I’m just going to toss this out as an alternative option.
I’ve done a fair amount of curve fitting and have often taken depth measurements at specific distances to plot out the control points for a set of curves—like what you’re doing here.
My first attempts of doing this were usually to level off a straight reference edge (typically one that covers the full distance of the contour—so I never have to reposition anything)… and then measure down from there for the depth readings, using vernier calipers.
This approach can be pretty accurate… but there’s room for improvement here since those reference points ultimately end up exerting an influence over the missing points that were never measured in the first place.
Resolution is of course the key here, and the closer together the points are… the more faithful the representation of the curve will be.
Generally speaking these days—and I say this begrudgingly (because I really like to find any reason at all to pull out any of my fancy and expensive measuring tools that I bought as a younger guy)—but, I don’t bother doing it this way anymore.
One can get better results by simply tracing the full contour of the profile onto any semi rigid surface that can easily be written on, and cut. Two of my favorites are 1/8 (or 1/16 MDF panel)… or even a suitably stiff bond paper (like whats often used in watercolor drawing books).
It’s not that hard to scribe and carefully cut to a scribe line. and from there You can photograph the contour and import that image into Rhino and start trace over the true outlines in that photograph if you wanted to.
What few mistakes are made can easily be fixed by remembering to always cut on the outside of the scribe line… and sand back to it using the grade of sandpaper to control how much you take off. It’s pretty hard to screw up a template with or 220 grit… and likewise you don’t have to sand too much if your using 60 and 80 grits.
Scissors and an x-acto knife take care of the paper templates (…and paper of the right weight can be sanded too, it just needs a little backing board, or finesse by sanding in-line along the papers edge—and not strongly across it, to point where you start to bend and fold it).
But the shortcut here is that don’t even have to cut the material you trace your line on… That’s only needed if your making a template which you want to do work off of.
For just importing an image into Rhino. Having the accurate scribe line is all you really need.
Below is an example of a panel sitting on two spacer blocks showing that you don’t even have to be in direct contact with the surface of the contours your looking to copy.
With a little extra work and preparation those block can be rigged so that they form L-Brackets and can hold the scribe panel in the position you want to use it in. Nothing fancy needs to be done here.
The only rule is that you want all of your scribe line to fit on the panel… and you don’t want anything to shift or move while your making your tracing (or scribe).
My last comment here is simply going to be a small apology… or justification perhaps. The thought did occur to me early on that if your talented enough to be making instruments… you probably don’t need to read my rambling about how to scribe a template.
My entire comment could have been reduced down to…
Hey, Mike why don’t you just scribe a template… and trace over it in Rhino.
and I’m sure you’d know what I was talking about. But others might not… so for better or worse I over explained things to cover the basic ideas for those who might not already know. This could be a mistake, hence my apology, in case this happen to rub you the wrong way.
Take Care, and good luck with the project,