# Compression of Multibeam Echosounders Bathymetry and Water Column Data
## 7 Conclusions and Future Work  
In this paper, we have proposed a data compressor pre-processing stage for bathymetry and water column data acquired by Multibeam Echosounders (MBES), adapted to the KMALL format from Kongsberg Maritime, but easily adaptable to other vendors as well. From all the systems that acquire information from the seafloor and water column, MBES are probably the most demanding in terms of processing power and data acquisition rates, being able to acquire several gigabytes of information per hour. The use of efficient data compression algorithms allows to drastically reduce the storage needs and to speed up and lighten data transfer in remote operations that require relatively slow and expensive narrowband satellite transmission, hence reducing the associated costs. We have tested the bespoke stage with the FAPEC entropy coder on three datasets formed by different echosounder models: Kongsberg Maritime EM304, EM712 and EM2040. When comparing FAPEC with the general-purpose compressors _gzip_, _Zstd_ and _bzip2_, we obtain, on average, the best compression ratios on water column data. Additionally, FAPEC always obtains remarkably better ratios in bathymetry data. This is even more notorious in combined bathymetry and water column files such as the ones in the third dataset (D3-304). Regarding compression time, _Zstd_ is faster when compressing water column data, although the compression ratios are clearly worse. For bathymetry data, FAPEC is even faster than _Zstd_. Lossy compression has also been treated, but only for water column datagrams. The level of losses is selectable by the user, with each level reducing the number of gray shades, hence the contrast. We have shown that even at high levels with only 11 shades, important features such as gas flares are still detectable. This option provides a powerful tool for water column exploratory surveys. FAPEC is a valuable asset, particularly for remote technical assistance and monitoring of crewed vessels and autonomous vehicles. During the study presented in this paper, we have identified some possible improvements that could increase the compression ratios, yet some of them would also increase the compression time. For instance, the algorithm for SKM could be improved in the sense that the prediction errors for common fields are stored in a buffer and sent together to the entropy coder. This approach is the one followed for MRZ datagrams, which are more relevant in terms of percentage. On the other hand, there are several datagram types that are not pre-processed. For these, instead of sending them directly to the entropy coder, a generic compression strategy could be applied, for instance, the Lempel-Ziv-Welch (LZW), which is already implemented as a FAPEC stage. Conceptualization, J.P., A.M. and D.A. ; methodology, J.P.; software, A.M. and J.P.; validation, J.R., D.A. and J.P.; formal analysis, A.M.; investigation, A.M. and J.P.; resources, J.P.; data curation, J.P.; writing--original draft preparation, A.M.; writing--review and editing, M.V., F.d.C., D.A., J.P. and G.M. ; visualization, A.M., D.A., M.V. and F.d.C. ; supervision, J.P. and J.R.; project administration, J.P. and D.A. ; funding acquisition, J.P. and D.A. All authors have read and agreed to the published version of the manuscript. This work was (partially) funded by the ERDF (a way of making Europe) by the European Union through grant RTI2018-095076-B-C21, the Institute of Cosmos Sciences University of Barcelona (ICCUB, Unidad de Excelencia Maria de Maeztu) through grant CEX2019-000918-M, the Spanish State Research Agency (PID2020-114322 RBI00), the European Union's Horizon 2020 research and innovation programme ([PERSON] grant 658358), the Catalan Government Excellence Research Groups Grant to GRC Geociencies Marines (ref. 2017 SGR 315), the Spanish Ministry of Science and Innovation project RODIN (PID2019-105717 RB-C22/AEI/10.13039/501100011033), and Fellowship FI 2019 by the Secretary for University and Research of the Generalitat de Catalunya and the European Social Fund.