auto zoom setting garmin 478

The documentation (sic) is pretty much silent on the menu settings.

In auto mode, in the map page, menu takes you to map setup.

You can set autozoom off or on.

On a later page, you have three autozoom setting for (something like)
local, highway, freeway, each of which is a list of distances which
appear to correspond to map resolutions.

I can't figure out what these do.

I want to set a max zoom for each condition. But that's not what
these settings seem to do.

As best I can tell, they set a cut off. If the resolution is greater
than the setting, you get almost no map detail.

What's going on?

Is this stuff documented anywhere?