I don't think I'm understanding the properties of a viewshed tile. Each tile is sized so that the viewshed size (in square metres) of every point within the ti

I don't think I'm understanding the properties of a viewshed tile. Each tile is sized so that the viewshed size (in square metres) of every point within the tile can be found just from elevation data within the tile itself? But that doesn't seem right from the example you gave, because along the edge at the mouth of the river, the viewshed presumably includes the rest of the river outside of the tile as well?