They are not perfect. When I bought my last farm, I noticed a lot, an in-holding, shown on my tax maps that I did not know that existed. Puzzled, I studied the map and then noticed the one in-holding that I did know that existed was not shown. I measured from the nearest road intersection and, voila!, I found that the extra one and the missing one were both the same distance to the corner - they obviously measured the right distance, but on the wrong road. Dreading bureaucratic hassles, I went in to check on this mistake. They saw what I meant immediately and without question, made the correction, thanking me for bringing it to their attention. Afterwards, I began to think, "If changes can be made so simply, what keeps people honest?" Still later, I realized that they would probably check the deeds at a later time to make sure that what I said was correct. They had written my changes in red pencil.
Surveys are not perfect either, but they are the best we have available. A case in point is my own home. When I bought it, I asked the owner if he could show me the boundaries. He gladly agreed and during the course of the walk, it became obvious to me that he did not really know a single one of them. So, I had a survey done. It's a small property and I did not want trouble with any neighbors. The results were surprising. No major differences, but several tiny ones. The most surprising one was the back boundary which is clearly defined by big trees on my side separated by a wall and fence from an abandoned field on the neighbor's side. Yet, the survey formed a skinny X with the obvious boundary, I went beyond it one one side and the neighbor's land went into "my" side at the other end. We are talking 15' or so, nothing much. But still it was not obvious. In another area, I ended up with a line running through a woods that I figured had to be mine. He had found an old fence, so old I had never seen it, and determined that the boundary was there (it corresponded to what he found written on paper). I was disappointed not to get that part of the woods but when everything was all calculated, I ended up with one tenth of an acre more than I had figured on, more than the tax map and deeds stated. A surveyor spends a great deal of time researching recorded papers before he sets foot on the property, then uses what he has learned and combines it with what he observes (lines of extra-large trees, field boundaries, stone walls, old fences...) to make the final map, one which he always may be required to defend in court. That actually happening is rare, but it is something they have to consider.
The tax map people do not research the deeds quite so thoroughly but the big difference is that they do not actually set foot on the property. But, they are pretty good. I would say that, in my experience, 90% of the time they are perfect and maybe 2% of the time they are off enough to be easily noticed on maps. Local attorneys routinely accept tax map boundaries as legal and sufficient for conveying property. They do check the deeds and maps and make sure they do not see abnormalities before clearing to close. There was one case where the owner's knowledge of the boundaries corresponded exactly to the tax map. Yet there was something that the attorney found that caused a question. Not wanting to delay the closing for this, the buyer and seller agreed that they would pay a certain sum per acre to the other if a survey, made later on, showed more than a 5% discrepancy. The survey, when done, merely corroborated the tax maps as it turns out. the next time that property was sold, the same question came up, but this time we had the survey map in hand to assuage the attorney's concerns.
The reason to survey on subdivisions? Quite often they do not follow historic boundaries and can cut right across fields, to give an example. Eventually, there may be a hedgerow that grows up along the new boundary, giving us new "historic" lines - but it takes time and, until this happens, we need the survey. And the lawyers can use the survey's language to draw up a good deed description, one which will not be in question. The historic deed descriptions are rarely questioned largely because they have been so well established over so long a period. It can be confusing, but, in the end, it all works out.