Thanks. When you say overlay maps... you mean you still get the onscreen navigation (with other metrics like speed/distance) and are able to zoom in and see the trail as opposed to just breadcrumb on a line?
There's a lot of flexibility in how these things work. But for one, let me just stop the notion that a bike GPS will navigate you like a car GPS. If you do the legwork and program a route (especially on ridewithgps), you can program a Garmin COURSE to mimic that sort of navigation. It takes some time to do, but it works pretty well on the road. I've done it a number of times. It does not work well on mtb trails. There are many reasons why. Just suffice to say that it doesn't work well.
People who use these devices to navigate mtb trails use a different method to navigate than that. This involves displaying the file you want to ride on the map. Digital maps are composed of layers. This is somewhat simplified on a Garmin, but you can select different maps that can display at the same time. So you can load a topo map from gpsfiledepot.com and then layer a trails only map from Trailforks on top of it (because the Trailforks map has a transparent background, but you can also get a map from Trailforks where they've already layered the trails on the basemap for you). Then, your device will layer your planned ride on top of all of those things. That's what I mean by "overlay". A map layer that "overlays" another.
The map is on its own screen. The 520 models and higher can display metrics at the same time, but they reduce the area the map will show. Models with bigger screens can show more data fields at the same time as the map. But typically, those extra fields are going to be on different "pages" on the device. You can have several different pages with different data fields on them.
I don't know about the 530, but the 520 does not let you control the map much. The zoom controls are buried under menus, so they're not easy to use. And you cannot pan the map. If you move up to the touchscreen models, you have more/easier control over how the map displays.
I don't know what you mean by "see the trail as opposed to a breadcrumb on a line". These things aren't displaying satellite imagery, if that's what you're asking. The maps are all vector maps. Just points, lines, and polygons. Satellite images require massive amounts of storage space for a pretty small area or an active cellular data connection. That's why phone apps display them - the active data connection. No data connection, no maps. On a mapping Garmin, the maps are stored on the device so are always available. Vector maps are able to convey lots of information with minimal storage and processing requirements. So the trails you are riding will
always be displayed as lines on the map.
My Oregon 450 can display satellite images. But like I said, they require massive storage space for a small area. And the display quality is generally not good enough for them to be useful. So I don't use them for anything.
