Depth soundings
Wherever the bottom is of such a nature that it forms clear contour lines, your echo sounder is a source of PLs. Reduce the depth to soundings, then inspect the chart. Your PL is the relevant contour, none the worse for not being so straight as a transit (Fig 18.4). If your depth places you between two charted contours, you'll have to judge where yours is most likely to be.
The nature of things is such that contours are rarely so precisely defined as a transit leading mark. Even if your PL is effectively a cable wide, however, it may be all you have to make up the second PL of a two-point fix, so don't despise it.
Every time you take a fix you should hit the button of the echo sounder as a guard against gross error. It isn't usually necessary to reduce to soundings for these purposes. Just have a quick look. If the depth in the vicinity of your fix is seriously adrift from what the chart says it should be, then as with an FT, something is wrong.
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