A span of land is a contiguous group of adjacent land sites, regardless of who controls those sites. If a card refers to 'its span of land' or 'this span of land,' it means the span of land occupied by that card. If that card doesn't currently occupy a span of land, then it doesn't have a span of land. To choose a span of land (e.g. Minecart Madness), choose one land site. Every land site (regardless of controller) that's connected to the chosen site via adjacent land sites is in that span of land. If a site becomes disconnected from the chosen land site (e.g. Earthquake in the middle of the turn), the disconnected site is no longer in the span of land. If the chosen site is no longer a land site (e.g. Flood in the middle of the turn), then the 'chosen span of land' no longer exists.