Can old analogue sidescan sonar data still be useful? An example of a sonograph mosaic geo-coded by the DECCA navigation system

In the Baltic and the North Sea area the collection of sidescan sonar data began in the early 1970s. Although positioning systems at that time were much less accurate than today, such analogue data archives are a valuable source of comparison for estimating spatial and temporal changes in sediment patterns. This paper describes how geographic information systems (GIS) and remote sensing methods are being applied for more accurate positioning and geometric correction of analogue sidescan sonar profiles geo-referenced by the DECCA navigation system (DNS). A 20 km2 area in the North Sea, where even after 25 years a complicated sediment pattern has only changed slightly, was selected for the comparison. Data from two sidescan sonar surveys, one from 1977 and the other from 2002, were available. The 2002 data, acquired with a positioning accuracy of estimated 25 m, were mosaicked using modern digital processing techniques. This dataset was used to estimate positioning errors in the analogue sidescan sonar profiles of 1977. After the application of a geo-referencing method of ‘rubber sheeting’ based on an irregular network of ground control points to generate a mosaic of the analogue profiles, positioning errors of 211 m in the E–W and 98 m in the N–S directions were obtained. The positioning error of the DNS calculated from equations provided by the DECCAN Navigator Company for this area, was as large as 200 m for the ‘purple’ station transmission, 197 m for the ‘green’, and 66 m for the ‘red’ one. Consequently, we conclude that the observed positioning errors were mainly caused by systematic deviations of the DNS rather than simplifications used in our methodology. Since such large errors can be corrected locally with some confidence, analogue sidescan profiles geo-referenced by DNS can be used as a significant source of information after digital image processing at appropriate map scales.