A couple of quick comments regarding Datums and Coordinate Systems.

NAD27 is the older datum used in N American. Older USGS topo maps used this datum.

NAD83 is the current datum for newly published USGS topo maps. WGS84 is the default datum on most GPS units when they come out of the box. I believe all current nautical and aeronautical charts are published using WGS84. It is important to understand that NAD83 and WGS84 are functionally equivilent for us as map users. The difference is at most a meter or so anywhere in N America.

Coodinate systems such as UTM, US National Grid (a UTM variant), and the various versions of Latitude and Longitude (Decimal Degrees, Degrees & Decimal Minutes, Degrees & Minutes & Seconds) can be used with either NAD 27 or NAD83/WGW84.

However, a given coordinate in any system will plot to a different lcoation in NAD27 than it will in NAD83/WGS84. The difference in N America varies, but is usually less than a couple of hundred meters at most. For example, in Alaska the surface location of N 61 deg, W 149 deg 15 min in NAD83 is about 63 meters north and 118 meters west of the same Lat and Long in NAD27. (About 134 meters WNW.)

I know this stuff can get very confusing. The key things to remember is that for ordinary navigation purposes, NAD83 and WGS84 are the same. And the difference between NAD83/WGS84 and NAD27 is at most a couple of hundred meters. This is true in ANY coordinate system. As long as you set your GPS to the same datum as your map, you will be fine. If someone gives you coordinates of a particular location, make sure you know what datum they are using. A particular peeve of mine are guidebook authors who give GPS coordinates, but don't specify the datum!