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The main principle of LIDAR is to measure the accurate time of the laser pulses sent from the system to the target surface. In the operation, laser pulses gradually scan the water surface and in combination with aircraft speed they should perform almost simultaneous soundings of each strip. Vectors sent from aircraft to the Sea are linked to the position of the aircraft. Coordinates of the points - X, Y, Z, are calculated at the time of each measurement. LIDAR crosses the surface of the sea while other impulses pass through the water column and, depending on the depth of the water, reflect from the seabed. Optical receiver on board of the aircraft detects pulse reflections from the seabed and sea surface. On the tidal water basins lidar strips must be adjusted by the changes in sea level. The operation should be reduced to a few hours during low water level. Typically, a surface of 20 to 30 km2 should be covered in an hour. The Baltic Sea is an inland sea, and the surveyed area is located in its South - western part, where meteorological and hydrological conditions affect the sea level changes in a short period of time. A lidar measurement of sea surface, that was done within 2 days, in the coastal zone of the Baltic Sea and the sea level measured 6 times a day at 8, 12, 16, 20, 00, 04 by a water gauge located in the port of Dziwnów (Poland) were used for this study. On the basis of the lidar data, strips were compared with each other. Calculation of time measurement was made for each single line separately. Profiles showing the variability of sea level for each neighboring and overlapping strips were generated. Differences were calculated changes in sea level were identified and on such basis, an adjustment was possible to perform. Microstation software and terrasolid application were used during the research. The latter allowed automatically and manual classification of the point cloud. A sea surface class was distinguished that way. Point cloud was adjusted to flight lines in terms of time and then compared.
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