No problem is your target accuracy is +/-30ft/10m. You will get 3 to 5m [10-16ft] SA with most outdoor Garmins in good average conditions. Very easy to locate corner and line pegs in open country with that accuracy provided they still exist of course. If there's been a recent resurvey it'll be easy to find them even in the bush.

GIS is quite a generic term and although i think most maps are in a basic raster format there may be individual built-in protection which makes them 'proprietary'. However i don't think that's the case with Local Government GIS maps in the US, i have a recollection of reading there are legal precedents where the courts found that the basic GIS information is part of the public domain and as such should be made available by the authority. Therefore it should be no problem to covert their digital GIS mapping data to a form usable by your Garmin. One such program is ExpertGPS:
Code:
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Info on others including Open Source/free software is here: [Only registered and activated users can see links. ]. Have a look at the other Stickies here too: [Only registered and activated users can see links. ].

I'm by no means a map making expert but we have members who are, perhaps you're get more targeted info in further posts.