Greg:

In general, you won't need to worry too much about using the correct datum for automotive navigation. The differences that exist between WGS84 (what the GPS constellation and your GPSR uses by default) and datums used on any paper map that has been printed in the last 20 years will not be big enough to cause you grief if your objective is simply to navigate on roadways or well established trails.

You might encounter differences of, for example, 100 feet or so between WGS84 and a datum used on a map printed in the last 20 years. It is very unlikely you will encounter a larger difference unless you are using a very, very old map - for example, something printed before 1970 or so. This is because most map publishers have already made the appropriate adjustment to the lat/long lines that appear on the map to bring them into fairly close conformity to WGS84.

Mariners need to pay attention to datums, because in many cases, maps that are printed today are still based on datums developed 150 years ago, and the lat/long lines on these nautical maps have NOT been corrected.

The two biggest reasons you don't have to worry much about datums are as follows:

1) Road maps are generally very large scale maps (e.g. 1:300,000 or more), which means that the tip of a very sharp pencil will have a greater error than the map datum will. Nautical maps are often 1:20,000 or less - at these very detailed scales, you will notice the datum difference.

2) If you are riding along a road, and see that because of a datum difference, there is a huge cliff where the map suggests there is a bridge, you can rely on your common sense to not lead you off the cliff. This is more difficult for mariners, who cannot see a big rock that is 5 feet below the waterline.

Basically - forget about datums, unless you have a very, very detailed map, such as an old map of a city center or something like that. In such a case, look in the margin of the map to see if there is a note there indicating the datum used. If there is no note, then the lat/long overlay on the map has probably already been (roughly) corrected to WGS84 by the company that prints the map.

Michael