9 July 2010 15:37
More from Andy about soundings…
The traditional lead line sounding made point measurements. The cartographer positioned them on the chart and interpolated between them by eye to draw depth contours. Cartographer and navigator alike had to assume that the gaps between the soundings were not a significantly different depth from the soundings themselves.
A worried cartographer could possibly order the surveyor to take more soundings but there would always be gaps and assumptions.
Modern sounding techniques are fundamentally different – side scan sonar imaging is effectively continuous – there are no gaps. No rocks will be missed. This can give the navigator a much higher degree of confidence than from point soundings. The soundings drawn on the chart can be positioned at the shallow bits such as isolated rocks.
So on a chart produced from a modern survey, the soundings are worst case, whereas from a traditional survey they are probably average and the spread of depth may be quite uncertain.
(caveat. Where it was particularly important to know there was a safe depth, traditional soundings were bolstered by using a drag line – if nothing caught on the line, which was set at a fixed depth with floats, then the area was entirely deeper than that.)
The source Data Diagram (SDD) on a chart tells us which type of survey was used:
A typical SDD from a BSB NOAA chart showing areas of full bottom coverage (modern survey techniques) and partial coverage (traditional survey techniques)
An SDD from Nuno Navigator, showing the S-57 equivalent (though for a different location). A2 is a full bottom coverage survey and U is ‘unknown but probably not’.
The Source Data Diagram for the chart containing the single line of soundings (above) contains the tracks of the individual ships that contributed soundings to the chart.