"don't understand why mbtiles is only considered an overlay."
We have been over this before, and my opinion does not change.
MBTiles should not be considered "real" charts because:
1. They do not contain the correct metadata to ensure safe navigation. Example: Depth soundings, if they exist: Feet, meters, or fathoms? You say, well, obviously photo
overlays do not contain soundings, so this is not relevant. But it is. There is no way to know what the content of an MBtiles represents. Could be a screen grab from some other charting app, could be Google
data, could be SAP. The tiles are simply georeferenced PNG images
, without backup information.
2. They are mostly homemade, with absolutely no provenance. Could be georeferenced totally in error, and there is no way to know, or even test for this.
In a place where only GSHHS and MBTiles are present, I would say that OCPN is simply a useful visualization tool, but certainly not suitable for navigation.