I pull the assessor's card and sketch on every property (if there is one) and print the their sketch on the back of my inspection form. I then pull a tape to "verify" the measurements. Sure does make the inspection go faster.
Most real estate agents no longer personally measure residential properties any more. The MLS sheet has a box to indicate where the square footage used comes from...Personally Measured, Public Record, or Appraiser. The majority use public records.
Since about 1980 builders are required to present the plans to the assessor's office. Their sketches come from those plans. Are they always right? Of course not, but I will put them up against most appraisers IN MY MARKET.
If you are going to be reviewed, where do suppose the reviewer is going to get the GLA data? Public Records. Where do you get the GLA information on the comparables you use in your appraisal reports? Please don't tell me you personally measure the comparables
Do you measure to the nearest foot, half foot, quarter foot, inch, 1/10th of a foot, or 10th of an inch? Sure is fun when you have two old listings, one current listing, the public record, and an old appraisal...and all of the measurements are different.