If State Plane coordinates are multiplied by the DAF to get ground coordinates, then the DAF is the reciprocal of a combined factor. Calling it something else does not change the fundamental conversion between grid and ground.
Take the reciprocal of the DAF for the last project you worked on. Compare it to a fairly representative NGS control point within the project's area and see if it closely matches that point's combined factor. If it doesn't, then there's a serious flaw in your operation.
You're right about the angles and lessening the convergence angle is a strong point in favor of LDPs. Not cured, but lessened.
I think the only thing that limits convergence angle change is the east-west extent of your projection zone, with little dependence on the type of projection.
The Iowa RCS cited in these discussions has about 0.75 minute per mile of convergence at latitude just above 41 degrees.
We aren't really discussing the "proper" way to do this, rather the convention that has developed over the years, surveyors are different I guess, we will express coordinates in northing, easting and are often told it should be easting, northing even though we are well aware of that, we use us survey foot and not meters or international feet, and so on....
It's just how this has always been done so by golly we will continue to do it this way;-)
That somebody would be a consortium of professional land surveying, engineering and GIS organizations. They agreed that 20 ppm linear error was a worthy goal that would meet most every engineering, surveying or GIS need just as North Carolina engineers agreed 100 ppm would meet every need in the 1930's. They also agreed that it was more important to get all positioning interests on the same page with respect to scaled or un-scaled coordinates. Surveyors are usually the biggest offenders of providing modified state plane coordinates with not enough information to get them back to a true geographic position. What's better? To have a zillion modified coordinate systems or 14 truly geographic systems?
If it ain't broke, don't fix it. By the way, a Google search for DAF turned up this item. I think it's paragraph 6 that describes the DAF in terms of the combined factor.
Now if somebody setting up a project fails to do the reciprocal of the combined factor to get the DAF ......
The one that started this thread covers only a few thousand square meters.
I'm not a surveyor, so my opinion on such things shouldn't count for very much. But, from a mathematical perspective, I would prefer a geodetically correct surface, regardless of the area it covers.
There must be thousands of projects that have their own adjusted coordinate systems. There's probably a good reason why thousands of projects with geodetically correct coordinates is a bad idea in comparison, but it's hard for a layman to see why.
Exactly:
This is an example of what the program Glen is discussing looks like and how it sets up the WY DOT form of working plane coordinates, many other programs are similar:
the "ground scale factor" needs to be put into the program in this form, and it would be documented as the DAF. The convention to apply it this way has been so long used it is second nature to me, but of course if I see 0.99978 as a scale factor I would have no problem using it either;-)