Ran into this woman at the Great Lakes National Cemetery in Holly, Michigan (42.853453°N, -83.616289°W) while we were volunteering to pick up flags placed at veterans' graves over Memorial Day weekend.
The device she's holding has a small camera at the bottom that takes a picture of the gravestone, while the high-accuracy GPS unit at the top records the location. She said this device is accurate to within about 10cm. What she was doing was actually a backup to the higher accuracy work that has already been completed (accurate to a couple cm). Behind her are part of the gravestones of one section of the cemetery. There are 15 or so sections and about 30,000 internments. There are a total of 142 national cemeteries in 40 states and Puerto Rico.
Makes the job of documenting 26 mile marks seem kind of small (in more ways than one).