Aspects of terrestrial laser scanning data acquisition and post-processing

Ostatnio coraz więcej uwagi poświęca się dokumentacji architektonicznej obiektów zabytkowych, zwłaszcza gdy są one obiektami kultu religijnego. Dane pozyskane z Naziemnego Skaningu Laserowego (NSL) pozwalają na wykonanie różnorodnych produktów i opracowań architektonicznych. Między innymi model elewacji i rysunek wektorowy powstały na bazie trójwymiarowego modelu są często wykorzystywane przez architektów i konserwatorów zabytków. W większości przypadków służą one jako dokumentacja niezbędna do odtworzenia stanu faktycznego obiektu sprzed renowacji, wykonania rysunków 2D, wizualizacji lub odtworzenia geometrii obiektu. W artykule przedstawiono wybrane aspekty pozyskiwania i przetwarzania danych z naziemnego skaningu laserowego na przykładzie opracowania zabytkowego kościoła drewnianego w Żukowie. Pomiary zostały wykonane skanerem impulsowym ScsanStation2.Terrestrial laser scanning is very useful tool for architectonic structures inventory. Thanks to its high precision and speed of data acquisition, we can collect information about geometry and dimensions in a short period of time. Recently, more and more attention is being paid to create architectonic documentation of cultural heritage structures- especially sacral structures. Data acquired from terrestrial laser scanner can be used in architecture. For example, facade models and vectorial sketches based on the 3D models are used very commonly by conservators for reconstruction or renovation purposes. In most cases, they are used as documentation indispensable to reconstruct the real structure condition from before renovation, create 2D sketches, and to reconstruct geometry of particular elements as sculptures, ceiling etc. Data was acquired by terrestrial, pulsed laser scanner Leica ScanStation2. The examined structure is a wooden, historic church in Żuków. To scan all building parts (both interior and exterior) we used six stations. We obtained the scans resolution adequately: for exterior 5 mm, and interior 7 mm (for some details 3 mm). The basic problem of measurements of such structures is adequate location of stations and tie points in the way that will enable further merging of both parts of the structure. Very often, external conditions make measurements difficult and force using particular methods of scanning and data fitting, what result in accuracy of the final products. In our tests, the RMSE of scan registration was equal to 2 mm. Errors on particular points (targets) have not exceeded the value of 3 mm (the same have not exceeded the accuracy of TLS measurements). The final products of TLS data post-processing are: 3D models of spatial structures, orthoimages, but also vectorial sketches, especially important and useful for architectonic or conservatory purposes. In the paper we described also some basic methods and 3D modelling problems on some chosen examples. Modelling of irregular structures, such as monuments and sculptures, causes quite serious problems. The most common trouble are "shadows" connected with the field of view of the scanner (data inconsistencies, which are visible on the top of high monuments or in places where some elements obscure other details). In such cases, adequate modelling method should be used. For modelling of complex 3D shapes we used some standard methods of solid modelling. We compared modelling using basic solids and meshing function. Our experiments have been conducted with the use of Cyclon and CAD soft ware. In the paper we will present the most important aspects of acquisition and post-processing of the data acquired with the use of terrestrial laser scanner