NavList:

David McN, you wrote:
"I haven't been able to satisfactorily come up with a quick and easy interpolation method to answer that, either by eyeball inspection or brief pencil and paper. I wonder if any NavListers have already done this and can help?"

First, I wouldn't call this an interpolation question. I think you're describing an "inverse lookup" issue. David P has already described how to do this by calculation, and a navigator a century ago might have done it just that way. You might think that a calculator is "cheating" but then so are logarithms. These are tools that yield the right answer by following an organized series of steps. A navigator who uses logarithms is not smarter than a navigator who uses a calculator. Neither really understands what's going on inside the "little black box", right?

For an inverse lookup in a two-dimensional table, you can poke around until you find a matching answer, but it's much quicker if you can get a close approximation to one of the numbers you're hoping to extract. You can do that by drawing your triangle. The Lat (really dLat) here is just "Dy" in a cartesian coordinate triangle, and the Dep is just "Dx". So get out a clean sheet of graph paper (or plain paper with a little more work) and draw the triangle. The distance travelled is then the hypotenuse, and the course (arbitrary to a quadrant) is the angle at the corner of the triangle. You could calculate the length of the of the hypotenuse by square root of the sum of the squares of Dx and Dy, but really for the practical problem at hand, drawing it is good enough. Use dividers or a strip of paper and read off the length directly. Then you can scan down the proper distance columns until you find the Dx, Dy that are close. Alternatively, you could calculate the corner angle as the inverse tangent of Dy/Dx (or Dx/Dy), but if you have a protractor, you can read off the angle from your drawing ...if you're working carefully. Once you have either the distance or the angle, then you have a simple one-dimensional lookup problem in place of the two-dimensional hunt and peck. Of course, you'll probably realize at about this point that you don't need the table anymore in the first place. Read the distance and course from the drawing, and you're done.

All of this description is based on some assumptions about what you're trying to do. But how would I know what you're trying to do?? It would be helpful when you post a question like this to describe the practical navigation problem that you're trying to work on instead of phrasing it abstractly. Just tell us what you're up to, what are you trying to achieve in the process you're working on? Makes life easier for all of us! :-)

Frank Reed