I thought I was beginning to understand this stuff, but now the mysteries are multiplying.
I backed up and created what I thought as a simple map, importing a shapefile of roads into GPSMapEdit. All the roads are imported as type 0x000a (unpaved) at level 0. I add another level (otherwise I get an error that about generating the preview). Here are the various settings:
And here's what a small piece of the map looks like in GPSMapEdit
Here's the corresponding region in MapSource - notice the extra straight lines. What are these? They appear to be some sort of simplification of the curves, but why are they there? They seem to relate to my zoom levels, but how?
And the final kicker - when I zoom out, I get this:
I've tried this process numerous times, tweaking zooms, changing type, use of external .typ file, etc, and keep getting this nonsense. I've been careful to delete all my intermediate files (rename outputs) so I know I'm seeing the results of my new run. I suspect these are newbie errors that are familiar to the more practiced, but I'm baffled.