help request: hatch areas between topographic lines
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The concept makes sense, but I think the execution will be more complicated than you expect. In this example image, the solid red and yellow Splines are arbitrary contours. The even-dashed ones of the same color are Offset from those, at a target distance [your "n"], to find where the other solid one is exactly "n" away.
The dashed red Spline is exactly "n" away from the solid red one all the way along, and likewise with the yellow ones. The magenta Line is where the red Spline thinks the yellow Spline is exactly "n" away from it, and the green Line is where the yellow Spline thinks the red one is exactly "n" away from it. Both the magenta and green Lines are exactly "n" long. They obviously represent different opinions about where "being that far apart" actually happens.
So as an attempt at a split-the-difference compromise, I put in the solid blue one, which is an averaged path between the solid red and yellow ones [via a routine I have been developing], and I Offset that by half of "n" in both directions to get the dashed blue ones, which are exactly "n" away from each other all along. The cyan Line runs from where the solid blue path thinks the solid red is half of "n" away from it and where it thinks the yellow one is half of "n" away from it. Its endpoints are not at the same place relative to the center blue path [if you project from them perpendicular to it]. Also, that cyan Line is not "n" long [in this case it's about 7% longer].
I didn't want to clutter up the image too much, so this isn't in it, but I also tried Offsetting the red and yellow Splines by half of "n" toward each other. That gives intersection points that are half of "n" from both Splines. But Lines perpendicular from those intersections to each Spline, while the sum of their lengths is "n," are not collinear. In this vicinity that intersection is even farther to the right than the cyan Line, there's about a 30-degree bend between the two Lines heading perpendicularly to the contours from there, and the straight-line distance between their outer ends [where they meet the contours] is about 4% shorter than "n." That is also about the length of a bisector between the green and magenta Lines, trimmed to the contour Splines.
You would need to come up with some rule whereby the determination of where the contours are, in fact, "n" apart is to be made, and in what way [i.e. to what locations on the contours themselves] the Line marking that location should be drawn. Maybe you give in to the ambiguity, always Offset the "higher" one only, and just go with where it thinks the next lower one is exactly that far away. But you may have reason to use other criteria. In any case, I don't think it would be easy [if even possible] to automate something like that.
[But it wouldn't surprise me if the Civil Engineering overlay program has some kind of capability along those lines.]
Kent Cooper, AIA 
