Interpolator

A real table is built the chained way. Interpolating properly in N directions at once needs 2^N corner values — two factors is four numbers, three is eight, five is thirty-two. Nobody prints that. So a sounding table gives you one figure against the sounding and then a separate little table of corrections for trim, and another for heel, each needing only two numbers. That is what a reading plus corrections is for, and it is the mode you will use most.

Two factors at once is for the rarer table that really is indexed by two things together — a grid with the columns headed by one factor and the rows by another, one value in each cell. Four corners, read along the top row first, the way it is printed.

Subtract is not add-with-a-minus. Your table prints “deduct the trim correction” and a positive number beside it. So you pick Subtract and type that positive number. Making you type a minus sign onto a figure the table prints as positive is exactly how a sign error gets in, which is why the two are separate choices and why the step list shows the correction as the table gives it.

Extrapolation is flagged, not forbidden. If your figure falls outside the two rows you typed, the straight line is being carried on past its anchors and it stops being evidence of anything. Sometimes that is what you want. More often it means a row was mistyped. Either way the step is marked and so is the answer.

A blank is not a zero here. Leave a point out and there is no answer — the result goes blank and says which field is missing, rather than quietly computing from a gap. If any one correction in the chain cannot be read, the whole chain is unanswerable: a partial total that looks finished is worse than no total at all.