The surveying equipment is very accurate—to one hundredth of a foot.
But the problem is usually not with the equipment. It’s with the assumptions used by the surveyor about how to use the equipment—specifically, which data to use, and which base points to use.
If he can find ,say, three of the four original steel pins which were set in the ground decades ago at the four corners of the lot, he has a strong base to begin with.
But if he only finds one of them, or if he finds two of them, and the distance between them today is a couple inches different than what it should be, the surveyor will use his professional judgement and experience to create the best fit of the data. It is not unusual for two surveyors to disagree.
A surveyor will give you the mathematical data; a lawyer will ask all the questions in Sparky’s post above. It can be simple, or it can get messy.So before you hire another surveyor (and then a lawyer), ask yourself whether it’s worth fighting over.