A bearing line is one piece of a route
Set a direction from an invented harbor. The ray shows that bearing across a flat schematic sea. This is not a portolan facsimile, navigational tool, or model of historical magnetic correction.
Why the model is incomplete
A direction is not a distance, a safe passage, or a coastline survey. The grid is a modern explanatory aid and not a claim that portolan charts used a uniform latitude-longitude graticule. Real charts combine place names, coastlines, scales, and bearing networks.