I think this thread has demonstrated that since appraisers will never have access to the method or standard of measurement of the homes we use as comparables, that it is REALLY the realtors and assessors (add builders in as well?) who ought to be getting a standard forced upon them. And training to boot.
The NAR could easily come out with guidance on this. I served as treasurer on my local realtor board one year, and I was amazed at the amount of mis-information and downright confusion that realtors have concerning GLA, above grade, below grade, living area, etc. From what I have seen in assessor records, I think some of that confusion stretches to them as well. Add to that the simple fact that in many, if not most, jurisdictions, the assessor never steps foot into a home. This is especially true of older homes. How many homes have we all appraised where the assessor's GLA is different (more than nominally) from what we discover--additions, open to below spaces, finished upper levels, garages which are no longer garages. Living area that USED to be garages, etc.
I think this is actually a major issue in appraising, is should be addressed somehow. I have no easy answers, but force feeding one standard upon appraisers doesn't seem like it will solve a single thing. The GLA of a home really doesn't matter in any absolute sense. It only matters RELATIVE to the GLA's of homes used as comparables. That does tend to be the largest line item adjustment in the SCA.
Desktops and hybrids where measuring is done by less experienced persons will only exacerbate these issues, IMHO.