I've been meaning to follow up on our prior discussion about GPS accuracy. I have had a few chances to experiment with four different GPS devices: one handheld Garmin (perhaps 15 years old), one handheld Garmin (perhaps two years old), an iPhone, and an iPad. I thought give some feedback on that experience.
First, in every test I did with all four side by side, each device always displayed uniquely different coordinates. In almost all cases, however, the first three digits where in agreement, and the fourth was usually reasonable close. Well, to be more exact, sometimes the third digit was off as well, but the overall difference in the numbers tended to be low when measured in thousandths.
Second, the iPhone and iPad display six digits, and the GPS devices only display five. I expected it to be the other way around, and I suspect that in spite of the extra digit the Apple products are less accurate than the Garmin ones.
Third, even though each device reported slightly different coordinates, if I used the same device to mark a spot, I was able to return to that spot quite easily whenever I used the same device. Whenever I used another device to navigate to the coordinates of the marked spot, however, I never quite got back there, but I never landed too far away either.
Fourth, to my surprise, I found it easiest to do geocaching with my iPhone than it was with the two GPS devices. (The iPad would probably have been the same as the iPhone if my 3G data service had been activated, so it didn't load the background maps.) I say this because I found it particularly helpful to have the coordinates drawn onto a map/satellite view. Of the two Garmin devices I had, the older one doesn't support maps, and the newer (borrowed) one came to me only with marine maps. I suspect my experience would have been different had I had a similar display on the GPS devices.
In summary, this experimentation just confirmed for me that this project is worth doing--and that it can be done with a smartphone. That last bit is something I like, because I want to use the phone to capture the GPS coordinates, and I suspect that most others who will use the coordinates will also be guided by a smartphone. They just need to be cautioned that the coordinates will guide them to the general area, not the exact burial plot.