In the past, church belfries, due to their shape and higher layout, often served as reference points in surveying. With the development of satellite based positioning methods and the establishment of the D96/TM coordinate system, they slowly lost their significance. Nevertheless, unlike ground trigonometric points, they have remained largely intact. As such, they enable the determination of transformation parameters between the old and the new coordinate systems. Therefore, we need to determine their positions in the new coordinate system. In this paper, we describe the process of determining the position of church belfries using terrestrial laser scanning (TLS) in the case of both belfries of the Trnovo church. The process involves determining coordinates of the tie points for georeferencing the point clouds, which requires the determination of the geodetic datum by a combination of GNSS measurement and geometric levelling, and the classical geodetic survey of the geodetic network with free-network adjustment and further use of the S-transformation. Based on the georeferenced point clouds, we determined the position of the trigonometric point by sphere parameters adjustment. The data is filtered using the RANSAC so that we only have points that belong to the sphere in the adjustment process. The results were compared with the positions determined based on the classical terrestrial geodetic survey.

Key words: church belfry, national coordinate system, geodetic network, terrestrial laser scanning, georeferencing, point cloud, least-squares adjustment, RANSAC