Comment by mothballed

In my county they set them to values that seem proportionally roughly correct but they don't really attempt to figure out the market value.

It all works out because the county uses N appropriated dollars and everything flows from that (people that think they start at some percentage of property values and then work to what the county can spend have it backwards); you currently own $X assessed value out of $Y total assessed value in the whole county, the property taxes you pay are N * X / Y, and the property tax rate just deducted from that. Whether the actual amount assessed is accurate is immaterial to paying a proportional share.