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PDF Maps is the app that I use more extensively, mostly when on foot, but it works ok off-road when you're moving slow, too. It's designed to utilize PDF files with embedded geospatial data (GeoPDF) and does the job very well, in my opinion. It's also a free app, and while you can make in app purchases of map tile sets, it also works very well with the free geopdf maps that are downloadable through the USGS map locator and downloader. Quadrangle maps (7.5x7.5 minute) are pretty good resolution and most (in my region, at least) have been updated in the last 2-3 years. These maps are next to useless for on-road navigation though, and do not in any way shape or form support turn by turn, route planning, etc... unless its in the form of a GPS track. This is where you need the microSD, though, as the maps take up quite a bit of memory. The lower peninsula of Michigan west of US-131 adds up to about 5gb if I remember correctly, so it will eat up the internal memory of an 8gb tablet very quickly.
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