucerne).
Gruen's main recent research interests include: automated object reconstruction with digital photogrammetric techniques, building and line feature extraction, 3D city modeling, image matching for DTM generation and object extraction, Three-Line Linear Array sensor modeling, industrial quality control using vision techniques, motion capture, body and face reconstruction for animation, imaging techniques for generation and control of VRs/VEs, especially for cultural heritage recording and modeling, 3D processing of very high resolution satellite images, photogrammetric UAV research.