**2. Materials and Methods**

While preparing a master image, such as the depth area taken from a bENC, the problem of data processing time is not critical. For recording and processing images intended for dynamic comparison with the master image, data reduction is the key problem. A dynamically registered image should be processed for a DTM generated in close to real time. DTM construction is an important step in generating the depth area of the acquired image to be compared with the master image of the bottom. A DTM of the bottom can be generated based on data obtained from GNSS measurements connected to a single-beam echosounder or a MBES, which is currently the most popular technology. The acquired data is used to create a DTM of the bottom of the river, lake, channel, or harbor area. The DTM not only models bottom area processes but also detects objects under the water surface and eventually helps with their visual inspection. Handling such a large volume of data is time-consuming and labor-intensive. Therefore, we propose the use of the OptD method [35–37] to reduce the data set. Reducing the number of observations allows the 3D model and depth area to be generated much faster. Moreover, the OptD method allows the data set to be divided into points representing the bottom of the river, lake, or channel and points representing objects that are not the bottom (items under the water surface).
