I've used a 30 minute vernier, a 20 minute vernier, but never a 15 minute vernier.
Here is a link to a 15' one.
It makes me wonder. Did anybody make a 10' vernier?
These old instruments are a part of the genre of forgotten survey devices, I'm afraid. Along with drafting machines, planimeters, and (cough) pencils!
It seems to me that this trend is a one-way trend. We ain't goin back to them, and I have no desire to.
Ah! Nostalgia! It's a sentimental thing.
With a clean 30" direct reading vernier it is possible to interpolate to 15" and less when the grooves were polished to your liking.
I cleaned and applied paste black boot polish to the verniers of a Dietzen on an as-needed basis. the other Iman liked nothing on the surface. We worked rain or shine in all environments. Construction projects and other barren soil laden areas required the most cleaning.
If we had to send an instrument into a shop we had to use a crusty ole K&E Paragon he had toted back and forth across Texas running some of the original electric transmission lines. The boss would not allow anyone to attempt to clean or open a cap.
Certain lots of instruments were superb and others were ok and all passed the same tests.
The vernier was an interchangeable and adjustable part with few special order spare parts to have picked from and they were short in supply.
Sorry to poop on anyone's parade, but early vernier geodetic transits were optically readable to 15" and if used in a stable environment by using a "winding up" technique could be good to less than 5". Kern & Wild T-2s, etc., with optical microscopes viewing both sides of a glass plate simultaneously eliminated most plate errors, and micrometer coincidence adjustments were highly accurate for sub minute estimation, maybe good to 2-3 seconds with inversion techniques, again, within environment and stable platforms, when observing distant targets.
Modern robots approach such accuracy; the limit is atmospheric considerations, target acuity and instrument stability. I assert the T-2/T/3 type optical instruments and larger pure optical instruments are more accurate in angle determinations than modern robots, with their prism target inaccuracies. Let's not boast about our robot's angular accuracy when observing 1,000' or shorter foot shots.
Of course, GPS is much more accurate in the 30,000+ foot shot arena, where angles are less important than distance and the game devolves into +- a few tenths of foot, good enough, But if you're staking 2,000 feet of curb, a simple optical theodolite will perform to +- 0.02', GPS observations be damned,
I have many more hours behind a transit, than a T-2. But, I've got hours behind both.
The T-2 is much better. But we are not trying to mess anybody up. We are surveyors. We use the tools we've got.