[dread78645]

I'm not that familiar with Navy technology, but what DOES one use to get a "sounding" for depth, if not sonar?

Fathometer. Low power, high frequency, short duration ping directed straight down.

[SauronOfMordor]

Fathometer. Low power, high frequency, short duration ping directed straight down.

Thanks, I get it now. So what the Navy investigators are implying is that, once he saw the fathometer readings did not jibe with his chart, he should have gone to dead slow and taken more readings (which may have shown him that the charts were out to lunch)