The importance of the number of points, transect location and interpolation techniques in the analysis of bathymetric measurements

Abstract

Determining the bathymetric properties of a river, lake or reservoir is important for many reasons because it identifies location of sections with sedimentation or erosion which indicate the key processes and biophysical habitat status. This study used observational data of a 5 km section of the Danube River (transformed into a reservoir after 1972) to determine the best GIS interpolation method and the optimal number of sampling points to get an accurate description of the riverbed. Nine interpolation methods were used to generate bathymetric maps of the Danube course, between km 955 and km 950, in the area of Iron Gates reservoir, using a variable number of points (n = 1943; n = 4328; n = 17139), obtained by single beam echo sounder. Six methods of deterministic interpolation have been used: Inverse Distance Weighting (IDW) – criteria automatically determined (CAD) and manually determined (MD); Local Polynomial Interpolation (LPI) – criteria automatically determined (CAD) and manually determined (MD); Radial Basis Function (RBF) – fully regularized spline CRS; Radial Basis Function RBF – spline with tension SWT, and three geo-statistics methods: Ordinary Kriging – OK, Simple Kriging – SK, Universal Kriging – UK. Cross-validation and bathymetric Digital Elevation Models (DEM s) analysis were used to evaluate the accuracy of the estimated data and maps. The two interpolation methods, Inverse Distance Weighting (IDW-MD) and Simple Kriging (SK), provide satisfactory results, even in the case of a small number of points (n = 1943). If the number of measured points grows and a section grid (perpendicular/longitudinal) is developed, the DEM s bathymetric accuracy improves significantly, especially in the case of complex morphology, even if the positive and negative forms of the bed lead to sudden changes in the measured values. The obtained results revealed differences between the interpolation methods used, particularly in the case of a small number of points measured per perpendicular trajectories, but by the addition of points on longitudinal paths, a significant improvement of the bathymetric maps is obtained.