I've just started playing with high resolution data from a surveyor we hired for our exploration project. Before that, all locations were based upon two primary sources: a USGS 1:24,000 (1 inch = 2000 feet) topographic map as a basemap (digitized into AutoCad) and readings from a high-end, commercially availiable gps.

I was curious to find out how the survey (which has a resolution of tenths of an inch) matched the other two sources. So far, the gps readings are pretty good, usually in the error limits of a USGS topo map, and often within a pencil width (in other words, on a 1:24,000 scale map, the width of a dot made by a semi sharp #2 pencil). Even at field map scale (1 inch = 100 feet) the gps works really well for putting a dot on the map. In mapping using many different sources*, I have to decide a pecking order of what data to trust first. Right now, that order is Survey, GPS, USGS, Air Photo. I have got to get some better Air photos!

The biggest factor seems to be the amount of time you let the gps get the signal.

The gps will never replace the Brunton compass in this line of work (unless it gets a built in 3D clinometer, and a mirror), but it has earned a place in the pocket of my field vest. I need to get a spare battery for longer days; the Brunton never runs out of juice. But I use the darn gadget all the time while travelling. It was quite useful when I wasn't quite sure if we were on the right train in Rome recently. My increasing concern over the gps readings diverging from our intended destination lead to my wife talking to some fellow passengers with a few words in Italian and lots of hand gestures and showing of tickets. They told us we were on the wrong train and showed us where to catch the right one. Nice people. (Disclaimer: I picked the train to begin with.)

Oh, do play with the overlay feature of Google Earth if you have maps of your own you'd like to see in 3D. Coolest thing ever.

*The sort of compilations I do use maps from many different sources: USGS topos, maps from USGS professional papers, figures in published articles, hand drawn maps from the field, ancient mine maps, air photos....and at many different scales, projections, and quality. The interesting thing about this sort of compilation, especially in CAD, is getting comfortable with error; because error is always there when your drafting can resolve thousandths of a foot. The consideration is always, "what is the resolution I need to answer the question?"