I've used a G 590 for about 2 yrs. The built in map sys is a N.Am. street map, and it has the vast bulk of Forest service roads. Garmin sells a DVD of NW TOPO and other regions. You own the license unlike all the downloadable map sys. So, I have the Topo maps on my HDD and I have installed it into G Base Camp and the Z 590. In the 590, I can select either the street or the topo map sys but not both. I can use either map sys for route planning in Base Camp. Findings:
First, A route planned in Topo in BC and used on the device in either map sys will work but the magenta line will usually be displaced somewhat. Second, the Topo will show contour lines everywhere and Nat'l Forests will be green, everthing else white; there is much less detail than with the street map sys. Third, I eventually found use of the Topo map sys resulted in nav errors like the device freezing and requiring a hard reset. Fourth, all the forest roads with two numbers or four numbers are in the street map sys and all I lose with using that is the contour lines. Fifth due to the level of detail presented, I can't use contour lines for nav unless I also have a paper map, and I always have paper maps and have taken lat & long cell shots a few times and plotted the result on a paper map (this is a datum issue because the world wide datum for the GPS constellation is WGS 84 and the Lat & Long you get from the Zumo will not plot correctly if the paper map is based on some other datum). Finally, I stopped using the Topo map sys because it had problems and offered no advantages. The device tells you where you are on the planet but without a map you do not know your positions in relation to any fixed point like an intersection of two roads. You can put the device into on off road mode and use it to nav to a lat Long position but it will go as the crow flies and will not stick to roads. If you ride on a road with a WGS 84 datum map in the device, it will show you on a road moving somewhere. If you use the updated lifetime map in the device, the device will show you where you are on the map. It will show you what lane you are on on the fwy, but you know where you are. If you are out in the boonies where you have never been before, you can't check or verify your position from the device with out a decent paper map: you will cross streams, go over bridges, and make turns and all that is on the paper map, and it could be correctly displayed on the device but only if it has the correct, compatible map sys. Trust and verify. You've heard of the woman (man) driving into a lake following the GPS. The GPS position was correct to a foot or so, the map in the device was not. Most of us cannot orient ourselves to a lat and Long without a map. Mariners do it as they have no choice but not without charts. Lubbers never use lat and long for orientation because it is hard and cannot be done without a paper map, AND, if you had a paper map that just displayed lat and long, you would have to have overlays containing the terrain, roads, and places or not know where you are in relation to the things you can see. If I was going to use a topo sys in the Zumo, I'd get the map sys from Garmin.