The lack of this function is for example significantly slows the analysis of OH arcLines files from SuNSS.

This function exists, but is named findWavelength, probably for historical reasons. So I'll take this as a request to clean up the interface in regard to get* vs find*. I propose:
1. Move findWavelength to getWavelength.
2. Move findPoint to getPoint.
3. Move findFiberId to getFiberId.

I'm a little concerned that get* implies a fast return without much in the way of calculation, which is not the case for any of these, but it's certainly the name we look for first, so perhaps convenience trumps that?

Oh, good. I think the name's OK; the user doesn't know that we don't have these things pre-computed to logically it's just a `get`

I don't see the corresponding vectorised function to return the fibre position (in x) at a set of wavelengths for a given fibre. This could be done by adding a vector overload to findPoint (returning a 2xn array), or a new function (getTracePosition?)

There is a vectorised version of findPoint already. If it's not working for you, check that your types are correct (fiberId arrays should be int32, wavelength arrays should be float64) and the arrays are contiguous.

Oh, that's great. I did look at the .h file, but didn't realise that you can vectorise over both fiberId, lambda which is exactly what I needed.

OK, so what's the verdict on what work needs to be done?

It sounds as if we should have a chat about a consistent API, but that all the needed functionality is present

Can this ticket be closed, and a separate ticket on the API update filed?

Problem is due to confusion over API. This is the subject of a separate ticket.