## 5 Conclusions  
The study conducted on the development of spatial digital twin of rivers and inland areas showed that with the approach used, the achievable accuracy of integration of ALS, ALB, MLS, MSLS, and MBES point clouds is at the level of 0.1 m.  
The proposed method, thanks to the temporal integration of measurements through a time server and spatial integration through a modified Q-ST algorithm, made it possible to increase the accuracy of developing a digital twin of inland rivers by more than 20% compared to mapping methods used without sensor integration. In addition, it has been shown that aerial bathymetric scanning is comparable in accuracy to classical measurement methods using echosounders and is a valuable source of elevation data acquisition. The transformation used in this research is an optimised approach in terms of cloud point selection to a small number of points, where the advantages of the Q-ST transformation over the traditional approach of transformation by similarity (Helmert) become apparent. Various platforms and measurement systems were used to create a geometric digital twin of rivers and inland areas: aerial, surface, terrestrial, and underwater. At the same time, a new approach was used to determine the offsets of measuring devices on hydrographic boats using integrated Total Station-TLS measurements and a modified Q-ST transformation. Time synchronisation of the measurements through a time server made it possible to reduce the uncertainty of the determined coordinates of points obtained in the fusion of MBES and MSLS data. The robustness criterion used led to the creation of a data fusion algorithm with improved accuracy. According to the authors, this is a step toward increasing the accuracy of autonomous navigation on rivers and inland areas. It can also be an important step towards the construction and development of Smart Digital Reality, by means of which the combined digital world with the real world will create a single, frequently updated, data-enriched, and machine-learning-model-enhanced reality of the future. This work was performed under the project \"Research and development work on the creation of a complete, multimodal mapping system for the needs of inland and sea waterways and exploitation areas\". The results will enable further research into the development of dynamic digital models of water areas and the synchronisation of the obtained data with public database systems to design technological and systemic solutions for the management and control of retention on surface waters for the development and availability of marine and inland navigation through the use of available hydrotechnical facilities, including junctions and water stages, whavres, and bridges while ensuring the required level of flood protection. Conceptualisation, A.T. and L.J. ; methodology, A.T., G.S. and T.K. ; software, A.T. and G.Z. ; validation, A.T., G.S., I.B.-O. and T.K. ; formal analysis, A.T., G.Z. and G.S; investigation, A.T., G.S. and T.K. ; resources, A.T. and G.S. ; data curation, A.T., G.S. and G.Z. ; writing--original draft preparation, A.T., G.S. and T.K. ; writing--review and editing, A.T., G.S. and T.K. ; visualisation, A.T., G.Z., I.B.-O. and M.L. ; supervision, A.T. and G.S. ; project administration, A.T. and L.J. ; funding acquisition, A.T. and L.J. All authors have read and agreed to the published version of the manuscript. This work was supported by The National Centre for Research and Development, Poland, through the project \"Research and development works on the creation of a complete, multimodal mapping system for the needs of inland and sea waterways and exploitation areas\", project number POIR.01.01.01-00-1372/19. Not applicable. The original contributions presented in the study are included in the article; further inquiries can be directed to the corresponding author. With this article, we pay homage to our deceased [PERSON], who did not manage to become a hydrographer. Author [PERSON] was employed by the company GISPRO SA.