The issue is that the conversions don't actually "get on" the point. It's been my experience that they get you close, if .5 to 1' is good enough then that's probably what you'll find. The real way to "get on" NAD27 is to get on the monuments. It depends what you're trying to do. Old highway jobs-get on that control. Old city work-get on that control. Oil and gas geological data maps-doesn't really matter since they aren't on great control anyway.
@mightymoe i see what you are saying now. That most definitely could be true of points that have moved or whatever. The point was only as good as the last time it was measured. In my case once I reverse engineered there scaling the coordinates it was bang on less than .15 ft most were less than a tenth. Only one was about.15 ft . So ncat was correct and tbc now is. At least in my area. But I ended up not being able to hold the true values as its all relative to the scaled coordinates. But I promise you everything i saved has a big note stating it. It site control not nad 27 or nad 83. Lol.
3 feet? Check on the US feet, Int'l feet thing before going further.
Ellipsoid, US feet, and why aren't you just doing a local calibration anyway? Who knows how well they fit the actual datum.
@dmyhill I was almost ready to do that. But its very linear and once i started investigating and following what all had been done prior it was easy to figure out. And now its all cleared up and in the clients hands. Hopefully i will get to redo all the control so its correct. I tied it all down and did what was needed.
I'd be skeptical of creating a custom calibration or transformation unless I had plenty of monuments to go off of. As a general rule if a someone wants NAD27 they are getting NCAT- or TBC-converted values based off of a good, solid NSRS survey, plus a narrative explaining the pitfalls of NAD27.
It's rare that clients actually, truly need NAD27, and if we can get them in a meeting prior to finalizing the contract, it's not uncommon to convince them to go with a more reliable datum, or at the very least deliver both current NSRS as well as NAD27.