A position obtained by sounding is usually approximate. Accuracy of this type of position depends on the following:
How completely and accurately depths are indicated on the chart.
The irregularity of the depths.
It is impossible to obtain a position by soundings if the ship is located in an area where depth is uniform throughout. In practice, position by soundings ordinarily serves as a check on a fix taken by some other means.
Suppose you have only one spot on or near your DR track where water depth is 6 fathoms and the depth over the rest of the area for miles around is 20 fathoms. If you record 6 fathoms, you can be certain you are located at the one point where a 6-fathom depth was shown on the chart.
Piloting by soundings is not that simple. What you really do is get a contour of the bottom you are passing over and try to match it up with a similar contour shown by the depth figures on the chart. One of the best methods is:
Draw a straight line on a piece of transparent paper or plastic. Calculate how far apart your soundings will be, in other words, the length of the ship’s run between soundings and mark off distances on the line to the scale of the chart. Alongside each mark representing a sounding, record the depth obtained at that sounding. The line represents the ship’s course. The line of soundings recorded on the overlay should fit the depth marks on the chart somewhere near your DR track. If it makes an accurate fit, it probably is a close approximation of the course the ship is actually making good.
