The use of linear smoothing methods to remove artefacts resulting from the seabed’s DTM lossy compression

Abstract

The article describes research on the use of local linear smoothing methods to remove artefacts resulting from the lossy compression of seabed’s digital terrain model (DTM). In practice, when creating seabed models, DTM based on a regular grid is most often used. When recording larger surfaces, the amount of data collected in the structure can be very large (millions or even hundreds of millions of points) as discussed by Maleika et al. (2011). In such a case, it is possible to significantly reduce the amount of this data by using lossy compression methods. The vast majority of these methods divide the entire surface into small blocks and compress each of them independently. In the process of reconstruction (decompression), clearly visible distortions called artefacts form at the boundaries (edges) of these blocks. In the study, the author described the methods of linear data approximation, enabling the removal of distortions at the boundaries of blocks in the lossy compression/reconstruction process, while maintaining high model accuracy and International Hydrographic Organization (IHO) standards. During the research, methods based on polynomials (from the 1st to 9th degree) and linear approximation, cubic approximation and smoothing spline interpolation were tested. The developed smoothing method was then modified to work locally in places where compression artefacts occur. In the next stage, distortion-dependent smoothing was additionally developed so that the power of the smoothing method would be dependent on the amount of the distortion present. All tests were carried out with the use of three different test surfaces, assessing the obtained results both objectively (calculating the model error at the 95% confidence level) and subjectively (by visually assessing the distortions at the interface of the compression blocks). The results obtained were presented on many figures and tables and interpreted. Finally, the test plots after the developed distortion-dependent local smoothing method were shown in order to assess the obtained effects. The experiments presented in the paper and the results obtained show the true potential of linear smoothing methods in removing distortions resulting from the use of lossy compression methods of seabed’s DTM.