One canvas for everything spatial. Use the polygon tool (top-left) to draw the field boundary, the 🏪 tool to place the Trading Post, and the ⚓ tool to add Harbours. Drag any point to move it; use the delete tool to remove one. Everything saves automatically.
To move a point, use the edit / drag tools on the map; to remove one, use the delete tool (top-left).
Place it with the 🏪 tool on the map. Set how close players must be to trade.
Use the ⚓ tool on the map to add one; drag to move, delete tool to remove.