Wojciech Maleika

Moving average optimization in digital terrain model generation based on test multibeam echosounder data

The paper presents a new method of digital terrain model (DTM) estimation based on modified moving average interpolation. There are many methods that can be employed in DTM creation, such as kriging, inverse distance weighting, nearest neighbour and moving average. The moving average method is not as precise as the others; hence, it is not commonly comprised in scientific work. Considering the high accuracy, the relatively low time costs, and the huge amount of measurement data collected by multibeam echosounder, however, the moving average method is definitely one of the most promising approaches. In this study, several variants of this method are analysed. An optimization of the moving average method is proposed based on a new module of selecting neighbouring points during the interpolation process—the “growing radius” approach. Tests experiments performed on various multibeam echosounder datasets demonstrate the high potential of this modified moving average method for improved DTM generation.

The influence of track configuration and multibeam echosounder parameters on the accuracy of seabed DTMs obtained in shallow water

This article presents research that aims to assess the accuracy of seabed bathymetry models on the basis of measurements carried out using a multibeam echosounder. It discusses the problem of accuracy assessment which occurs at particular stages of the creation of digital terrain models (DTMs), and analyzes the influence of both the individual multibeam echosounder parameters and the procedures adopted during the survey on the modeling process’s accuracy. Factors that may affect accuracy include the device’s measurement accuracy, the boat speed, the track configuration, the beam width and the selected interpolation method. The article does not deal with errors due to navigation and vessel motion, these being taken to be of the highest possible accuracy. For the research we developed a virtual survey simulator. Recommendations for how to optimize the measurement process while ensuring the created models are of a high quality are presented later in the paper.

The influence of the grid resolution on the accuracy of the digital terrain model used in seabed modeling

Modern digital terrain models (DTM) are widely used in the exploration of water areas. The models are often based on bathymetric data from a multibeam echosounder. DTM creators should properly select model parameters, firstly the grid resolution. High grid resolution enables creating very accurate models, however, they require high computing power and the data gathered in the grid occupy much more memory space. Low grid resolution means significantly less data describing the model, but, naturally, its accuracy will also be lower. The author proposes a method permitting to examine the accuracies of DTMs that depend on adopted grid resolution. Further the article will present search for an accurate grid resolution for three selected real surfaces of the seabed. The obtained results were visualized and interpreted. The author also proposes tips to be used while creating DTMs. Conclusions from the research may be helpful in digital terrain modeling of the seabed.

Effect of density of measurement points collected from a multibeam echosounder on the accuracy of a digital terrain model

Digital terrain models (DTMs), finding a wide range of applications in the exploration of water areas, are mainly created on the basis of bathymetric data from a multibeam echosounder. The estimation of DTM accuracy dependent on the choice of the survey parameters is difficult due to the lack of reference surface. These authors have developed the methodology of simulation called virtual survey, which enables examining how various parameters of the echosounder, survey and DTM construction algorithms affect the errors of the created models. They are aimed at precise estimation of the model accuracy and the optimization of depth measurement work. The article includes the results of the examination of the effect of parameters determining the density of measurement points on the accuracy of the obtained GRID model. It has been proved that a significant reduction of recorded data density leads to only a slight increase in the modeling error, which makes the bathymetric survey much more cost-effective.

Multibeam echosounder simulator applying noise generator for the purpose of sea bottom visualisation

Hydroacoustic data form almost the only basis for seabed imaging [1]. The accuracy of scanning devices is therefore crucial for the reliability of the images. In the paper, the precision of the Simrad EM3000 multibeam echosounder was examined by means of statistical methods on real data. The maximum and mean errors were estimated, the correlation of error value and angle between the beam and the vertical line could be observed as well. The results of the experiments will help to increase the accuracy of terrain modeling of the sea bottom what will result in more realistic visualizations. The results of those experiments were implemented in the multibeam echosounder simulator.

Near-Lossless PCA-Based Compression of Seabed Surface with Prediction

The paper presents a compression method based on Principal Component Analysis applied to reduce the volume of data in seabed Digital Terrain Model. Such data have to be processed in a manner very different from typical digital images because of practical aspects of analysed problem. Hence, the developed algorithm features a variable compression ratio and a possibility to control a maximal reconstruction error. The main objective is to build an orthogonal base and find a number of PCA coefficients representing analysed surface with an acceptable reconstruction accuracy. We present two variants of processing: an iterative compression approach and an approach predicting a number of coefficients before compression starts. It yields much lower computational demand and is faster. The later algorithm employs several statistical measures of an input surface describing its complexity at the prediction stage. Employed, simple classifier based on Classification and Regression Tree do not introduce high additional time overhead. Performed experiments on real data showed high compression ratios, better than for typical DCT-based methods. The possible application of developed method is modern data management system employed in maritime industry.

Evaluation of KLT method for controlled lossy compression of high-resolution seabed’s DTM

The paper presents Karhunen-Loeve Transform-based compression method, which can be applied to reduce the volume of data describing seabed topography. Developed algorithm allows for variable compression ratio and a possibility to limit the maximal reconstruction error. These properties have been introduced to the method because of the practical aspects of analysed problem. During the development, many experiments have been performed. The results presented in the paper show that the application of KLT in a conjunction with a set of so called common eigensurfaces gives high compression ratio for data representing seabed. In comparison to other well-known methods (Discrete Cosine Transform or Wavelet Transform based) the compression ratio is, in many cases, much higher. A novel solution for the problem of blank nodes occurrence and of storing irregular surface shapes is presented. The final compression algorithm presented here can become an important element of modern storage and management systems (handling vast data) that are used nowadays in maritime technology. Thanks to the open architecture, developed algorithm can be extended with other solutions aimed at seabed topographic measurement and visualisation.

Visualisation of Multibeam Echosounder Measurement Data

During a sea survey session performed using a multibeam echosounder millions of measurement points are generated. Sea surveys should be carried out in such a way, that the maximum accuracy of created seabed models (DTM) is achieved and the standards specified by the IHO S-44 guidelines are met. One of the requirements is so called full sea floor search, which means the ability of a system to detect all the cubic features at least 1 m in size. Spatial distribution of measurement points is irregular and the distances between closest data points are varying, depending on many factors (on survey parameters, depth or distance between the beam and the vessel). Due to those reasons, it is difficult for the users of hydrographic systems to evaluate the degree of coverage of seabed by measurement points, and therefore to confirm fulfilment of the normative requirements. As a solution we propose visualisation methods for measurement data collected in sea surveys. Specific features of such a visualisation are explained and a method for creating the images is presented, along with some exemplary results.

A Multiresolution Grid Structure Applied to Seafloor Shape Modeling

This paper proposes a method of creating a multiresolution depth grid containing bathymetric data describing a stretch of sea floor. The included literature review presents current solutions in the area of the creation of digital terrain models (DTMs) focusing on methods employing regular grids, with a discussion on the strong and weak points of such an approach. As a basis for the investigations, some important recommendations from the International Hydrographic Organization are provided and are related to the accuracy of created models. The authors propose a novel method of storing DTM data, involving multiresolution depth grids. The paper presents the characteristics of this method, numerical algorithms of a conversion between a regular grid and the multiresolution one, and experiments on typical seafloor surfaces. The results are discussed, focusing on the data reduction rate and the variable resolution of the grid structure. The proposed method can be applied in Geographical Information Systems, especially for the purposes of solving sea survey problems.

Lossless Compression Method for Digital Terrain Model of Seabed Shape

Dealing with Digital Terrain Models requires storing and processing of huge amounts of data, obtained from hydrographic measurements. Currently no dedicated methods for DTM data compression exist. In the paper a lossless compression method is proposed, tailored specifically for DTM data. The method involves discarding redundant data, performing differential coding, Variable Length Value coding, and finally compression using LZ77 or PPM algorithm. We present the results of experiments performed on real-world hydrographic data, which prove the validity of the proposed approach.