Gridding digitized bathymetry in the Straits of Malacca

Abstract

Maritime charts in the form of bathymetry from the Straits of Malacca were digitised to form the data set that was operated on by the Kriging algorithm to provide gridded digital data sets. There were two forms of algorithms tested, the Kriging and the Minimum Curvature, of these two Kriging was found to be the most suitable. Along with these two algorithms different grid sizes were also experimented with, these were 1 minute and 5 minutes (I' & 5'). The I' grid size and the Kriging were found to be the best combination for the gridding of the digitised bathymetry. The bathymetry was digitised 'on screen' using Golden Software's Didger3. The maps were of mixed scales 1:200000 and 1:300000, scanned at a resolution of 150 dpi, this was done in order to decrease the size of the digital file, namely the TIF format. Affine polynomial calibrations were performed on all charts and were within acceptable limits. A total of 18357 points were digitised induding soundings and coastlines, the minimum height being -358.86 m the maximum being 0 m. All soundings were transformed to Mean Sea Level (MSL) using the tidal information published by the Department of Surveying and Mapping Malaysia (JUPEM) Tide Tables for the year 1998. A comparison of the gridded (Kriging) digitised points is accomplished through comparing an independent set of digitised points (approximately 10%) with the gridded digitised data. Also a comparison with ETOP05 is made, the results are as follows: a mean of the differences of -0.332 m and the standard deviation of the mean of ±7.784m, for the digitised data set and a mean of the differences of -0.281 m and the standard deviation of the mean of ±15.812m for ETOP05. This tends to show that ETOPOS is not as representative of the bathymetry of the Straits of Malacca and that the gridding process followed by the authors is suitable enough for the data to be used in other applications, such as tidal studies or current analysis for offshore and coastal engineering processes.