Untitled attachment https://digipres.club/system/media_attachments/files/113/744/931/497/006/439/original/68f9eb0acad0e2a6.png
So this is interesting: Changing that byte made one vertex of the triangle change so that it matches a vertex from the river.I bet that means this is a format where you define vertexes, then define shapes by connecting those vertexes together.