Somewhere along the line the home team coach has to provide to you what the prevailing ground rules are for his field. The umpire does not SET ground rules, only enforces them and decides what they are if BOTH coaches cannot come to an agreement on what they should be. Part of the pre-game conference is to talk about existing conditions and if they differ - for example - for a gravel area next to a dugout at the end - is it live ball or dead ball territory. I worked one game where there was an area within the confines of the fence but tarped over with old tires holding it down. It was made clear by HT that the area was DBT however it got there, even though fielders had full access. I said OK, let's go, and that was that.