How are GRM's on appraisals determined?

[Johann Jells]

I have a long held commercial multi-family in a city with a prospective revaluation coming, and I'd like to know what kind of tax hike I'm in for. Last I knew appraisals for commercial properties were by income and not by comps. But how else do they come up with the appropriate GRM if not by looking at sales, which amounts to comps?

 I guess what I'm afraid of is the city appraiser using the same methodology as most appraisers I've dealt with recently: 1st decide what the target is and fudge the numbers to get there.